HomeMy WebLinkAboutDSHW-2024-005377ROUTINE LANDFILL SETILEMENT MONITORING
RESERVOIR AND LANDFILL WASTE MANAGEMENT AREAS
CHEVRON SALT LAKE REFINERY
October 2, 2012
PURPOSE AND BACKGROUND
Two landfill cells have been constructed on Chevron Salt Lake Refinery property in Salt
Lake City, Utah. The first of these was completed in 1994 and is referred to as the Reservoir
Waste Management Area ("RWMA") landfill cell. The RWMA landfill contains approximately
141,000 cubic yards of stabilized hazardous and non-hazardous waste with a footprint of
approximately 9 acres and a maximum waste thickness of 18 feet. The bottom of the RWMA
landfill is lined with a 1-foot thick compacted-clay liner constructed to an original surface
elevation of 4215 .0 feet1. The landfill's cover is lined with compacted clay and a geomembrane
material (EarthFax, 1995). The second landfill cell, referred to as the Landfill Waste
Management Area ("LWMA") landfill cell was completed in 1998. The LWMA landfill is larger
with approximately 239,000 cubic yards of stabilized non-hazardous waste covering an area of
16 acres with a maximum waste thickness of 25 feet. The bottom of the LWMA landfill is also
lined with a 1-foot thick compacted-clay liner; however, this liner was constructed to a higher
surface elevation of 4218.5 feet. The landfill's cover is lined with a geosynthetic clay liner
(EarthFax, 1999).
Upon completion of the RWMA and LWMA landfill cells, numerous survey monuments
were installed on the surface of the cell to allow for settlement to be monitored over time. The
settlement monuments were constructed of galvanized pipe and extend approximately 2 feet
above and at least 2.5 feet below the surface of the landfills to avoid any potential disturbance
from frost heave. The baseline surveys for the RWMA and LWMA settlement monuments were
performed on August 1994 and November 1998, respectively. Settlement monuments were
then routinely surveyed throughout the following years, including the most recent survey
performed in February 2011. The purpose of this report is to summarize the results of this
most-recent survey.
METHODS
Both the RWMA and LWMA landfill settlement monuments were surveyed by Diamond
Land Surveying on February 15, 2011. The surveys were performed using an electronic-read
differential level and were based off of the USGS section corner near the northeast corner of
Pond 3 (elevation 4216.85 feet). The closure error of the RWMA and LWMA surveys was 0.006
and 0.026 feet, respectively. The elevation of the USGS section corner was also surveyed
relative to other benchmarks within the refinery. This survey indicated that the section corner
1 All references made to elevation within this report are relative to the Chevron Refinery datum
1 EarthFax Engineering, Inc.
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
October 2, 2012 Management Areas
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may have settled approximately 0.1-foot since its original elevation was established. However,
for the purposes of this report and to maintain consistency with past surveys, the section
corner was assumed to be located at elevation 4216.85 feet.
SURVEY RESULTS
The results of the February 2011 settlement survey are included in Tables 1 and 2,
respectively. These tables also include the results of past settlement surveys for each landfill.
The locations of the settlement monuments identified in these tables can be found on Figure 1
for the RWMA landfill and Figure 2 for the LWMA landfill. These two figures also provide the
location of existing monitor wells and settlement contours for the monuments based on data
from the most-recent settlement survey.
As shown in Table 1 and Figure 1, the RWMA landfill monuments have settled between
0.4 and 1.6 feet since their installation in August 1994. As shown by the settlement contours
on Figure 1, settlements are generally greater near the center and southern end of the landfill.
This difference in settlement is primarily due to two factors. First, the stresses induced on the
underlying soils are greatest near the center of the landfill where waste thicknesses and stress
overlap are greatest, thus resulting in increased settlement. Second, the northern end of the
landfill was partially pre-consolidated by stabilized waste and stockpiled materials during its
construction. As a result, by the time the cover was completed and settlement monuments
installed, a portion of the settlement subject to occur at the northern end had already
occurred.
