DRC-2015-002245 - 0901a06880523f8c

MWH
BUILDING A BETTER WORLD
MEMORANDUM
DRC-2015-002245
TO: Harold Roberts and Kathy Weinel
Energy Fuels Resources (USA) Inc
DATE: April 24, 2015
FROM: Melanie Davis and Clint Strachan REFERENCE: 1009740
SUBJECT: Responses to Review Comments on Energy Fuels Resources (USA) Inc., White
Mesa Mill Tailings Data Analysis Report (January and March 2015) and
Probabilistic Seismic Hazard Analysis Report (March 2015)
INTRODUCTION
This memorandum presents responses to review comments provided by the Utah Division of
Radiation Control (DRC) and their contractor AECOM (formerly URS) on the Energy Fuels
Resources (USA) Inc. (EFRI) White Mesa Mill Tailings Data Analysis Report (TDAR), versions
dated January and March 2015, and Probabilistic Seismic Hazard Analysis Report (PSHA),
version dated March 2015. These responses have been prepared for EFRI by MWH Americas,
Inc. (MWH) for submittal to DRC. Comments and responses are provided for the following:
• DRC March 2015 TDAR review comments and MWH responses
• AECOM March 2015 TDAR review comments and MWH responses
• AECOM March 2015 PSHA review comments and MWH responses
• DRC January 2015 TDAR review comments and MWH responses
• AECOM January 2015 TDAR review comments and MWH responses
DRC and AECOM comments are listed in italics followed by MWH's response.
DRC MARCH 2015 TDAR REVIEW COMMENTS AND MWH RESPONSES
1. Section 2.1 - CPT Soundings. As pointed out in our January 22, 2015 Technical
Memorandum (DRC 2015) cross-sections (profiles) depicting the stratigraphy of the tailings
material at each CPT sounding within the tailings impoundments were absent from the
October 2014 TDA report. MWH included three cross-sections in the MWH 2015 report
showing vertical CPT plots. The cross-sections should be drawn with horizontal
interpretation of the interlayered nature of the tailings materials beyond the CPT plots. The
CPT plots have been interpreted per MWH revised Larson and Mitchell [L&M] (1986)
scheme. The simplified two category scheme has discounted the thicker sequences of
apparent Interim Fill sand and/or tailings sand at the surface of Cells 2 and 3; and appear to
diminish potential effects of Sensitive Fine Grained soil. Additionally the water level data is
plotted on the cross-sections at an elevation lower than measured by pore pressure
dissipation (PPD) testing, and instead per MWH's interpretation of dynamic PPD profiles.
The DRC continues to have questions regarding this classification scheme and the depicted
water level profile. To help facilitate resolution of these concerns, please refer to the
MWH Responses Memo_24Apr2015.docx PAGE 1

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comments on the classification scheme provided below in Section 4 of this Technical
Memorandum, and the attached AECOM Technical Memorandum dated March 31, 2015 for
the water level, respectively.
Per a request in DRC 2015, MWH has reduced data gaps on Figure 2-1 – Location Map, by
adding depths penetrated by each CPT sounding in Cell 3, however there is still missing
data for CPT-2W3, CPT-2W4-C, and CPT-2W6-S in Cell 2. Please review and update the
map with the missing information.

MWH Response

Tailings profiles developed from the CPT results are summarized in Figures 4-3, 4-4,
and 4-5 of the TDAR. The profiles show significant interbedding of the tailings along
each tailings profile and significant variation in tailings classification between profiles.
This vertical and lateral heterogeneity is consistent with the method of tailings discharge
from various locations throughout the cells and not just from the perimeter. This
discharge method was used to reduce large-scale segregation and minimize large zones
of fine-grained tailings. The interbedding and lateral variability of the tailings does not
allow horizontal interpretation of tailings characteristics between CPT locations.

Please refer to responses to DRC’s March 2015 comment no. 4 regarding classification
of tailings and DRC’s March 2015 comment no. 5 and AECOM’s March 2015 comment
no. 4 regarding water level data.

Figure 2-1 of the TDAR has been revised to include data requested by DRC.
2. Section 2.2 - Direct Push Sampling. DRC 2015 requested clarification with regards to
symbol usage on representative boring logs within the “Run” column appearing to indicate
that there were sample runs over 24 inches in length up to 36 inches in length, when the
Work Plan described the sampler as being 12 to 18 inches in length. MWH has revised the
TDA report and logs to indicate samplers available during the Direct Push sampling could
accommodate sample runs 24 or 36 inches in length. The DRC considers this item
adequately addressed.
In a response to a request made in DRC 2015, MWH clarified instances where the push
sample symbols were absent from the “Push Samples” column on the logs of CPT-2W3,
CPT-2W4-C, and CPT-2E1. However, the depiction is still unclear on the upper two
samples of the log for CPT-2W6-S(3). Please clarify/revise instances where the push
sample symbol appears to be absent from the “Push Samples” column on this log.
DRC 2015 also inquired on the perception that on initial review there seemed to be a bias to
placing the recovered sample as representative of the bottom of a 24 inch sample run and
that this procedure could incorrectly place material that was captured at the initial
penetration to the bottom of the sample interval. MWH considered this comment and
revised the boring logs according to the following explanation on page 9, 3rd paragraph, of
MWH 2015: “The original field logs recorded the depths of samples from the bottom of the

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sample run. The sample depths have been revised on the logs to represent depth from the
top of the sample run. This revision is documented in the notes.” MWH indicates the
sample identifications within the TDA report and laboratory results were revised accordingly
and the DRC considers this item adequately addressed.
In response to a DRC 2015 request for clarification from MWH with respect to what would be
appropriate sample recovery criteria, MWH indicated that as described in the Work Plan,
approximately 30 6-inch long samples were to be collected based on the direct push
sampling frequency and laboratory testing program. A total of 49 samples were collected.
46 samples were selected for testing, and 38 of the samples had lengths of 6 inches or
greater. Therefore it can be inferred that MWH exceeded the expected Work Plan goals.
The DRC considers this item adequately addressed.
DRC 2015 inquired as to why sampling and testing and interpretation within the upper sand
section (interim fill) of each tailings cell were nearly absent from MWH 2014. MWH
indicated in Section 1.3 of MWH 2015 that the interim fill was evaluated extensively in
Denison, 2011 and EFRI 2012 and therefore not included as a part of the tailings
investigation. While gathering and summarizing the data into one document would
have been desired the DRC considers this response adequate.
DRC 2015 also inquired as to how the geotechnical properties of the sequences of Sensitive
Fine Grained soil identified by the CPT soundings are to be treated/modeled (See CPT plots
for SP2W3; SP3-3S; and SP3-6N and other plots which depict sequences of Sensitive Fine
Grained soil). This material falls within Zone 1 of typical soil behavior classification charts
(e.g., Figure 2 in L&M 1986). The plot of data in Figure 1 of L&M 1986 differentiates three
general (broad) tailings material categories. Figure 2 in L&M 1986 further distinguishes
Sensitive Fine Grained material as “material behavior type” Zone 1. The licensee should
describe how these sensitive fine grained materials are to be classified, treated, and
modeled in future analyses/models. To help facilitate resolution of these concerns;
please refer to the comments on the classification scheme provided below in Section 4 of
this Technical Memorandum.
DRC 2015 highlighted several editorial review comments within Section 2.2 and Appendix C
of the MWH 2014 report. The DRC considers that MWH 2015 has adequately addressed
each of these review comments.

MWH Response

Text was added to subsurface exploration log for CPT-2W6-S(2) to note that samples
were not collected from the upper two sample runs. In addition, this log was revised to
provide the correct shading for the push sample from 16 to 16.7 feet.

Please refer to response to DRC’s March 2015 comment no. 4 regarding classification of
tailings and discussion of sensitive fine grained materials.

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3. Section 3.0 - Laboratory Investigation. DRC 2015 identified four procedural aspects of
the laboratory program that should have been better acknowledged within the body of the
TDA report. MWH expanded the narrative of the report to adequately address the
initial two DRC comments on Delayed Testing and Shipping Disturbance. With
regards to the third comment on the potential for sample disturbance caused by
substandard sample size, MWH responded that they were following approved Work Plan
means. That being noted the results are nonetheless not compliant with the
requirements of the ASTM D2435 standard. A preliminary review of geotechnical
literature on sampling and sample disturbance would also appear to indicate that samplers
providing specimens larger than the minimum diameters referred to in two references
provided in MWH 2015 (Kongkitkul et al, 2014; and Shogaki, 2006) should be used for
consolidation testing. Furthermore, one reference is for soft Bangkok clay deposits which
typically classifies as CL or CH sediments and the other considered clays from a marine
environment, respectively. The tailings depositional environment produces a somewhat
courser [sic] material and therefore the references do not appear to be directly applicable to
this project. DRC requests that the non-standard results from the tailings
consolidation testing be noted and clearly described as such in the TDA report. DRC
also requests that a qualifying statement be added to indicate how the licensee
anticipates that these data will be treated in downstream technical analyses/models.
Gypsum Presence
The fo[u]rth comment DRC 2015 pointed out that the L&M 1986 paper is silent on whether
their test data accounted for gypsum. MWH adequately addresses this comment by
including personal communication with Ned Larson (co-author, L&M 1986) that gypsum was
not accounted for in preparing the classification scheme. The DRC considers this item
adequately addressed.
General Laboratory Review Comments
DRC 2015 identified several technical or editorial items associated with the Laboratory
Investigation section of the TDA report. In response MWH replotted division lines at the
lower left corner of the standard plasticity chart on Figure E.4-1, Summary of Atterberg
Limits Tests Results. The DRC considers this response acceptable.
Consolidation test result data associated with CPT-2W6-S(2)@12.3 feet continues to be
incorrectly classified to be representative of tailings slimes. The total weight of the specimen
used in the consolidation test set-up is indicative of a sand–slime specimen. This is further
revealed in the value of the consolidation coefficient cv from this testing series. This value
results in a time of consolidation for slime that is less than that reported for typical tailing
sand–slimes. In other words the supposed slime material is draining faster than the
representative cv value for sand-slime specimens. The consolidation test result data
associated with CPT-2W6-S(2)@12.3 feet needs to be removed from being averaged
with the other slime consolidation test result and included, as appropriate, with sand-
slime data. Please review and revise any report component (such as Table 3-2, Figure
E.1-1, and Table 4-7) that relied on this data or interpretation.

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The DRC has revisited the consolidation test data associated with CPT-2W6-S(3)@14.5 feet
and considers the results to be representative of tailings sand–slimes, as previously
classified in MWH 2014 and is no longer a concern.
The units for cv in Table 3-2, Summary of Laboratory Testing, have been incorrectly
identified below Note g of the table as cm/s = cubic meters per second. Please review this
and make corrections as appropriate.
DRC 2015 also inquired about eleven ASTM D422 lab test sheets that report an increase in
the percent passing the #200 sieve from the result of the ASTM D1140 test to the
subsequent D422 test result. The increase ranges from 2.2% to 11.8% with an average
increase of 4.9%. MWH responded that the deflocculant sodium hexametaphosphate
caused the dissolved gypsum to flocculate and thereby be retained during the #200 wash
test and subsequently these bonded solids to be shaken apart during the mechanical sieve
analysis ASTM D422. Possible other explanations include insufficient initial washing; a
clogged initial #200 screen, and an otherwise out of tolerance #200 screen during the
mechanical shaking. Given the unusual results, they emphasize the importance of
considering a range of uncertainty in the geotechnical parameters. The DRC concerns with
regards to the subject gradation testing procedures have been adequately addressed.
However, future geotechnical analysis will need to incorporate a range of possible
input parameters and present the results in terms of a sensitivity analysis.

