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Evaluation of a final cover slide at a landfill with recirculating leachate
Geotextiles and Geomembranes 35 (2012) 100e106
Contents lists available at SciVerse ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Evaluation of a final cover slide at a landfill with recirculating leachate Craig H. Benson*, Tuncer B. Edil, Xiaodong Wang Geological Engineering, University of Wisconsin-Madison, 2205 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706-1691, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2012 Received in revised form 19 July 2012 Accepted 26 July 2012 Available online 27 August 2012
A forensic investigation was conducted to determine the mechanism causing the slide of a final cover on a 4:1 (horizontal: vertical) slope at a municipal solid waste (MSW) landfill where leachate had been recirculated. The slide occurred as a veneer displacement along the interface between the geomembrane (GM) and geosynthetic clay liner (GCL). Site observations suggested that elevated gas pressures were a significant contributor to the failure. Laboratory tests were conducted to determine the shear strength parameters of the GMeGCL interface and the reduction in normal stress required to cause displacement of the GMeGCL interface. Hydraulic conductivity and gas permeability of the GCL were also determined. Slope stability analyses were conducted to determine the gas pressure required to cause a slide and the factor of safety that would have existed if the gas pressures was at zero gage. Good agreement was obtained between gas pressures measured in the field, measurements of the reduction in normal stress required to cause sliding on the GMeGCL interface in a large-scale direct shear test, and the gas pressures corresponding to FS ¼ 1 (imminent sliding) from the slope stability analysis. The findings from this study, and a similar case history, illustrate the importance of managing gas at an acceptable level beneath the cover at MSW landfills. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Interface shear Geomembrane Geosynthetic clay liner Landfill gas Slide Leachate recirculation Bioreactor landfill
1. Introduction In early July 2011, a 0.6-ha section of final cover slid unexpectedly on the eastern slope of a recently completed cell at a municipal solid waste (MSW) landfill in northern Wisconsin, USA. The slide area was within a 4.25-ha section of final cover constructed in Fall 2010, and a post-slide survey indicated that the slope was oriented at 4:1 (horizontal: vertical), 0.1. The cover profile consists of a composite barrier overlain by a geocomposite drainage layer (GDL), 760 mm of outwash sand as a drainage/protective layer, and 150 mm of topsoil (Fig. 1). The composite barrier consists of a needle-punched geosynthetic clay liner (GCL) overlain by a textured 1-mm-thick linear low-density polyethylene (LLDPE) geomembrane (GM). Both geotextiles in the GCL were non-woven. The geomembrane was manufactured as a blown film with nitrogen blowing agent coextruded to create texturing. The GDL was comprised of a geonet (GN) with non-woven needle-punched geotextiles (GT) heat-bonded on both sizes. The final cover was constructed on a 600-mm-thick layer of silty clay placed directly on the waste. A schematic of the area where the slide occurred is shown in Fig. 2. The slide occurred as a veneer displacement along the interface between the GM and GCL, which exposed an area
* Corresponding author. Tel.: þ1 608 262 7242; fax: þ1 608 890 3718l. E-mail address: [email protected] (C.H. Benson). 0266-1144/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.geotexmem.2012.07.006
approximately 14 m long (slope direction) and 37 m wide (cross slope direction) (Fig. 3a). The surface soils (top soil and drainage/ protection layers) were bunching downslope (Fig. 3b), and cracked upslope of the exposed area (Fig. 3c). The total area where the slope was deformed is shown in Fig. 2 along with the area where the GCL was exposed. Tensile failure of the GM and the GDL were evident near the upslope edge of the exposed area, where the GM was exceptionally thin and distorted (Fig. 4a) and the ribs of the GN and the GT were torn (Fig. 4b). Test holes excavated in the subgrade soil of the exposed area showed evidence of perched water (leachate) as well as gas bubbling (Fig. 5). These observations were consistent with recent operational conditions. The landfill had been recirculating leachate vigorously in the cell prior to installation of the final cover, which had “watered out” the gas wells. Pumps for extracting leachate concurrent with gas collection were installed in the gas wells, but had been shutdown prior to the slide for repair. Consequently, gas extraction in the region was limited prior to the slide. Moreover, gas was problematic when this section of cover was constructed. During cover construction, eleven gas relief holes were intentionally placed in the GM to ensure it remained in contact with the ground surface during construction (these holes were sealed prior to placement of overlying geosynthetics and soils). Based on these observations, the failure mechanism was initially hypothesized to be a reduction in shear resistance at the interface between the GM and GCL due to elevated gas pressure. Water
C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
Fig. 1. Schematic cross-section of final cover with composite barrier.
pressure (from perched leachate) acting on the lower side of the composite barrier was considered a potential secondary cause. This paper describes the field observations, sampling and laboratory testing, and stability analyses conducted to evaluate the hypothesized failure mechanism. 2. Sampling and inspection 2.1. Test areas Two test areas were opened south of the area where the slide occurred and near the edge of any visible distress of the cover soils.