Monuments on the LWMA landfill have settled between 0.3 and 1.8 feet since their
installation in November 1998 (see Table 2 and Figure 2). Settlements are generally higher near
the center and western end of the landfill. This is expected to be occurring for the following
three reasons: (1) stresses induced on the underlying soils are greatest near the center of the
cell (see above), (2) the eastern end of the cell was partially pre-consolidated by stabilized
waste and stockpiled materials during construction, and (3) the original ground surface at the
eastern end of the cell was typically higher than that at the western end, also pre-consolidating
the underlying, compressible soils. Additionally, an abandoned H-pile foundation is located
below the landfill along its eastern end (see Figure 2). The concrete pile cap associated with
this foundation was removed during landfill construction but the piles themselves could not be
removed and were cutoff below grade. With the H-piles remaining in place, stresses induced
by the landfill’s loading are transferred to deeper, less-compressible soil layers, thus resulting in
a decreased settlement in the immediate vicinity.
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
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Settlement versus time curves for each of the settlement monuments are provided in
Figures 2 and 3 for the RWMA and LWMA landfills, respectively. As shown in these figures, the
monuments on each landfill have been settling at a relatively constant rate of approximately
1/4-inch to 3/8-inch per year for the last 8 to 10 years. This settlement is suspected to be what
is known as “secondary settlement.” Three types of settlement occur upon loading from the
landfill cells: (1) elastic settlement where the soil deforms (like a spring) immediately after the
load is placed, (2) primary settlement where the soil settles as a result of excess pore pressures
(induced by the landfill’s loading) dissipating from the pore spaces in fine-grained soils, allowing
them to consolidate, and (3) secondary settlement which occurs at a relatively slower rate over
a longer period of time due to compression of bonds between clay particles on a micro-scale
(Holtz and Kovacs, 1981).
In the case of loading induced by construction of the RWMA and LWMA landfill cells,
elastic settlement is considered negligible. Based on the settlement data presented in Figure 2,
the RWMA landfill appears to have reached the end of primary settlement in 2000,
approximately 6 years after completion of the landfill and installation of the settlement
monuments. The LWMA landfill appears to have reached the end of primary settlement by the
end of 2002, approximately 4 years after completion of the landfill and installation of
settlement monuments (Figure 3). Both landfills will continue to experience secondary
settlement indefinitely, although the rate of secondary settlement will decrease with time. In
practice, secondary settlement is not well understood; however, mathematical models of
secondary settlement calibrated to settlement observations at the RWMA and LWMA landfills
indicate that each will experience approximately 6 to 8 inches of additional settlement over the
next 50 years (Qian et al., 2002). One exception to this is at the eastern end of the LWMA in
the immediate vicinity of the abandoned H-pile foundation, where settlement will be
considerably less.
LOCATION OF THE BOTTOM CLAY LINER
All settlement observations presented thus far are for the top surfaces of the RWMA
and LWMA landfills. As discussed previously, the bottom clay liner of each landfill would have
experienced settlement during construction, prior to completion of the cover and installation of
the settlement monuments. Therefore, the settlement of each landfill’s bottom liner is greater
than the settlement measured at the top surface. Due to the dynamic nature of landfill
construction, the settlement of the liner can not be accurately modeled and could not be
measured during construction. As an indication of the bottom liner’s settlement during
construction, however, surveyor’s measurements near the end of construction on the LWMA
landfill (with fill heights near their maximum) indicated that the landfill was settling as much as
0.1 to 0.15 feet per month (Brooks, 1999). These estimates provide an indication that the
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
October 2, 2012 Management Areas
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bottom clay liner may have settled between 6 and 12 inches over the course of landfill
construction; however, these estimates contain a high degree of uncertainty.
The maximum elevation of the bottom clay liner surface (contact point between the
stabilized waste and the bottom liner) for the RWMA and LWMA landfills are included in their
respective Tables 1 and 2. These elevations were obtained by subtracting the surveyed
settlement of the monuments located on the top surface from the original surface elevation of
the bottom clay liner. Keep in mind that, for reasons discussed above, the actual elevation of
the bottom clay liner will be lower that the values presented in these tables.
A hydrograph of the groundwater surface elevation beneath the RWMA landfill as
measured in the RWMA compliance monitor wells is included as Figure 5. Since 2004, these
measurements have been taken in July during the refinery-wide annual groundwater sampling
event. Sampling events and water level measurements prior to 2004 occurred on a semi-
annual basis as shown on the hydrograph. During the past 5 years, groundwater levels have
generally been around elevation 4215 feet beneath the landfill’s eastern end and 4214 feet
beneath the western end. As shown in Table 1, the maximum surface elevations of the RWMA
landfill’s bottom clay liner are all at elevation 4214.6 feet or less. Given the fact that the
bottom liner would have experienced additional settlement during construction that is not
accounted for in Table 1, it can reasonably be expected that the bottom liner of the RWMA
landfill has been saturated over much of the past 5 years.