MWH Response

As stated in Section 3.0 of the TDAR, the diameter of the collected tailings samples is
smaller than the minimum diameter recommended per ASTM D2435. Text has been
added to Section 3.0 to note that the consolidation test results are non-standard and
discussion is provided on how this may impact the test results. The use of studies
previously cited regarding the effect that sample diameter may have on consolidation of
fine-grained soils have been clarified. In addition, text has been added to Section 3.0
recommending that it should be recognized that utilizing a smaller diameter may
decrease the measured compressibility in the laboratory test results and that the test
results be interpreted conservatively in future technical analyses that rely on these
results.

The tailings sample CPT-2W6-S(2) at 12.3 feet has 97 percent passing the No. 200
sieve. Thus classification of this material as slime tailings based on the tailings
classification criteria used (slime tailings range from 60 to 100 percent passing the No.
200 sieve) is correct. Total unit weight is not a parameter used for classification of
tailings and it would be incorrect to revise the tailings classification based on this
parameter. The dry unit weight of the consolidation test specimen from CPT-2W6-S(2)
at 12.3 feet (80.8 pcf) is well within the range of measured dry densities for slime tailings
samples (61.0 pcf to 94.6 pcf) at the site. The results for consolidation parameters for
sample CPT-2W-6(2) at 12.3 feet are within the range of published test results for
uranium tailings samples classified as slime tailings. The measured cv values for the
White Mesa tailings slimes samples range from 0.0005 to 0.003 cm2/s, and are
consistent with the average measured value for slime tailings (0.0016 cm2/s) reported by

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Keshian and Rager (1986). The test results indicate that the range of cv values for the
sand-slime tailings (0.0005 to 0.002 cm2/s) are at the low end of the range reported by
Keshian and Rager (1986) for sand-slime tailings (0.001 to 0.05 cm2/s) and are similar to
the cv values obtained from testing of the slimes tailings. Thus it appears the
consolidation rate behavior may be controlled by finer fractions within the tailings
samples. We do not agree that there is a technical justification for using total unit weight,
in lieu of the actual gradation test results, to reclassify the tailings sample from CPT-
2W6-S(2) at 12.3 feet and have not made the requested modification to the report.

Units listed for Table 3.2 in the TDAR for cv were corrected.

As requested by DRC, future geotechnical analysis will consider uncertainty in the
laboratory measured percent passing the No. 200 sieve for the eleven samples
referenced by DRC in this comment. Text stating this was added to Section 3.0 of the
TDAR.
4. Section 4.1 - Tailing Classification – Correlation. As indicated previously, MWH adopted
a tailings characterization scheme developed by L&M (1986) to capture site-specific field
and lab data with adjacent CPT sounding data to classify material catalogued in the
remaining CPT soundings. Based on their interpretation of the data, MWH concluded that
an adjustment to the L&M-recommended material classification bracketing scheme is
necessary. MWH recommended a uniform lateral shift in the curve between the sand-slime
and slimes; a revision in the criteria for percentage fines content between the sand-slime
and slimes from 70% to 60%; and finally recommended removal of the curve dividing sand
from sand-slime material, resulting in a classification of the tailings into only two tailings
material types: sand-slimes and slimes.
Based on overall general revisions within MWH 2015, the DRC is no longer concerned with
the first and second recommended revisions. Those revisions being: 1) a uniform lateral
shift in the curve between the sand-slime and slime categories; and 2) the revision in the
criteria for percentage fines content between the sand-slime and slimes from 70% to 60%.
However, as discussed in the following paragraphs, DRC considers the MWH-
recommended removal of the curve dividing sand from sand-slime material to be
unwarranted, based on data that the two materials are present, and uncertainty or
deficiencies with field and laboratory data.
DRC 2015 identified several plotting errors in the main interpretation graph, Figure E.1-1
Friction Ratio vs. Cone Resistance Tailings Classification. In response MWH revised the
plot accordingly. DRC considers this response acceptable. The DRC has the following
observations with regards to removing the curve defining the transition from sand to sand-
slime: (1) The combination plots of CPT data from Cells 2 and 3 (Figure E.1-3 and Figure
E.1-4, respectively) and a number of individual CPT plots that clearly indicate there are
sands in the tailings profile, with or without a possible recommended lateral shift; (2) The
field program recovered tailings that classified as sand as indicated with 4 of the 20
gradation tests; (3) The (limited) number of samples does not appear to provide sufficient
justification to remove the published division line; (4) Furthermore, there are a number of

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Tables within subsections of Section 4 of MWH 2015 that include geotechnical data on
tailings sand that is geotechnically different than the data collected for sand-slime.
To further augment the DRC’s basis on the third item above the DRC has reviewed the
newly plotted data contained in the revised TDA report and continues to realize that MWH
had difficulty obtaining representative samples of the coarser tailings sand and therefore the
tailings sands appear to have been under represented in the laboratory data analysis
contained in the TDA report. An initial impediment is observed in the following six CPT
soundings which encountered sequences of material that behaved as sand to silty sand and
yet there were no adjacent borings: CPT-2W5-C; CPT-2W6-S; CPT-3-1S; CPT-3S CPT-3-
8N; and CPT-8S. A second two part difficulty was realized in instances where tailings sand
was expected at the following borings yet the field operations were apparently unequipped
to recover a sand sample or the sand sequences were passed by/skipped entirely: CPT-
2W6-S(2), CPT-2W6-S(3); CPT-2E1. It is widely common knowledge that samplers
experience difficulties recovering wet, loose sands and that experience has led to the
deployment of sand catcher devices on the sampler shoe or tip. There is no indication that
a shoe equipped with a sand catcher was used. Regardless of the factors that prevented
sampling in the upper 10 feet of borings CPT-2W6-S(2) and CPT-2W6-S(3), the DRC is
compelled to reiterate that CPT soundings from tailings Cells 2 and 3 indicate there are
sequences of sand that were apparently difficult to recover and therefore be tested and
included in the population of data for the development of a correlation scheme.
Considering the examples described above as well as the comments presented earlier in
this Technical Memorandum with regards to uncertainties with the Direct Push exploration
program and the laboratory data, it is not clear that the recommended removal of the curve
dividing sand from sand-slime material from the L&M 1986 classification scheme has been
adequately justified. DRC requests that the licensee: (1) re-evaluate the proposed
adjustment to remove the curve dividing sand from sand-slime from the L&M
classification scheme; and (2) revise the TDA report accordingly to reflect the
licensee’s final proposed classification of the tailings materials.
Sensitive Fine Grained Material
The CPT soundings revealed a soil behavior type known as Sensitive Fine Grained soil
(See SP2W3; SP3-3S; and SP3-6N) that may have not been adequately characterized in
the TDA report with the MWH-recommended correlation scheme. The Work Plan indicated
the CPT soundings would be used to develop profiles that characterize the tailings
stratigraphy and thereby allow for interpretation and modeling of the various tailing materials
both vertically and laterally. DRC requests that the licensee: (1) re-evaluate the
distribution of tailings sediments to provide interpretation of the interlayered nature
horizontally beyond the CPT plots and across each tailings cell. This depiction should
include Sensitive Fine Grained material as a subset of slimes. Without cross-sections
depicting the distribution of these tailings materials at each CPT sounding it is unclear how
the tailings should be geotechnically modeled for this soil behavior type; and (2) revise the
TDA report accordingly to reflect the licensee’s final proposed classification of the

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tailings materials.
Editorial comments on Updated Figures
The DRC noted during our current review that with the updated Figure E.1-2, Friction Ration
vs. Cone Resistance, Adjusted Tailings Classification, that several consolidation data points
are incorrectly shaded in the updated graph. Please review this and make corrections.

The DRC also noted that the legend within Figures E.1-21 through E.1-37 with blue shading
for sand tailings is misleading because with the current classification scheme adjustments
there would not be blue shading displayed on the CPT plots since the CPT plots are only
plotting the two categories. The DRC interprets that replacing the sand category curve will
allow for a better understanding of the extent of tailings sand. DRC requests that licensee
review the subject figures and re-evaluate the proposed adjustment to remove the
curve dividing sand from sand-slime from the L&M classification.

MWH Response

MWH agrees that tailings characterization results indicate the presence of both sand and
sand-slime tailings within the tailings Cells 2 and 3 at White Mesa. Removal of the
division between sand and sand-slime tailings for Figure E.1-2 was proposed to provide
a better correlation of CPT testing results with laboratory testing data. This adjustment
was not intended to indicate that sand tailings were not present. It is understood that
there should be a division between the sand and sand-slime tailings, however the
selection of a division line is not clear based on comparison of laboratory testing data
with the CPT test results. To address DRC’s concern with combining the sand and
sand-slime tailings within one division on Figure E.1-2, MWH added the sand/sand-slime
division line from Larson and Mitchell (1986) to this figure and associated figures
(Figures 4-3 through 4-5 and E.1-3 through E.1-37). Future technical analyses will
consider that this division is not correlated to the site-specific laboratory testing results
for the sand tailings and conservative adjustment of parameters to address uncertainty
will be evaluated.

DRC requests re-evaluation of the tailings classification to consider the soil behavior
type known as sensitive fine-grained material. As noted in Section 4.1 of the TDAR,
there are a number of criteria used to classify soils based on CPT results. One method
is the soil behavior type zone method. A version of this method is presented in Lunne,
Robertson, and Powell (1997). This method includes sensitive fine-grained material as
one of 12 soil behavior types. This method is provided as a default soil classification
method by the CPT contractor used for the White Mesa tailings investigation. However,
this criteria was not developed for classification of uranium tailings and was not used by
MWH to classify the White Mesa tailings. MWH used criteria developed by the U.S.
Department of Energy for classification of uranium tailings as presented in Larson and
Mitchell (1986) to classify the White Mesa tailings. Larson and Mitchell (1986) note that
classification of tailings using the soil behavior type method, such as the one presented
in Lunne, Robertson, and Powell (1997), results in a higher level of uncertainty than the
method presented in Larson and Mitchell (1986). In addition, it appears that materials

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that classify as sensitive fine-grained material from CPT testing are actually slime and
sand-slime tailings based on laboratory gradation testing. As such, it would be incorrect
to define a subset of the tailings slimes as sensitive fine-grained material based solely
on the CPT-based soil behavior type.

Figure E.1-2 was revised to correct shading for consolidation data points.
5. Section 4.2 - Pore Pressures. The DRC continues to have questions regarding the
interpretation of water levels measured during this study, please refer discussion on this
item contained in the attached AECOM Technical Memorandum dated March 31, 2015 and
respond accordingly.