During the past 5 years, groundwater elevations beneath the LWMA landfill have varied
between 4215.2 and 4217.7 feet at the eastern end of the landfill. At the landfill’s western end,
these levels have varied between 4209.9 and 4212.2 feet (see hydrograph in Figure 6). Similar
to the RWMA measurements, these groundwater levels were also measured in July during the
refinery-wide annual groundwater sampling event. As shown in Table 2, the maximum
elevation of the bottom clay liner surface is as low as elevation 4218.0 feet near the eastern
end of the landfill. In light of the fact that the bottom liner would have experienced some
settlement during construction and that groundwater levels are seasonably higher than those
measured during the July sampling event (Figure 6), it can reasonably be expected that the
surface of the bottom clay liner is below the potentiometric groundwater surface on an
intermittent, seasonal basis at the eastern end of the LWMA landfill. The maximum elevation
of the bottom liner surface is around elevation 4217.1 feet at the LWMA landfill’s western end,
well above the underlying groundwater.
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
October 2, 2012 Management Areas
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DIFFERENTIAL SETTLEMENT
The potential for adverse impacts to the bottom liner or cover system as a result of
differential settlement was analyzed at the locations shown in Figures 1 and 2. These locations
have the largest measured distortion (differential settlement divided by the distance between
measurement points). Of the locations analyzed, analyses indicate that the largest tensile
strain induced on the landfill occurs at LWMA-DS2 with a value of 0.0014%. This location is in
the immediate vicinity of the abandoned pile foundation at the eastern end of the LWMA
landfill. According to Qian et al. (2002) the tensile strain at failure for a compacted-clay liner is
typically between 0.1% and 4%. Allowable tensile strains for geomembranes (RWMA landfill
cover) or geosynthetic clay liners (LWMA landfill cover) are significantly higher. Based on these
analyses, there are no indications of the potential for damage to either the bottom clay liner or
cover liners in either landfill as a result of differential settlement measured to date.
RECOMMENDATIONS
It is recommended that the settlement monuments on the RWMA and LWMA landfills
be re-surveyed in 2015. This survey should begin and end at the USGS section corner near the
northeast corner of Pond 3. The survey results should be used to update the settlement tables
included within this report and re-evaluate/update the report’s conclusions.
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
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October 2, 2012 Management Areas
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REFERENCES
Brooks, R.K., 1999. Installation Methods and Survey Results for Settlement Monuments at the
Landfill Waste Management Area at the Chevron Products Company, Salt Lake Refinery.
Letter report submitted to Mr. Mike Pope of the Chevron Salt Lake Refinery. Midvale,
Utah.
EarthFax Engineering, Inc. 2012. 2012 Annual Groundwater Sampling Report. Chevron
Products Company, Salt Lake Refinery, Salt Lake City, Utah. Midvale, Utah.
EarthFax Engineering, Inc. 1999. Final Remedy Corrective Action Report, Landfill Waste
Management Area. Chevron Products Company, Salt Lake Refinery, Salt Lake City, Utah.
Midvale, Utah.
EarthFax Engineering, Inc. 1995. Closure Report, Reservoir Waste Management Area. Chevron
U.S.A Products Company, Salt Lake Refinery, Salt Lake City, Utah. Midvale, Utah.
Holtz, R.D. and W.D. Kovacs. 1981. An Introduction to Geotechnical Engineering. Prentice-Hall,
Inc. Englewood Cliffs, New Jersey.
Qian, X., Koerner, R.M., and Gray, D.H. 2002. Prentice-Hall, Inc. Geotechnical Aspects of Landfill
Design and Construction. Upper Saddle River, New Jersey.
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
October 2, 2012 Management Areas
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TABLES
Chevron Salt Lake Refinery
Salt Lake City, Utah
October 2, 2012
Routine Landfill Settlement Monitoring
Reservoir and Landfill Waste
Management Areas
EarthFax Engineering, Inc.
Feb.
2000
Nov.
2000
Dec.
2002
Dec.
2005
Feb.