MWH Response

MWH acknowledges that the most effective method to measure water levels and
porewater pressures in the tailings would be from installed and equilibrated piezometers.
However, due to access conditions in the tailings cells for both installation and
monitoring, and the time necessary for porewater pressures in the piezometers to
stabilize to static conditions, measurements were made from the CPT program. These
measurements were from pore pressures measured during advancing the cone as well
as measurements from the dissipation tests. These methods typically provide
conservatively high values due to the generation of excess porewater pressures from
movement of the cone.

MWH revisited the interpretation of estimated water levels within the tailings cells and
revised the maximum elevations of the top of saturated tailings to be based only on the
pore pressure dissipation test results. This is discussed further in MWH’s response to
AECOM’s March 2015 comment no. 4.

Pore pressure dissipation test results provide information to conservatively estimate
saturated tailings thicknesses and in some cases appear to provide overly conservative
results. This approach is consistent with what was agreed upon between DRC, AECOM
and EFRI regarding the tailings characterization approach presented at the meeting held
at AECOM’s Denver office on April 30, 2013 and in all versions of the work plan (MWH,
2013a, 2013b, and 2013d).   Installation of monitoring instrumentation as recommended
by AECOM was not included in the work plan and would result in considerable delay in
completing the analyses required for the preparation of the EFRI responses to DRC’s
February 2013 comments ((DRC, 2013a, b) on EFRI’s White Mesa Reclamation Plan,
Version 5.0 (Denison, 2011) and Infiltration and Contaminant Transport Report (MWH,
2010).

6. Section 4.3 - Tailings Density. It was noted during the DRC’s current review that in four
places the figures called out in the second paragraph of Section 4.3 appear to be incorrectly
directed to figures labeled “E2” in Appendix E instead of “E3”. Please review this and
make corrections as appropriate.
It was also noted during our current review that the data presented on Figure E.3-12

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PAGE 10

appears to be incorrectly plotting the interpretation of data. Instead of the data scattering
over the plot area, the predicted Dry Density Estimated from CPT data is falling into three
linear concentrations with unit weights of approximately 76 pcf, 82 pcf and 86 pcf. Please
review this and make corrections as appropriate.
Section 4.3-Tailings Density of MWH 2015 discusses and tabulates geotechnical test results
that are associated independently with sand tailings. Figure E.3-1, Friction Ratio vs. Cone
Resistance–Tailing Classification and Table.4-2, Summary of In-Situ Tailings Density form
Laboratory Testing include data from all three categories of the L&M scheme. This is
consistent with the DRC judgment that the sand tailings curve should be included on the
MWH-recommended correlation scheme. DRC requests that licensee review this remark
with the review comments on Section 4.1 above when considering the proposed
adjustment to remove the curve dividing sand from sand-slime from the L&M
classification.

MWH Response

Figure number designations have been corrected in Section 4.3.

Figure E.3-12 shows correct values for dry densities estimated from CPT data using the
method presented in Lunne et al. (1997). This method was the CPT Contractor’s
default method to estimate density based on CPT results. This method uses typical
density values based on soil type. The results show that this method does not provide a
good correlation with measured data at the same depths.

Section 4.3 of the TDAR was updated to be consistent with the tailings divisions shown
on the revised Figure E.1-2.
7. Section 4.5 - Consolidation Properties. As described above in first paragraph of Review
Comment 3, the consolidation test results are not compliant with requirements of the ASTM
D2435 standard. As previously stated, the DRC requests that the non-standard results
from the tailings consolidation testing be noted and clearly described as such in the
TDA report. DRC requests that a qualifying statement be added to indicate how the
licensee anticipates that these data will be treated in downstream technical
analyses/models.
And to be consistent with earlier comments described in subsection General Laboratory
Review Comments of Review Comment 3; Table 4.7, Summary of Laboratory Measured
Consolidation Parameters has tabulated the cv for slime as being more rapid than that for
sand—slime. Please review this and make corrections as appropriate.

MWH Response

Please refer to the response to DRC’s March 2015 comment no. 3 regarding
consolidation testing results and discussion on the cv results. Text was added to Section
4.5 to note that the consolidation test results are non-standard and recommendations

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are provided on how to interpret the results for future technical analyses.

8. Section 5.0 – Summary. MWH has revised this section to include reference to the October
2013 work plan. Please provide the DRC with a copy of the October 2013 work plan as
it was only shared between MWH and EFRI.
The second paragraph of Section 5.0 in MWH 2015 states that “…the tailings within Cells 2
and 3 are relatively uniform…” This appears to be internally inconsistent with the conclusion
made in the seventh paragraph of Section 4.1. “Review of these figures, as well as the
boring logs and laboratory results indicate there is significant interbedding …”. Please
evaluate the difference and make revisions to render the two conclusions internally
consistent.

MWH Response

Hard copies and electronic copies of the October 2013 work plan (MWH, 2013d) are
provided with this memorandum.

The second paragraph of Section 5.0 of the TDAR has been revised to clarify the
description of tailings characteristics to indicate that although there is significant
interbedding in the tailings profiles, the tailings are similar in characteristics between Cell
2 and 3, and do not have large segregated zones of tailings slimes or sands within each
cell.

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AECOM MARCH 2015 TDAR REVIEW COMMENTS AND MWH RESPONSES

1. General Comment. An updated “redline” version of the Tailings Data Analysis (TDA) Report
dated March 2015 was submitted by EFRI for review indicating changes that were made to
the previous (October 2014) version of the report based on EFRI’s consideration of technical
comments contained in the URS Technical Memorandum dated January 22, 2015.
However, a document containing specific responses to those individual technical comments,
which would facilitate review and determination of whether and how each individual
comment has been addressed, was not provided. The remainder of this Technical
Memorandum therefore focuses (only) on URS (AECOM)’s review comments based on a
review of the content contained in the March 2015 “redline” report. For improved
transparency and traceability, it is requested that the licensee provide a
Comment/Response document or Comment/Response Matrix documenting the response to
each review comment previously submitted to help facilitate the review and closeout or
continuance of those previously furnished (January 22, 2015) technical review comments on
the October 2014 version of the TDA report.

MWH Response

As requested by DRC, comment and responses are included in this memorandum for
AECOM’s January and March 2015 comments on the TDAR. Please note that EFRI did
not provide a comment/response document for AECOM’s January 2015 comments with
submittal of the March 2015 TDAR because DRC specifically requested responses “be
in the form of revised reports.”

2. Sec 3.0, 6th para. p. 12. As noted in Comment No. 19 in the January 22, 2015 Technical
Memorandum, according to ASTM D2435 the minimum specimen diameter or inside
diameter for testing consolidation properties shall be 2 inches. This was not the case for the
tested consolidation samples with a reported diameter of 1.4 inches. Typically push-in
samples obtained using the CPT rig are for visual confirmation only and in some cases
index property testing if enough sample quantity can be obtained. The March 2015 redline
version of the report references two published papers that indicate that a reduction in
diameter from 2.4 to 1.2 or 1.4 inches for fine-grained soils has an insignificant impact on
measured consolidation. Both papers involved testing of clays (one involving marine clays),
rather than tailings having grain sizes/classifications ranging from slimes to sands. Based on
the information provided, this issue will remain open. (See also Comment No. 6 below.)

MWH Response

For background, it should be noted that the sampling method proposed for the tailings
characterization was first discussed with DRC and AECOM (formerly URS) at a meeting
held at AECOM’s Denver office on April 30, 2013. It was agreed upon in the meeting by
DRC and AECOM that EFRI’s proposed tailings characterization plan approach,
including the sampling method, was reasonable. EFRI committed to provide a work
plan to DRC for concurrence before proceeding with the investigation. All versions of
the work plan included the aforementioned sampling method and listed the sampling

MWH MEMORANDUM

PAGE 13

diameter range of 1.0 to 1.5 inches.
The initial work plan for the proposed tailings investigation (MWH, 2013a) was provided
to DRC on June 24, 2013. DRC provided comments on the work plan in a letter dated
July 2, 2013 (DRC, 2013c). A revised work plan (MWH, 2013b) and responses to DRC
comments (MWH, 2013c) were provided to DRC on August 1, 2013. DRC provided
approval of the work plan verbally to EFRI on September 12, 2013 (documented in
EFRI, 2013).
Prior to starting the investigation, MWH provided a final update to the work plan to EFRI
on October 10, 2013 (MWH, 2013d). The final update included the following procedural
revisions: 1) improved sample handling and shipping procedures, 2) replacement of the
recommended geotechnical laboratory with two alternative certified laboratories (due to
the previously listed laboratory no longer having a valid radioactive materials license), 3)
added text to note that settlement had been checked prior to the investigation, and 4)
updated schedule for the field investigation.
As requested by DRC, the electronic and hard copies of the October 2013 version of the
work plan (MWH, 2013d) are provided with this memorandum.
As stated in Section 3.0 of the TDAR, the diameter of the collected tailings samples is
smaller than the minimum diameter recommended per ASTM D2435. Text was added
to Section 3.0 to note that the consolidation test results are non-standard and discussion
is provided on how this may impact the test results. The use of studies previously cited
regarding the effect of sample diameter on consolidation of fine-grained materials soils
have been clarified. In addition, text was added to Section 3.0 recommending that it
should be recognized that utilizing a smaller diameter may decrease the measured
compressibility in the laboratory test results and that the test results be interpreted
conservatively in future technical analyses that rely on these results.

3. Sec. 4.1, Tailings Classification, para.8, p.16. Cross sections have been provided but no
explanation is presented as to how slime and sand-slime zones will extend in plan view.
Also sand zones have not been identified but grouped in with sand-slime zones, without
sufficient justification. For example, previous studies (including, but not limited to, a tailings
grinding report referenced in the Cell 4B Design Report [Geosyntec Consultants 2007])
indicate that the tailings range in grain sizes from silt to medium sand, with the largest
percentage of the tailings having gradations indicative of fine sand. Additionally, combination
plots of CPT Data from Cells 2 and 3 (Figure E.1-3 and Figure E.1-4, respectively) and a
number of CPT Plots indicate that there are intervals of sand contained within the tailings
profile. A conclusion that there are no sands and that the tailings are predominantly made
up of sand- slime tailings appears to be not adequately supported by the available
laboratory data and the associated data uncertainties and given that a portion of Tailings
Cell 3 was not investigated. It is recommended that cross-sections be provided depicting the
specific stratigraphy of the tailings materials based on specific profiles encountered in the
CPT penetrations. It is also recommended that the licensee re-evaluate the proposed
adjustments to the L&M classification scheme and revise the TDA report accordingly to
reflect the licensee’s final proposed classification of the tailings materials, or, alternatively,
provide additional detailed rationale to support the proposed adjustments.

MWH MEMORANDUM

PAGE 14

MWH Response

Tailings profiles developed from the CPT results show significant interbedding of the
tailings along each tailings profile and significant variation in tailings classification
between profiles. This vertical and lateral heterogeneity is consistent with the method of
tailings discharge from various locations throughout the cells and not just from the
perimeter. This discharge method was used to reduce large-scale segregation and
minimize large zones of fine-grained tailings. The interbedding and lateral variability of
the tailings does not allow horizontal interpretation of tailings characteristics between
CPT locations.

Please refer to responses to DRC’s March 2015 comments no. 4 regarding classification
of tailings.

4. Secs 4.1 and 4.2 and Figure 4-3 through 4-6. URS does not agree with established
elevation of saturated tailings as shown on Figures 4-3 through 4-5. The CPT is not
conclusive to establish this, as shown on Figure 4-6. If the lower bound is considered, then
the points at depth are not used and if the upper bound case is used, the points in the cover
are not considered. Could there be higher pore pressures present in the tailings than in the
upper 5 feet (cover)? Dynamic pore pressure during CPT advancement does not indicate
static water level at sounding location. State of practice includes piezometers to monitor
pore pressures. Piezometers are recommended to be installed to measure changing water
levels with time and to establish the pore pressure regime with the tailings for further
geotechnical analysis. Measuring water level at sump location has limited relevance to
characterizing pore pressures in the remaining cell.

MWH Response

MWH agrees that dynamic pore pressure during CPT advancement does not indicate
static water level at sounding locations; however, continuous generation of positive
dynamic pore pressures during CPT advancement is a strong indicator of saturated
conditions. The location where saturated conditions occurred based on dynamic pore
pressure measurements were used in conjunction with pore pressure dissipation test
results to estimate water levels in the March 2015 version of the TDAR.

To address DRC’s and AECOM’s concerns with MWH’s interpretation of water levels in
the tailings, MWH revised the interpretation of the estimated elevations of the top of
saturated tailings shown on Figures E.2-1 through E.2-16 to conservatively base these
estimates solely on pore pressure dissipation test measurements instead of using both
pore pressure dissipation test and generation of positive dynamic pore pressure
measurements. This revision was recommended by AECOM in their January 2015
comments. Please note that Figure 4-6 was provided to show the lower and upper
bound estimations for the top of saturated tailings using pore pressure dissipation tests.
This figure does not indicate that the CPT results are not conclusive, but does indicate
that in some cases the results of the pore pressure dissipation testing may be overly
conservative and not representative of actual conditions. Use of the pore pressure
dissipation tests to estimate the top elevation of saturated tailings is conservative for use
in future analyses.

MWH MEMORANDUM

PAGE 15

MWH recognizes that installation of piezometers can provide measurements of water
levels within the tailings. However, the estimates of the top of saturated tailings as now
presented in the TDAR should be sufficient to provide conservative estimates for use in
technical analyses. This approach is consistent with what was agreed upon between
DRC, AECOM and EFRI regarding the tailings characterization approach presented at
the meeting held at AECOM’s Denver office on April 30, 2013 and presented in the work
plan (MWH, 2013d).

Please see response to DRC comment no. 5 for additional background information on
the approach used to estimate saturated tailings thicknesses from the CPT program.

5. Sec 4.4, Hydraulic Conductivity and Table 4-4. It is recommended that Table 4-4 show
each laboratory test value and what material type it is considered to be instead of showing a
range of values. In Table 4-4, sand-slime is shown as less permeable than slime. Please
clarify that the information is correct and accurate and provide an explanation for this
apparent discrepancy, or revise the information if necessary. (See also Comment No. 7
below.)

MWH Response

Table D-1 in Appendix D provides individual laboratory test results for all the laboratory
tests conducted. Results listed in Table 4-4 are correct and indicate that the laboratory
vertical hydraulic conductivity results for the sand-slime and slimes tailings are similar
and potentially controlled by the finer fraction of the tailings samples.

6. Sec 4.5., Consolidation Properties and Table 4-7. It is recommended that Table 4-7 show
each laboratory test value and what material type it is considered to be instead of showing a
range of values. In Table 4-7, the cv value is greater for slime than sand-slime, indicating
more rapid consolidation in slime than sand-slime, which is most likely not realistic. Based
on consolidation and permeability testing results, please provide a discussion of whether
there is a significant difference in behavior between slimes and sand-slimes. Please clarify
and explain whether the difference should be between sand-slime/slime and sand.

MWH Response

Table D-1 in Appendix D provides individual laboratory test results for all the laboratory
tests conducted.

Please refer to response to DRC’s March 2015 comment no. 3 regarding consolidation
testing results and discussion on the cv results.

7. Sec. 4.4., Hydraulic Conductivity. In the last sentence of this section (p. 23) it is stated
that “It is expected that the hydraulic conductivities used for the tailings for future analyses
will be lower than the estimated hydraulic conductivities used in previous analyses”. This
sentence appears to be presented without accompanying full context or explanation.
Additionally, a subsurface investigation of tailings characteristics in the central southern
portion of Cell 3 (uncovered area) has not yet been completed. It is recommended that this

MWH MEMORANDUM

PAGE 16

sentence be deleted or, alternatively, a comprehensive analysis and discussion, including
an evaluation of uncertainties associated with the use of values of hydraulic conductivities
for all tailings types that may be present in different areas of cells 2 and 3, be provided to
support this statement. (See also Comment No. 5 above, and Comment No. 12 in the URS
January 21, 2015 Technical Memorandum).

MWH Response

The last sentence of Section 4.4 of the TDAR has been deleted to reduce confusion.

8. Sec 4.5, Consolidation Properties and Table 4-8. In the last paragraph in this section (p.
24) it is stated that “These results [estimated ch values in Table 4-8] are unreasonably high
and cannot be explained solely by anisotropy. It is recommended that laboratory measured
cv values be used in future technical analyses and that cv values not be calculated from
estimated ch values based on the CPT soundings.” Please provide additional explanation of
other factors that could affect the accuracy of the estimated ch values listed in the last
column of Table 4-8. Please also discuss the implications and possible uncertainties
associated with use of the reported laboratory measured values of cv in future technical
analyses. Include in this discussion an assessment of the reliability of use of cv values
measured for slimes, sand-slime mixtures, and sand tailings, given the small sample
diameters that were used in the laboratory consolidation testing (See also Comment No. 2
above).

MWH Response

Other factors that can impact ch values estimated from CPT data were added to the 3rd
paragraph of Section 4.5 of the TDAR.

Please refer to the response to AECOM’s March 2015 comment no. 2 regarding cv
values and the diameter of tailings samples

9. Section 1.2, Historical Tailings Data. In the first sentence, reference is made to “Denison
2009”; however, that reference is missing from Section 6.0, References.

MWH Response

The Denison (2009) reference has been added to the revised TDAR.

10. Sec 3.0, 11th para. p. 13. The second sentence in this paragraph states: “It is not expected
that natural moisture contents will not be used in any future technical analyses for the
Reclamation Plan and ICTM Report.” Please clarify that this statement is correct and
accurate as written and revise the sentence as necessary.

MWH Response

The second sentence in the 11th paragraph of Section 3.0 of the TDAR has been
corrected.

MWH MEMORANDUM

PAGE 17

AECOM MARCH 2015 PSHA REVIEW COMMENTS AND MWH RESPONSES

1. Section 4.2.1. It is not clear why a truncated exponential model was not used in calculating
the recurrence for the Colorado Plateau and Intermountain Seismic Belt (ISB). This
comment was also in the first review of the report. Can the authors explain why it wasn’t
used?

In the attached Figures 1 and 2, we calculated the recurrence for the Colorado Plateau
using the truncated exponential model and the earthquake counts per magnitude bin
provided by MWH in Table 2 in their report. MWH calculated a b-value of 0.88 compared to
our value of 0.90. This difference will have very little if any impact on the hazard at the site.
However, note what happens when the Mmin is changed to M 3.5 (Figure 2) instead of M
3.0 (Figure 1) as used by MWH. The b-value changes from 0.90 to 0.97 and the a-value to
3.54 and the fit to the data is slightly better. This change illustrates the uncertainty in the
recurrence parameters that could impact the hazard. This uncertainty was not accounted for
by MWH in their PSHA and they may want to evaluate its impact on their hazard results.
MWH Response

A truncated exponential was used in the hazard code. The plots for the Colorado
Plateau and Intermountain Seismic Zone are shown in Figures 1 and 2.

There is a small variation between the b-values calculated in the PSHA report and those
shown in Figures 1 and 2 below. This difference has very little impact on the seismic
hazard at the site and has not been revised in the report.

As suggested by the reviewer, the variation of the minimum magnitude (3.0 as compared
to 3.5) used in the calculations was investigated. A test run was performed to evaluate if
increasing the minimum magnitude would impact the hazard. Using a minimum
magnitude of 3.5 to calculate the recurrence resulted in a decrease in the overall hazard
from 0.19g to 0.17g, as shown in Table 1. Therefore, this uncertainty in the recurrence
parameters was evaluated and the results are not significant enough to revise the
hazard calculations. No changes have been made to the report.

MWH MEMORANDUM

PAGE 18

Table 1:
Comparison of Uncertainty in the Recurrence Parameters
Return Period (Years) Vs30 (m/s) Mean PGA (g)
10,000
Original Analysis
(Mmin=3.0)
a-value=3.3
b-value=0.88
580 0.19
Revised Analysis
Including
(Mmin=3.5)
a-value=3.54
b-value=0.97
580 0.17

2. Section 4.2. The background seismicity in the Colorado Plateau and the ISB is assumed to
be uniformly distributed in the MWH study. Most state-of-the-practice PSHAs use Gaussian
smoothing of the seismicity with or without uniform seismic source zones such as the USGS
in the National Seismic Hazard Maps. This probably does not have a significant impact on
the hazard computed by MWH but they should acknowledge this fact in their report. If MWH
Figure 1: Figure 2:
Earthquake Recurrence for Colorado
Plateau
Earthquake Recurrence for Intermountain
Seismic Belt
0.0001
0.001
0.01
0.1
1
10
100
1000
3456789
Cu
m
u
l
a
t
i
v
e
 Ra
t
e
/
Y
e
a
r
Magnitude
log(N)=3.3‐0.88M
Activity Rate (M≥5)=0.07
0.0001
0.001
0.01
0.1
1
10
100
1000
3456789
Cu
m
u
l
a
t
i
v
e
 Ra
t
e
/
Y
e
a
r
Magnitude
log(N)=3.4‐0.84M
Activity Rate (M≥5)=0.15

MWH MEMORANDUM

PAGE 19

has this capability, it would be prudent for them to run the PSHA with Gaussian smoothing
to assess the impact on the hazard at the site.
MWH Response

Section 6.2.1 of the PSHA report was revised to state that the analysis does not include
Gaussian smoothing.

3. Section 1.1, 2nd paragraph, 1st sentence. The sentence is awkward and needs to be
reworded. It should read something to the effect: “The PSHA was performed to estimate the
probabilistic hazard at the site by characterizing potential seismic sources and assessing the
likelihood of earthquakes of various magnitudes occurring on those sources and the
likelihood of the earthquakes producing ground motions over a specified level.”
MWH Response

Section 1.1 of the PSHA report was revised and now reads: “The PSHA was performed
to estimate the seismic hazard at the project site within a probabilistic framework by
characterizing potential seismic sources.”