2011
Northeast Corner Topsoil-267 4229.31 4227.22 0.44 0.44 0.51 0.52 0.67 4214.3
North Edge (Center)Topsoil-271 4229.48 4227.45 0.58 0.58 0.65 0.68 0.78 4214.2
Northwest Corner Topsoil-87 4228.91 4226.99 0.30 0.31 0.35 0.37 0.43 4214.6
East Edge (Center)Topsoil-213 4229.36 4227.50 0.72 0.74 0.80 0.83 1.00 4214.0
West Edge (Center)Topsoil-53 4229.01 4227.05 0.66 0.66 0.72 0.75 0.88 4214.1
Southeast Corner Topsoil-238 4228.63 4226.83 0.85 0.84 0.90 0.93 1.12 4213.9
South Edge (Center)Topsoil-250 4228.83 4226.88 0.87 0.88 0.93 0.98 1.15 4213.9
Southwest Corner Topsoil-104 4228.69 4226.89 0.81 0.81 0.86 0.91 1.07 4213.9
North Peak Topsoil-84 4234.34 4232.45 0.90 0.90 0.99 1.04 1.18 4213.8
Center Peak Topsoil-95 4234.40 4232.49 1.09 1.09 1.18 1.22 1.36 4213.6
South Peak Topsoil-101 4234.52 4232.53 1.26 1.26 1.34 1.39 1.56 4213.4
(a) See Figure 1 for monument locations
(b) Settlement monuments stickup between 1.8 and 2.1 feet above the top surface elevation
(c) The 1-foot thick bottom clay liner was constructed with an original surface elevation of 4215.0 feet.
Value is presented as a maximum since some unmeasured settlement of the bottom clay liner would have occurred during cell construction.
TABLE 1
CHEVRON SALT LAKE REFINERY
RWMA Landfill Settlement Monitoring
Settlement of RWMA Landfill Monuments Since August 1994
Location Monument
ID (a)
Monument
Elevation
August 1994
(ft) (b)
Top Surface
Elevation
August 1994
(ft)
Survey Date and Settlement of
Monuments Since August 1994 (ft)Maximum Elevation
of the Bottom Clay Liner
Surface (ft) (c)
NORTH EDGE
EAST EDGE
WEST EDGE
SOUTH EDGE
PEAK
Chevron Salt Lake Refinery
Salt Lake City, Utah
October 2, 2012
Routine Landfill Settlement Monitoring
Reservoir and Landfill Waste
Management Areas
EarthFax Engineering, Inc.
Nov.
1999
Apr.
2000
Nov.
2000
Dec.
2002
Dec.
2005
Feb.
2011
Northeast Corner 24179 4231.50 4229.44 0.20 0.27 0.32 0.39 0.44 0.53 4218.0
Southeast Corner 24158 4231.71 4229.93 0.05 0.10 0.13 0.17 0.21 0.33 4218.2
North Edge 23787 4232.06 4230.15 0.38 0.48 0.60 0.78 0.84 1.03 4217.5
South Edge 23800 4232.37 4230.41 0.27 0.35 0.45 0.58 0.67 0.85 4217.7
Northwest Corner 23654 4232.35 4230.16 0.70 0.81 0.93 1.13 1.25 1.43 4217.1
West Edge 23661 4232.52 4230.47 0.58 0.70 0.85 1.08 1.20 1.41 4217.1
Southwest Corner 23668 4231.92 4230.02 0.59 0.71 0.87 1.09 1.24 1.38 4217.1
West Peak 23710 4245.35 4243.17 0.84 1.02 1.16 1.43 1.56 1.80 4216.7
East Peak 23917 4240.11 4238.11 0.04 0.12 0.15 0.24 0.30 0.43 4218.1
H-Piles 24154 4232.59 4230.74 0.00 0.02 0.06 0.10 0.13 0.25 4218.3
North of H-Piles 24178 4232.18 4230.29 0.15 0.22 0.28 0.34 0.41 0.52 4218.0
West of H-Piles 24173 4235.33 4233.46 0.05 0.10 0.12 0.16 0.19 0.27 4218.2
South of H-Piles 24134 4234.06 4232.17 0.06 0.11 0.13 0.18 0.21 0.33 4218.2
(a) See Figure 2 for monument locations
(b) Settlement monuments stickup between 1.8 and 2.2 feet above the top surface elevation
(c) The 1-foot thick bottom clay liner was constructed with an original surface elevation of 4218.5 feet.