4. Section 2.1, last sentence. The northern Naciimiento fault is located in northwestern New
Mexico, not northeastern New Mexico.
MWH Response

Section 2.1 of the PSHA report was corrected.

5. Section 6.3, 1st paragraph. The paragraph describes the PGA results but references Figure
9 which are the Uniform Hazard Spectra (UHS). Please reword to acknowledge that UHS
were computed.
MWH Response

Section 6.3 of the PSHA report was revised to acknowledge that the UHS was
computed. Figures 9 and 10 were switched such that Figure 9 shows the total hazard
curve and Figure 10 shows the UHS.

6. Section 7, last paragraph. The USGS National Seismic Hazard Map methodology is no
different than the methodology used in the MWH report or any other PSHA. Hence the
statement that the estimation of hazard at 10,000 years is “outside the intended use of the
data and likely explain the differences in the PGA” is incorrect. There are probably legitimate
reasons that the USGS PGA value is higher. One reason could be the smoothing window
used by the USGS is generally 50 km, which tends to spread the hazard to greater
distances. Hence for a particular site, the hazard will be higher due to contributions coming
from a larger range of distances than the MWH study where smoothing was not performed.

MWH MEMORANDUM

PAGE 20

MWH Response

Section 7 of the PSHA report was revised.

7. Figure 11. The figure indicates the controlling earthquake at the site for a return period of
10,000 years. Is this the mean or modal magnitude and distance? Since the PGA computed
in the MWH study is to evaluate the liquefaction potential of the reclaimed tailings cells,
there should be a short discussion in the report on the controlling earthquake.
MWH Response

The last paragraph of Section 6.3 of the PSHA report was revised to include discussion
about the controlling earthquake.

MWH MEMORANDUM

PAGE 21

DRC JANUARY 2015 TDAR REVIEW COMMENTS AND MWH RESPONSES

1. Section 2.1 - CPT Soundings. The widely dispersed cone penetration testing (CPT)
soundings have provided a significant improvement in the available data to model the
geotechnical properties of the tailings soil profile within Cells 2 and 3. MWH refers to the
1986 paper by Larson and Mitchell (L&M) for the U.S. Department of Energy Uranium Mills
Tailings Remedial Action (UMTRA) Project which provides early experience interpreting
CPT data to characterize uranium tailings piles. Notwithstanding the variation of tailing soils
over small distances causing a soil sample taken at a given interval to potentially be quite
different from the soil penetrated by an adjacent CPT sounding, the paper is quick to point
out that predicted Unified Soil Classification System (USCS) material classifications within a
typical CPT classification zone may vary greatly with site specific classification testing
results. This paper highlights the importance of developing site specific correlations
between the CPT record and site specific laboratory classification and proposes a
classification scheme unique to uranium mill tailings. The L&M scheme utilizes three
traditional brackets to capture and categorize mine tailings: (1) Sand which is material with
0% to 30% passing the #200 sieve; (2) Sand Slime which is a mixture that has 30% to 70%
passing the #200 sieve; and (3) Slime which is a material with 70% to 100% passing the
#200 sieve. MWH has recommended adjustments to the L&M scheme which is to be
discussed later.
The work plan anticipated 7 CPT soundings in each cell for a total of 14 soundings. More
than 14 CPT soundings were completed. Each CPT sounding was to extend into the
tailings profile to at least within 5 feet of the predicted depth to the cell liner. The CPT
soundings within Cells 2 and 3 typically reached to within 2 feet and 5 feet of the predicted
liner depth, respectively. Several noted exceptions were CPT soundings CPT-3-8S, CPT-3-
4N, and CPT-3-3S which were each terminated at depths slightly more than 5 feet from the
predicted liner at approximately 7.5 feet, 8.3 feet, and 9 feet, respectively. In general, the
DRC acknowledges that the CPT soundings collected field data as it was intended to
undertake. However, the work plan indicated the CPT soundings would be used to develop
profiles that characterize the tailings stratigraphy and thereby allow for interpretation and
modeling of the various tailing materials both vertically and laterally. Cross-sections
(profiles) through the tailings impoundments are absent from the Tailings Data Analysis
Report. Without cross-sections depicting the stratigraphy of the tailings materials at each
CPT sounding it is unclear how the tailings material types are distributed and therefore
uncertain how the tailings should be geotechnically modeled. Please provide profiles that
depict the stratigraphy within each tailings cell both vertically and laterally.
The Location Map identified as Figure 2-1 does not consistently call-out the depth
penetrated by each CPT sounding. Please review and update the map with the missing
information.

MWH MEMORANDUM

PAGE 22

MWH Response
Please refer to responses to DRC’s March 2015 comment no. 4 regarding classification
of tailings.
The intention of recommending a maximum allowable probe depth was to provide a
buffer zone to minimize the potential for puncture of the tailings liner system in Cells 2
and 3 during the investigation. The estimated thickness of interim cover and tailings at
each CPT location is greater than the total depth of the CPT sounding at each location,
indicating that no CPT probes came close to the liner.
Cross-sections showing tailings profiles across Cell 2 and Cell 3 were added to the
March 2015 version of the TDAR to address DRC comments. Please see response to
DRC’s March 2015 comment no.1 regarding horizontal interpretation of the tailings.
Figure 2-1 of the TDAR was updated to address DRC’s January and March 2015
comments.
2. Section 2.2 - Direct Push Sampling. The DRC acknowledges that additional direct push
explorations were added over the work plan amount of 2 explorations per cell. Sample data
collected from the field direct push sampling program will be invaluable to understanding the
degree of variability of geotechnical physical properties within the material placed in Cells 2
and 3. This data goes to the primary goal of calibrating the abundant CPT soundings.
However, before acknowledging the Direct Push field program achieved the objectives of
the work plan the following review comments need to be addressed.
The eight representative logs in Appendix B need to be internally consistent with respect to
grammatical technique and symbol usage. Please indicate in a suitable place the
standard ASTM practice (2487 or 2488; or both) used to classify the soil encountered.
Symbols within the “Run” column appear to indicate on a few logs that there are sample
runs over 24 inches in length up to 36 inches in length, however most sample runs were 24
inches. Review this representation of 36 inch sample runs and confirm that it is correct.
The work plan described the sampler as being 12 to 18 inches in length and Section 2.2 of
the report indicates the sampler was 24 inches in length with an internal diameter of 1.5
inches. If it was possible to achieve sampling runs over 24 inches in length please
describe in further detail the longitudinal dimensions of available sampling jars, as
well as the length of the sample sleeves or rings. Provide details of the alternate
sampler set-up to assure that the sampler could accommodate accidental over
driving without disturbing (compressing) the sample. Please clarify/revise instances
where the push sample symbol is absent from the “Push Samples” column on the
following logs: CPT-2W3; CPT-2W4-C; CPT-2W6-S(3); and CPT-2E1.
Please describe the rationale that was used to determine what portion of the tailings
profile is represented by a typical 24 inch sample run that recovered less than 50% of
the penetration length attempted, especially for the longer sample attempts. For

MWH MEMORANDUM

PAGE 23

example explain how 2 to 6 inches of recovered material from a 24 inch sample run was
accurately positioned on the log. On initial review there seems to be a bias to placing the
recovered sample as representative of the bottom of a 24 inch sample run and then
scheduling and developing lab results to establish a correlation to the CPT data from this
designated “bottom depth”. This procedure could incorrectly place material that was
captured at the initial penetration to the bottom of the sample interval.
Difficulties with achieving decent sample recovery are a factor with every successful
exploration program. It is noted that based on current information on the Direct Push logs,
sample recovery achieved an overall success rate of approximately 40% recovered of the
sample run attempted. Furthermore of the nearly 160 lineal feet of Direct Push explorations
the total sample length recovered represents less than 20% of the lineal feet explored by the
Direct Push explorations. Ideal recovery rates would minimize introducing error and
uncertainty, below 50% recovery might be considered too uncertain given the narrative on
sampling procedures discussed in the preceding paragraph. MWH needs to clearly indicate
what recovery criteria would be appropriate for correlation and why.
These aspects of the sampling procedures as discussed in the preceding two paragraphs
are especially important to understand based on the adjacent CPT soundings the tailings
profile frequently changes classification vertically within several inches and certainly within a
24 inch sample run. Given the inherent frequent profile changes, the tailings
characterization report needs to explain clearly how any proposed correlation
scheme accounted for 1) an apparent overall low sample recovery; 2) an often limited
amount of material being recovered for testing; and 3) the apparent uncertainty of
sample location within the 24 inch interval, along with the associated biased to assign
samples to the bottom of a sample run.
It is noted that sampling within the upper sand section (interim cover / platform fill) of each
tailings cell is nearly absent, there are 2 possible representative samples collected at the
interface with the tailings soil, please indicate if this omission was intentional and
describe how this absence of data will be filled. To be complete the Tailings Data
Analysis Report should needs to include interpretation, past or present, on the geotechnical
properties of this sequence of material. Another material identified by the CPT soundings
that was not sampled and tested consists of sequences of Sensitive Fine Grained soil (this
item was also identified in URS, 9/24/14). See CPT plots for SP2W3; SP3-3S; and SP3-6N
and other plots which depict sequences of Sensitive Fine Grained soil. This material falls
within Zone 1 of typical soil behavior classification charts. The L&M plot of data does not
appear to have to account for this zone as they didn’t have data to plot within this zone.
Please review the CPT data within this zone and clearly justify within the report how
the geotechnical properties of the Sensitive Fine Grained soil are to be modeled.
The following are several editorial review comments. The elevation information is absent
from each log, please revise each log to include this information. The moisture content
and dry density for the sample from CPT-2W4-C @ 8.9 feet have been incorrectly posted to
the log of CPT-2W3. The moisture content and dry density for the sample identified as CPT-

MWH MEMORANDUM

PAGE 24

3-6N @ 10.5 feet have been omitted from the log. The columns for % Gravel - % Sand - %
Passing No. 200 sieve would be expected to add up to 100%. While minor there are a few
instances where the % Sand is off by 0.1% and appears to be associated with a rounding
error. A bigger deviation from the lab sheet result to the data placed on the log is noted for
sample CPT-3-6N @ 6.5 feet with the % Sand entered on the log. Please review these
comments and revise the logs and report as appropriate. The photo logs are very
helpful and appreciated. Please consider adding a running head and/or page numbering to
the pages of photos in Appendix C.

MWH Response

The ASTM standard (D2487) used for USCS classification of laboratory samples was
added to the field logs in Appendix B of TDAR as requested by DRC. The logs were
also revised to correct sample run lengths and revise push sample symbols where
applicable.

The original field logs recorded the depths of samples from the bottom of the sample run.
The sample depths have been revised on the logs to represent depth from the top of the
sample run. This revision is documented in the notes on the logs. In addition, the
sample designations provided to the laboratory were revised in the laboratory report
provided in Appendix D of the TDAR.

In regards to sample recovery, the work plan (MWH, 2013d) stated that approximately
30 6-inch long samples were to be collected based on the direct push sampling
frequency and laboratory testing program. A total of 49 samples were collected during
the investigation. Forty-six samples were selected for testing, and 38 of the samples
had lengths of 6 inches or greater. This text was added to the TDAR.

The tailings investigation focused on obtaining information on the tailings from the CPT
soundings and direct push sampling. Evaluation of the interim cover material was not
part of the tailings investigation. The intent of the tailings investigation was to provide
site-specific tailings data requested by the DRC. The tailings investigation approach
was presented in a meeting on April 30, 2013 with the DRC, AECOM (formerly URS),
EFRI, and MWH, and was also presented in the work plan (2013d). The interim cover
material was already evaluated extensively as documented in Denison (2011) and EFRI
(2012). Additional discussion on the objectives of the tailings investigation has been
included in the TDAR.

Please see response to DRC’s March 2015 comment no. 4 regarding the soil behavior
type sensitive fine-grained material.

Editorial comments provided by DRC on the field logs have been addressed as noted in
DRC’s March 2015 comment no. 2.
3. Section 3.0 - Laboratory Investigation. The following four paragraphs describe
procedural aspects of the laboratory program that were identified during the DRC review but
were not thoroughly acknowledged within the body of the Tailing Data Analysis Report.

MWH MEMORANDUM

PAGE 25

With the intent to develop a site specific correlation to CPT soundings, please review these
items and expand the narrative of the characterization report to account for them and
how they might or might not affect the correlation. The subsequent review comments
are based on technical or editorial items.
Delayed Testing
The DRC notes that with the delay in testing of often over 2 months, ordinary expectations
for timely geotechnical testing conditions were not observed. With an exception of one
consolidation test completed within approximately 1.5 months, the remaining four
consolidation tests where started more than 3 months after they were recovered from the
tailings. Ideally geotechnical laboratory testing for consolidation parameters would
commence directly upon returning from the field with the samples.
Shipping Disturbance
While understood that it was not originally anticipated, there is limited mention in the report
how the specimens were physically handled during the 1300 mile journey between Colorado
and Tennessee. Please indicate whether the samples were shipped commercially or
not. The DRC believes there would be considerable opportunity for sample disturbance
caused by the shipping of the samples to Tennessee in lieu of the proposed laboratory
situated roughly 70 miles south of MWH’s Fort Collins office in Lakewood, Colorado
(understood to be subsequently disqualified).
Sample Tool Disturbance
Please research and interpret published studies on the potential disturbance of Direct Push
samples with inner diameters equal to or less than 1.5 inches that are used for geotechnical
testing. Section 6.2.2 of ASTM D2435 (Consolidation test method) states that the minimum
specimen diameter or inside diameter of the specimen ring shall be 2 inches. The samples
obtained are 1.4 inches in diameter or approximately 70% of the specified minimum
diameter. To further understand the impact of a smaller sample consider if the outer 1/8-
inch perimeter of the 1.4-inch diameter specimen is disturbed by internal wall friction, this
results in 33% of the specimen area being disturbed.
Gypsum Presence
There is the concern of the influence of gypsum (CaSO4-2H2O) being present in the tailings
samples and thus affecting the accuracy of several laboratory test methods. The 2nd
paragraph of report Section 3.0 acknowledges the potential for high moisture contents and
high fines contents. The method to determine moisture content of soil, ASTM D2216,
specifically points out that standard lab procedure may dehydrate the crystalline water
contained in gypsum and suggests that a lower drying temperature of 60° C be utilized in
lieu of the standard 110° C. As acknowledged the higher drying temperature burns off the
hydrated water resulting in erroneous higher moisture contents and the creation of anhydrite
particles not normally present in the natural tailings material. The potential error enters in
the results of ASTM D1140 (#200 Sieve wash) with potentially higher fines contents; the

MWH MEMORANDUM

PAGE 26

results of ASTM D4318 (Atterberg limits) which are entirely based on moisture contents; and
the results of ASTM D422 (gradation) which would be affected similarly to ASTM D1140. It
is unclear what is causing the abrupt curvature behavior of the hydrometer gradation curves.
MWH states in the second paragraph of Section 3.0 “The measured laboratory data used in
Larson and Mitchell (1986) study did not account for gypsum in the tailings.” This
conclusion may not be correct in as much as the L&M paper is silent on whether their test
data accounted for gypsum. The reviewer concurs that this concern will affect certain input
parameters for liquefaction hazard analysis which benefit from fines content. Possibly the
correction for fines content will need to be conservatively reduced.
General Laboratory Review Comments
The following review comments are based on technical or editorial items noted during DRC’s
review of the Laboratory Investigation section of the report.
Figure E.4-1 Summary of Atterberg Limits Tests Results has incorrectly plotted division
lines at the lower left corner of the standard plasticity chart. The “A”-line has been extended
diagonally to the X-axis instead horizontally at PI = 4 from an LL = 0 to 25.5. The “U”-line
has also been extended diagonally to the X-axis instead of vertically at LL = 16 to a PI = 7.
This is clearly depicted in Figure 4 of ASTM D2487. Please review the details of the
standard figure and make corrections as appropriate.
The consolidation test identified as CPT-2W6-S(2)@13 feet has been classified to be
representative of tailings slimes, however the total weight of the specimen used in the
consolidation test set-up is indicative of a sand – slime specimen. Similarly, the
consolidation test identified as CPT-2W6-S(3)@15 feet has been classified to be
representative of tailings sand - slimes, however the total weight of the specimen used in the
consolidation test set-up is indicative of a slime specimen. Please research and review the
laboratory testing data as well as the groupings and graphs that included these results to be
sure it is being included with the appropriate soil grouping. These are examples of the
variability of the tailings profile within a short distance. If appropriate please review and
revise any other report component (such as Table 3-2 or Figure E.1-1) that relied on
this data or interpretation.
The eleven ASTM D422 lab test sheets report an increase in the percent passing the #200
sieve from the result of the ASTM D1140 test to the subsequent D422 test result. The
amount of increase ranges from 2.2% to 11.8% with an average increase of 4.9%. While an
increase in the % passing the #200 sieve from the initial wash (D1140) to the after dry sieve
wash is common, it is typically small. ASTM D6913 indirectly indicates that an increase
greater than 2% could be indicative of a problem such as degradation during mechanical
shaking; loss of sample during testing, or other issues such as the influence of the
dehydrating the gypsum crystals and thus appearing to pass the crystalline water as wash
water. Please research and review the laboratory testing data and procedures for the
eleven gradations with S&ME to be sure the tests were performed correctly. If needed

MWH MEMORANDUM

PAGE 27

please review and revise any other report component that relied on this data or
interpretation.

MWH Response

Text was added to Section 3.0 of the TDAR to provide information on delayed testing
and shipping disturbance. This text addressed DRC’s comments on this issue as noted
in DRC’s March 2015 comment no.3.

Please see response to DRC’s March 2015 comment no. 3 regarding the diameter of
samples tested for consolidation.

Additional discussion was included in Section 3.0 of the TDAR in response to DRC’s
concern regarding gypsum presence in the tailings. MWH also confirmed that the
measured laboratory data used for the Larson and Mitchell (1986) study did not account
for gypsum in the tailings. Based on DRC’s March 2015 comment no. 3, DRC’s
concerns regarding gypsum are addressed.

Revisions requested by DRC to Figure E.4-1 were addressed as noted in DRC’s March
2015 comment no. 3.

Please see response to DRC’s March 2015 comment no. 3 regarding the comment on
the consolidation test identified as CPT-2W6-S(2) @ 12.3 feet (previously listed at 13
feet in the January 2015 version of the TDAR).

DRC’s 2015 March comment no. 3 notes that classification of the consolidation test
CPT-2W6-S(3) at 14.5 feet (listed at 15 feet in the January 2015 version of the TDAR) is
no longer a concern.

MWH reviewed results of the percent passing the No. 200 test using ASTM D1140 prior
to using ASTM D422 for the 11 tailings samples tested with both procedures, as
requested by DRC. Text was added to Section 3.0 of the TDAR to discuss differences in
the results using the two methods. Please see response to DRC’s March 2015
comment no. 3 noting that future geotechnical analysis will consider the uncertainty in
the laboratory measured percent passing the No. 200 sieve for these 11 samples.
4. Section 4.1 - Tailing Classification – Correlation. As indicated earlier a characterization
scheme developed by L&M has been adopted by MWH to capture site specific field and lab
data with adjacent CPT sounding data and thereby making it possible to classify material
catalogued in the remaining CPT soundings. MWH has interpreted their data and
concluded an adjustment to the L&M brackets is necessary. MWH has recommended a
uniform lateral shift in the curve between the sand-slime and slimes; a revision in the criteria
for % fines content between the sand-slime and slimes from 70% to 60%; and finally the
removal of the curve dividing sand from sand-slime material, resulting in two material types
sand-slime and slime. As discussed in the following paragraphs the adjustments appear to
be without merit, based on laboratory and field data uncertainty or deficiencies.

MWH MEMORANDUM

PAGE 28

The classification curves by Larson and Mitchell are reported to be based on continuous
data which is neither the case for data presented in the report nor anticipated with the work
plan. The interpretation to adjust the L&M curves is based on 20 specimens from
approximately 160 lineal feet of exploration, that were selected for correlation purposes and
subjects of gradation testing. Of the 20 specimens, 8 specimens were from sample runs
with recovery rates less than 50%. Therefore nearly half of the specimens are subject to the
uncertainty discussed previously with regards to sample location within a 24 inch sample
run.
There also appears to be several plotting errors in the main interpretation graph, Figure E.1-
1 Friction Ratio vs. Cone Resistance Tailings Classification. The graph appears to
have incorrectly plotted or transposed gradation and Cc data for the sandier sample from
CPT-2W3 @ 7.0 feet with the more fine grained sample from CPT 3-6N @ 5 feet. Please
review and revise this figure and any other report component that relied on this data
or interpretation. While the plot of data from the sample at CPT-3-4N @ 9’ was excluded it
emphasizes the complex nature of the tailings. The specimen consisted of 9 inches of soil
from a 30 inch sample run. The gradation result of 19.6% fines content classifies the
specimen as sand. The adjacent CPT log SP-34N appears to interpret the following 4 soil
behavior transitions between the 9 to 11.5 feet interval: Silt / Sensitive Fines / Clay / Sandy
Silt.
The DRC has the following observations with regards to removing the curve defining the
transition from sand to sand-slime. The combination plots of CPT Data from Cells 2 and 3
(Figure E.1-3 and Figure E.1-4, respectively) clearly indicate that there are sands in the
tailings profile. The field program recovered tailings that classified as sand as indicated with
4 of the 20 gradation tests. The number of samples appears to be justification to not
remove the published division line. Furthermore, a conclusion that there are no sands and
that the tailings are predominantly made up of sand slime tailings may be an unsupported
conclusion. Without cross-sections depicting the stratigraphy of the tailings this may be an
unconservative simplification of the tailings profile.
With the examples above as well as the numerous comments presented earlier in this
review memo with regards to uncertainties with the Direct Push exploration program and the
laboratory data it is not clear that the adjustments to the L&M classification scheme are
adequately justified.
Interim Cover Material and Sensitive Fines Grained Material
Additionally, the CPT soundings revealed two soil behavior types that have not been
adequately characterized in the Tailings Data Analysis Report. The first being the surface
sequence of sandy soil with debris in Cells 2 and 3. The second being the sequences of
Sensitive Fine Grained soil (See SP2W3; SP3-3S; and SP3-6N). The work plan indicated
the CPT soundings would be used to develop profiles that characterize the tailings
stratigraphy and thereby allow for interpretation and modeling of the various tailing materials
both vertically and laterally. Without cross-sections depicting the distribution of these

MWH MEMORANDUM

PAGE 29

tailings materials at each CPT sounding it is unclear how the tailings should be
geotechnically modeled for these two soil behavior types. Please provide profiles that
depict the stratigraphy within each tailings cell both vertically and laterally.