Value is presented as a maximum since some unmeasured settlement of the bottom clay liner would have occurred during cell construction.
H-PILE FOUNDATION
TABLE 2
CHEVRON SALT LAKE REFINERY
LWMA Landfill Settlement Monitoring
Settlement of LWMA Landfill Monuments Since November 1998
Maximum Elevation of
the Bottom Clay Liner
Surface (ft) (c)
EAST EDGE
NORTH AND SOUTH EDGES
WEST EDGE
CENTER
Location Monument
ID (a)
Monument
Elevation
November 1998
(ft) (b)
Top Surface
Elevation
November 1998
(ft)
Survey Date and Settlement of
Monuments Since November 1998 (ft)
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FIGURES
A
~ ...,
~~ I~
G:l
....
·~
'~~&
SECTION CORNER
\ IELEV: 4216.85'
I
1 I
I , ,§
I ~
B c
-
~-----+-----+----l
DR SWF CH. SAM
(f) DR APP . ____ _
z1----------~--___. __ ,________.
0 ENGR. SAM
(fl OPR'G . DEPT. APPROVED
>1----------~--___. __ ,________. w 1-=========J
0::: ~-----1------1--~ EN G'R. DEPT .
D E F G
:i;
:a:.~ ::c ::c
4225 \ \ ~ 11
x x x
,.o-TOPSOIL-238
(1.12')
\ \
\ \
~ . I \
I I ,.s-\ I I I < . TOPSOIL-101
(1.56') -
-------1.0--.._,,,,;, _____ _
421
• TOPSOIL-84 SETTLEMENT MONUMENT ID
(1.18') AND SETTLEMENT SINCE
AUGUST 1994
S MONITOR WELL
---------.. ~ ... ,., -.~ • • ~'-'°J ..._-.........
EarthFax
SCALE t • = 80'
0
Lll
N
01
z
0
LEGEND
--1.5--
(RWMA-os1)
DATE OCT 2012
MONUMENT SETTLEMENT
CONTOURS (FT)
SINCE AUGUST 1994
DIFFERENTIAL SETTLEMENT
ANALYSIS TRANSECT
--4230--EXISTING CELL
TOPOGRAPHY (FT)
RESERVOIR WASTE MANAGEMENT AREA LANDFILL
SETILEMENT OF MONUMENTS
SINCE AUGUST 1994
C.C.
S.O. FIGURE 1
1
2
3
4
N 10 750
N 10 500
D
0
0 m
w
A
0
0
lfl
N
m
w
B
~---+---+------t
c
DR SWF CH. SAM
(f) DR APP. ___ _
Z>---------~---+---+------t
O ENGR. SAM
(fl OPR'G. DEPT. APPROVED
>>---------~---+---+------t w 1--=========.j
0::: ~---+----+---------! EN G'R. DEPT.
---------.. ~ ... r• ,.~ • • ~'-'°J ...._-........
EarthFax
SCALE
0
lfl r--.
m
w
D E
l' = 120' DATE
F G
x
L---~4--.31P=~~r---ABANDONED H-PILE
OCT 2012
FOUNDATION (CUTOFF BELOW
BOTIOM LANDFILL LINER)
LEGEND
x
• 24158
(0.33')
SETTLEMENT MONUMENT ID
AND SETTLEMENT SINCE
NOVEMBER 1998
S MONITOR WELL
--1.5--MONUMENT SETTLEMENT
CONTOURS (FT)
SINCE NOVEMBER 1998
0
i »' ( LWMA-DS1) DIFFERENTIAL SETTLEMENT
)//, ANALYSIS TRANSECT
~ -4230-EXISTING CELL
% x TOPOGRAPHY (FT)
LANDFILL WASTE MANAGEMENT AREA LANDFILL
SETILEMENT OF MONUMENTS
SINCE NOVEMBER 1998
C.C.
S.O. FIGURE 2
1
2
4
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Figure 3: RWMA Landfill Monument Settlement vs. Time
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
October 2, 2012 Management Areas
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Figure 4: LWMA Landfill Monument Settlement vs. Time
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
Salt Lake City, Utah Reservoir and Landfill Waste
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Figure 5: Hydrograph of RWMA Monitor Well Groundwater Levels
Chevron Salt Lake Refinery Routine Landfill Settlement Monitoring
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Figure 6: Hydrograph of LWMA Monitor Well Groundwater Levels