MWH Response

Additional discussion was included in the TDAR regarding the tailings classification
method used. Plotting errors identified in DRC’s January and March 2015 comment no.
3 for Figures E.1-1 and E.1-2 were addressed. Please see response to DRC’s March
2015 comment no. 3 regarding additional revisions made to address DRC’s concerns on
the tailings classification.

Please see response to DRC’s January 2015 comment no. 2 regarding investigation of
the interim cover material. Evaluation of the interim cover material was not an objective
of the tailings investigation. However, since CPT soundings were taken at depths within
the interim cover, tailings profiles were revised to show a layer of interim cover at the top
of each profile.

Please see response to DRC’s March 2015 comment no. 4 regarding the soil behavior
type sensitive fine grained material.
Cross-sections showing tailings profiles across Cell 2 and Cell 3 were added to the
March 2015 version of the TDAR to address DRC comments. Please see response to
DRC’s March 2015 comment no.1 regarding horizontal interpretation of the tailings.

MWH MEMORANDUM

PAGE 30

AECOM JANUARY 2015 TDAR REVIEW COMMENTS AND MWH RESPONSES

1. Section 1.2, Objectives, and Sections 4.0 and 5.0. Overall, there are no conclusions or
recommendations on how the CPT and laboratory data will be used, or how these data
compare to previous work. Additionally, it is unclear in Section 1.2 whether additional
objectives of this investigation include, for example: (1) To acquire CPT and laboratory data
to be used to assist in development of cross-sections through the existing tailings
impoundment providing interpretation between various tailings types (i.e., sands, slimes and
transitional tailings) and their distributions for use in final cover design; (2) To interpret the
over-consolidation ratio and sensitivity of the tailings using CPT data to assist in evaluation
of material behavior; etc…? Please provide cross sections showing inferred distributions of
sand, sand-slime and slime tailings types in the two cells investigated and indicating how the
CPT characterization is used on sections. Please also provide a summary of tailings data
collected to date and their intended use(s), including how the current data compare to
previous information/data provided on tailings properties and indicate whether data acquired
to date are considered adequate for fulfilling the intended use(s). (See also additional
specific comments below).

MWH Response

Discussion was added to Sections 1.2 and 1.3 of the TDAR to provide more background
on the objectives of the tailings investigation and TDAR. Although data presented in the
TDAR will be used for future technical analyses, it is not the intent of the TDAR to
provide specific recommendations on how tailings properties will be selected for each
type of analysis.
Cross-sections showing tailings profiles across Cell 2 and Cell 3 were added to the
March 2015 version of the TDAR to address DRC comments. Please see response to
DRC’s March 2015 comment no.1 regarding horizontal interpretation of the tailings.
Discussion on historical tailings data was added as Section 1.2 of the March version of
the TDAR. This historic tailings data will be considered for future technical analyses as
a subset of the tailings data presented in the TDAR. As noted in the TDAR, the intent of
the tailings investigation was to provide site-specific tailings data as requested by the
DRC and the investigation followed the work plan approved by the DRC. Results of the
investigation will be used to update technical analyses to address DRC review
comments on the Reclamation Plan Revision 5.0 (Denison, 2011) and the revised ICTM
Report (MWH, 2010).

2. Figure 2-2. The depths shown of Figure 2-2 are unclear. Were the CPTs pre-drilled and
hence the larger number shown on the figure? For example, sounding SCPT-2W2 shows a
depth of 20.34 ft on CPT log and on Figure 2-1. However, on Figure 2-2 it is shown as 21.53
ft, which would suggest the sounding was pre-drilled to a depth of 1.19 feet. Please clarify.

MWH MEMORANDUM

PAGE 31

MWH Response

The depth of the CPT soundings is shown on Figure 2-1 of the TDAR. The total
thickness of interim cover and tailings at each CPT location are shown on Figure 2-2 of
the TDAR. Figure 2-2 does not show the depth of CPT soundings. The estimated
thickness of interim cover and tailings at each CPT location is greater than the total
depth of the CPT sounding at each location, indicating that no CPT probes came close
to the liner.

3. All CPT soundings appear to show a very clear upper layer with higher tip resistance and
higher skin friction (could be the interim cover/working platform fill) than the underlying
tailings with lower tip resistance and lower skin friction. Please confirm the distinction
between cover and tailings. All laboratory tests were performed on samples with lower tip
resistance and lower skin friction, i.e. tailings. Please provide information on test data that
are currently available for the interim cover/platform fill, and indicate whether such data are
considered adequate for final cover design.

MWH Response

Please see response to DRC’s January 2015 comment no. 2 regarding investigation of
the interim cover material. Evaluation of the interim cover material was not an objective
of the tailings investigation. However, since CPT soundings were taken at depths within
the interim cover, tailings profiles were revised to show a layer of interim cover at the top
of each profile.

4. Section 3.0 (all). Recommend an explanation be added to the discussion as to how the
Specific Gravity values determined for the different tailings samples tested might have been
affected by gypsum concentrations in the tailings (owing to the low specific gravity of
gypsum) and how this might impact any analyses completed for the Reclamation Plan or the
Infiltration and Contaminant Transport Modeling Report that incorporate Specific Gravity
values. Approximately what ranges of gypsum contents are expected to be present in the
tailings, according to tailings fraction?

MWH Response

Additional discussion was included in Section 3.0 (paragraphs 9 and 10) of the TDAR in
regards to the potential impact of the presence on gypsum on specific gravity
measurements of the tailings samples.

5. Section 3.0: Tables 3-1 and 3-2. A total of 5 tailings samples were tested for hydraulic
conductivity, compared to the 6 hydraulic conductivity tests specified in the Tailings
Characterization Work Plan. Table 3-2 also indicates that no sand tailings were tested for
hydraulic conductivity. Please provide the following information with respect to the
characterization of hydraulic conductivities in the tailings:
i. A comparative analysis of the current hydraulic conductivity testing results (for sand-
slimes and slimes tailings only) relative to (higher) estimates of overall hydraulic
conductivity for the tailings previously developed based on White Mesa tailings testing

MWH MEMORANDUM

PAGE 32

data collected in 1987 and 1999 and a comparison to a different off-site tailings pile. In
particular, the previous estimates suggested that the White Mesa tailings consist of
approximately 55 to 57 % sand (e.g., MWH 2010; MWH 2011; Geosyntec 2007); and
ii. An assessment of the representativeness of the current tailings hydraulic conductivity
testing results with respect to the distribution of sand, sand-slime, and slime tailings
types in the various cells, with respect to the previously estimated tailings hydraulic
conductivity values, and with respect to dewatering and final cover design needs (see
also Comment No. 1 above).

MWH Response

The specific hydraulic conductivity values to use for analyses will depend on the type of
analyses and how the tailings will be modeled. It is expected that the hydraulic
conductivities used for the tailings for future analyses will be lower than the estimated
hydraulic conductivities used in previous analyses. Although the data presented in the
TDAR will be used for these future technical analyses, it is not the intent of this report to
provide specific recommendations on how tailings properties will be selected for each
type of analysis.

For reference, the sand percentage based on particle size distribution for the tailings
samples collected from the October 2013 tailings investigation is approximately 50
percent.

6. The top of saturated tailings (listed in Table 4-1) has been estimated at the depth where
continuous elevated dynamic pore pressures have been encountered. This is not consistent
with the static pore pressure measurements at some locations (e.g. SCPT-2W3, see Figure
1 below) and also the degree of saturation measured in consolidation tests (e.g. SCPT-2W2
at depth of 7 feet). It is not clear how these data would be used, but it is recommended to
establish a phreatic surface or zero pore pressure line for analysis. Please provide a
discussion on how the data will be used in future analysis.

MWH Response

Please see response to AECOM’s March 2015 comment no. 4.

7. Section 4.2. The report states that “Equilibrium pore pressures measured during the pore
pressure dissipation tests often yield values that are higher than actual steady-state pore
pressure at the specific location and depth”. The reviewer does not necessarily agree with
this statement. Typically, if sufficient time is provided to achieve equilibrium in tailings, there
is a good correlation between static dissipation tests and pore pressures measured by
vibrating wire piezometers, e.g., see Winckler et. al (2014). If no active piezometers are
available, then vibrating wire piezometers should be installed using the CPT rig to evaluate
pore pressures within the tailings. Piezometers would assist in evaluating the pore pressure
with time and provide guidance if drainage is occurring as predicted in analysis.

MWH Response

Please see response to AECOM’s March 2015 comment no. 4.

MWH MEMORANDUM

PAGE 33

8. Section 4.2. The report also states “Dynamic pore pressures typically represent the upper
bound to the actual equilibrium pore pressures since they are the sum of the equilibrium
pore pressure and excess pore pressures due to shearing.” This is not always true. Lower or
negative dynamic pore pressures could also be generated if the material is dilative i.e.
generates negative pore pressures upon shearing. The next sentence in the report states
“The pore pressures due to shearing are usually positive unless the tailings are heavily
overconsolidated.” This does not agree with the laboratory data that showed over-
consolidated behavior in soundings SCPT-2W2 at a depth of 7 ft and SCPT-2W3 at depth of
7.5 ft, and showed positive dynamic pore pressure at both locations. The dynamic pore
pressures may not reflect hydrostatic pore pressures nor the degree of saturation within the
tailings.

MWH Response

Please see response to AECOM’s March 2015 comment no. 4. In addition, text was
added to Section 4.2 of the March 2015 version of the TDAR regarding over
consolidation ratios of the tailings samples. This text is also included in the April 2015
version of the TDAR.

9. Section 4.2, p. 12. The statement is made that “there are also some lenses of elevated
pore pressures at shallow depths, but these are considered perched zones in the interim
cover and/or tailings due to seasonal fluctuations”. Are there additional data available that
would confirm that such perched zones are seasonal vs. ‘perennial’ in nature?

MWH Response

Please see text regarding perched zones added to paragraph Section 4.2 of the March
2015 version of the TDAR. This text is also included in the April 2015 version of the
TDAR.

10. URS has found that performing at least three static dissipation tests per sounding is helpful
in evaluating the in situ pore pressure profile. This has been performed at two sounding
locations within Cell 2. Three soundings had two dissipation tests and the remaining eight
locations have one dissipation test. Fortunately, it appears that the static dissipation tests
were run out long enough to reach equilibrium in all cases except for at sounding SCPT-3-
6N. If piezometer data is available the pore pressure profile should be confirmed with this
information. URS’s interpretation of the CPT static dissipation tests indicates that there is
near hydrostatic pressure below the ground surface, see Figure 1.

MWH MEMORANDUM

PAGE 34

Figure 1: Static dissipation test result for sounding SCPT-2W3 showing near hydrostatic
conditions below ground surface.
It also appear that there is a drainage toward the sump in Cell 2 based on dissipation test
results obtained in SCPT-2W6-S, -S(2), and –S(3), shown on Figure 2, which shows a
phreatic surface approximately 10 feet below the ground surface. These dissipation tests
were plotted together due to the proximity of the soundings.

0
5
10
15
20
25
30
35
0 10203040
De
p
t
h
 (f
e
e
t
)
Pore Pressure (feet)
SCPT‐2W3
Dissipation Test
Results
Estimated Saturated
Tailings
Interim cover
Tailings

MWH MEMORANDUM

PAGE 35

Figure 2: Static dissipation test result for sounding SCPT-2W6-S, -S(2), and –S(3) showing
near hydrostatic conditions below a depth of approximately 10 feet.

MWH Response

Please see response to AECOM’s March 2015 comment no. 4.

11. It is not clear how the delineation of sand, sand-slime, and slime tailings will be used for
future calculations/analyses/models. Five consolidation tests were performed; based on
fines content, two were performed on slime tailings (67.4 and 97 percent fines) and three
were performed on sand-slime tailings (percent fines between 46.3 and 58.1). The reviewer
recommends also looking at plasticity indices and densities to evaluate material behavior.
The interval at 15 ft for SCPT-2W6-S(3) might be more slime-like than sand-slime like due to
plasticity and density.

MWH Response

Discussion on tailings classification is provided in Section 4.1 of the TDAR. Criteria
used for classification of the tailings are based on the percentage of material finer than
the no. 200 sieve by weight. The use of other index parameters, such as Atterberg limits
and density, are not included in defining these categories.

12. Section 4.3, p. 14. Please clarify/verify what figure or figures (e.g., Figures E.3-1 and E.3-
2?) are relevant to the derivation of the recommended density values listed in Table 4-3.
Briefly describe the basis for selection of the listed average values.

0
5
10
15
20
25
30
35
0 10203040
De
p
t
h
 (f
e
e
t
)
Pore Pressure (feet)
SCPT‐2W6‐S, ‐S(2), and
‐S(3) Dissipation Tests
Estimated Saturated
Tailings
100% Hydrostatic
Interim 
Tailings

MWH MEMORANDUM

PAGE 36

MWH Response

Density values provided in Table 4-3 of the TDAR are average measured density values
for the tailings samples collected during the October 2013 field investigation. Figures
E.3-1 and E.3-2 show laboratory-measured total and dry density versus depth,
respectively, for the tailings samples tested.

13. Section 4.4. Based on the tested hydraulic conductivity values, there appear to be minor
differences between slime and sand-slime tailings. Please provide a discussion on how
these test data will be used in future analysis (see also Comment Nos. 1 and 5 above and
Comment No. 14). Also please comment on the tested confining pressures that the
hydraulic conductivity tests were performed at and how the tested confining pressures were
selected.

MWH Response

Please see response to AECOM’s January and March 2015 comment no. 5 regarding
the tested hydraulic conductivity values.

Text was added to paragraph Section 4.4 of the March 2015 version of the TDAR
regarding confining pressures used for the hydraulic conductivity tests. This text is also
is included in the April 2015 version of the TDAR.

14. Section 4.4, pp. 15-16 and Table 4-6. Please provide a discussion comparing the
estimated hydraulic conductivity values listed in Table 4-6 for sand tailings to previous
estimates of tailings hydraulic conductivity described under Comment No. 5 above.
Describe how the different estimates were developed and provide a discussion of the
reliability and representativeness of these estimates of in-situ conditions in the tailings as
they relate to characterization of areas/locations within tailings cells that may consist of more
sandy material based on the current investigation and previous tailings testing results.
Provide additional information regarding how the current and previous tailings testing data
will be used to represent the potential variability in hydraulic conductivity values across the
tailings management cells, especially with regard to sand tailings.

MWH Response

Please see response to AECOM’s January comment no. 5 regarding the hydraulic
conductivity values to be used for future analyses.

15. The report shows estimated values of the horizontal coefficient of consolidation. Please
provide a discussion on how these data will be used in future analysis. Also discuss how the
vertical coefficient of consolidation (cv) values listed in Table 4-7 compare with estimates of
cv that may be derived from horizontal coefficient of consolidation (ch) estimates/values
(e.g., estimated ch values in Table 4-8) using published empirical methods (e.g., Robertson
et al. 1992) and discuss implications, if any, of apparent differences.

MWH MEMORANDUM

PAGE 37

MWH Response

Please see response to AECOM’s March 2015 comment no. 8.

16. The direct push sampling consists of “piston-type” sampler deployed from the CPT rig. The
sampler have 1.5-inch inner diameter and are 24 inches in length. A total of 35 sampling
intervals were target with 24 of these locations with sampling recovery less than 16 inches.
Three of the 35 locations had sample recovery greater than the sampler length. Please
provide comment as to the reason for the poor recovery and also indicate where (at what
interval) the recovery was obtained.

MWH Response

Please see DRC’s March 2015 comment regarding sample recovery, which determined
that this issue has been adequately addressed by the additional discussion provided in
In Section 2.2 of the TDAR.

17. According to ASTM D2435 the minimum specimen diameter or inside diameter shall be 2
inches. This was not the case for the tested consolidation samples with a reported diameter
of 1.4 inches. Please comment on why larger samples were not obtained using a drill rig to
collect samples for engineering property testing. Typically push-in samples obtained using
the CPT rig are for visual confirmation only and in some cases index property testing if
enough sample quantity can be obtained.

MWH Response

Please see response to AECOM’s March 2015 comment no. 2.

18. Cell 3 was characterized by sampling at two locations, while Cell 2 was characterized by
sampling 6 locations. At Cell 3, one location show fines content of less than 13% while the
other show fines content greater than 67%. Please provide comment on the adequacy of the
sampling distribution within each cell and spatial variation within each cell.

MWH Response

The approved work plan (MWH, 2013d) listed two direct push sampling locations per
tailings cell (four total), which were to be selected during the field program based on the
results of the CPT soundings. Direct push sampling was actually conducted at six
sampling locations in Cell 2 and two sampling locations in Cell 3. The locations were
selected to span the range of material responses (e.g. pore pressures, soil behavior
types) measured during CPT testing, as well as to provide sufficient tailings samples for
laboratory testing.

EFRI addressed DRC comments regarding sampling distribution and spatial variation as
part of preparation of the work plan.

19. On drilling logs, the permeability should be raised to a negative number. There appear to be
other typos on the exploration logs with respect to reported cc and cv values.

MWH MEMORANDUM

PAGE 38

MWH Response

Typos on the field logs in the TDAR were corrected.

20. On Figures E.2-1 through E.2-8 it would be helpful to show the pore pressure in ft.
associated with the static dissipation test and not only the elevation where the test was
performed.

MWH Response

Figures E.2-1 through E.2-8 of the TDAR were revised to include this change.

21. Discrepancies between values obtained from the laboratory testing and values reported in
Table 4-7 have been noted. Please review and correct discrepancies.

MWH Response

Discrepancies between values obtained in the laboratory testing and values report in
Table 4-7 of the TDAR have been corrected.

22. The sample recovery noted on boring logs is different interval than noted on the laboratory
testing results. Please check for and correct discrepancies.

MWH Response

Discrepancies between sample recovery noted on field logs and laboratory testing
results have been corrected.

MWH MEMORANDUM

PAGE 39

REFERENCES

Denison Mines USA Corp. (Denison), 2009. Reclamation Plan, White Mesa Mill, Blanding Utah,
Revision 4.0, November.
Denison Mines (USA) Corp. (Denison), 2011. Reclamation Plan White Mesa Mill, Blanding,
Utah, Version 5.0. September.
Energy Fuels Resources (USA) Inc. (EFRI), 2012. Responses to Interrogatories – Round 1 for
Reclamation Plan, Revision 5.0, March 2012. August 15.
Energy Fuels Resources (USA) Inc. (EFRI), 2013. Email from Jo Ann Tishler (EFRI) to John
Hultquist (DRC) confirming comments from John Hultquist on September 12, 2013 that the
responses to DRC comments on the Tailings Characterization Work Plan were satisfactory
to DRC, that DRC had no further comments on the revised plan, and that MWH could
proceed with the field investigation. September 12.
Keshian B. and R.E. Rager. 1988. Geotechnical Properties of Hydraulically Placed Uranium
Tailings. Hydraulic Fill Structures, ASCE Geotechnical Special Publication No. 21, pp.227-
254.
Larson, N.B., and B. Mitchell, 1986. Cone Penetrometer Use on Uranium Mill Tailings. Use of
In-sit Tests in Geotechnical Engineering. Samuel P. Clemence, Editor, Proceedings of a
conference sponsored by the Geotechnical Engineering Division of the American Society of
Civil Engineers, Geotechnical Special Publication No. 6.
Lunne, T., P.K. Robertson, and J.J.M. Powell, 1997. Cone Penetration Testing in Geotechnical
Practice. Blackie Academic, EF Spon/Routledge Publ., New York.
MWH Americas, Inc. (MWH), 2010. Denison Mines (USA) Corp. Revised Infiltration and
Contaminant Transport Modeling Report, White Mesa Mill Site, Blanding, Utah. Report
prepared for Denison Mines. March.
MWH Americas, Inc. (MWH), 2013a. Energy Fuels Resources (USA) Inc. (EFRI) White Mesa
Mill Tailings Characterization and Analysis Work Plan. Report prepared for EFRI. June.
MWH Americas, Inc. (MWH), 2013b. Energy Fuels Resources (USA) Inc. (EFRI) White Mesa
Mill Tailings Characterization and Analysis Work Plan. Report prepared for EFRI. July.
MWH Americas, Inc. (MWH), 2013c. DRC Request for Information for White Mesa Mill Tailings
Characterization and Analysis Work Plan. Letter from MWH to EFRI. July 30.
MWH Americas, Inc. (MWH), 2013d. Energy Fuels Resources (USA) Inc. (EFRI) White Mesa
Mill Tailings Characterization and Analysis Work Plan. Report prepared for EFRI. October.
Utah Department of Environmental Quality, Division of Radiation Control (DRC), 2013a.
Radioactive Material License (RML) Number UT 1900479: Review of September 10, 2012
Energy Fuels Resources (USA), Inc. Responses to Round 1 Interrogatories on Revised

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Infiltration and Contaminant Transport Modeling (ICTM) Report, White Mesa Mill Site,
Blanding, Utah, report dated March 2010. February 7.
Utah Department of Environmental Quality, Division of Radiation Control (DRC), 2013b. Review
of August 15, 2012 (and May 31, 2012) Energy Fuels Resources (USA), Inc. Responses to
Round 1 Interrogatories on Revision 5 Reclamation Plan Review, White Mesa Mill, Blanding,
Utah, report dated September 2011. February 13.