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Test Area 1 was opened near the top of the slope and Test Area 2 around mid slope (Fig. 2). Both test areas were approximately 4 m 4 m in cross-section. Cover soils were removed with a backhoe until the excavation was within approximately 300e400 mm of the GDL. The remaining soils were removed using hand tools. When the cover soils were removed, the thickness of each layer was measured with a steel tape, density and water content were measured with a nuclear densometer, and grab samples of the cover soils were collected for use in the laboratory. The cover soils were moist but not saturated when they were excavated, with water contents ranging between 9.7 and 13.2% for the top soil and 2.7e6.4% for the drainage/protection layer. Only films of free water were present in the GDL when it was removed. The total density of the topsoil was 1.58 Mg/m3; for the outwash sand used for the protective/drainage layer, the total density was 1.92e1.94 Mg/m3. Additional clues regarding the failure mechanism became evident as the soil remaining over the GDL was removed using hand tools. The surface of the soil began to bulge and when the surface soils were completely removed, the GM bulged 170 mm off the underlying surface due to gas pressure, despite being anchored tightly around the periphery of the test area by the cover soils (Fig. 6). A small puncture through the GDL and GM made using a utility knife released the pressure, and the odor of the emanating gas was indicative of landfill gas. The geomembrane and GCL were cut with a razor knife around the periphery of the test area so that samples could be retrieved for laboratory testing. As soon as the geosynthetics were cut, they immediately separated by approximately 30 mm (Fig. 7), indicating that the geomembrane and GCL were in tension. After the geomembrane was removed from the test area, three GCL samples were collected using the procedure described in ASTM D 6072, with PVC plates (300 mm 300 mm 5 mm) used as the supporting layer. The GCL samples were sealed in plastic sheet in essentially undisturbed condition using the procedure described in Scalia and Benson (2011). Grab samples (2 kg) of the subgrade soil were collected from the area beneath each GCL sample and stored
Fig. 2. Schematic of final cover showing area where the cover displaced (dark shading), area where GCL and subgrade soil were exposed (light shading), test areas, and gas pressure monitoring locations (circled P1eP5). Elevations shown are in feet. Scale is in meters.
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C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
Fig. 4. Photographs from slide: (a) torn edge of geomembrane and (b) torn geonet and geotextile from the geosynthetic drainage layer.
Fig. 3. Photographs from slide: (a) exposed GCL, (b) bulging of cover soils downslope, and (c) cracks in cover soil upslope.
in sealable plastic bags. The remaining GCL was removed from the excavation. The subgrade soil was inspected after the GCL was removed. There was no evidence of saturation, free water, or seeps indicative of elevated pore water pressures. However, gas was present and was emanating from fine cracks and voids in the subgrade soil. For example, a whistling sound was audible in the test area and gas flow could be felt when a hand was placed near the cracks or voids in the subgrade soil. 2.2. Gas pressure monitoring The bulging of the GM when the cover soil was removed in the test areas, the presence of gas, and the absence of free water were strong indications that excess gas pressure was a primary factor responsible for the slide. The condition in the slide area was also consistent with this inference. Gas was bubbling vigorously in test holes, indicating that elevated gas pressure was present. Water was observed in test holes excavated in the subgrade soil, but free water
and seeps were not present at the surface of the subgrade soil. Perched or other sources of free water were not observed during the forensic examination or shortly thereafter when the cover was removed from the side slope during restoration, further confirming that water pressure was an unlikely cause of the failure. Based on these observations, excess gas pressure was considered to be the primary factor contributing to the slide. Thus, five gas probes were installed to measure gas pressures (P1eP5) so that analyses could be conducted to assess the impact of gas pressure on slope stability. Probes were driven into the waste directly beneath the subgrade soil, and the pressure was observed until a steady condition was achieved. P1 and P2 were located near the test areas, P4 and P5 were located within the area where the GCL and subgrade soil were exposed, and P3 was located at a transition area near the southern edge of the exposed area (Fig. 2). Gas pressures were nil (<1 kPa) at P4 and P5 (within exposed area). At P1 (farthest from open area, Fig. 2), the gas pressure was highest (7 kPa). At P2, the gas pressure was 4 kPa and at P3 (closest to exposed area, Fig. 2) the gas pressure was 3 kPa. The absence of measurable gas pressure in the exposed area was not unexpected. Gas pressures almost certainly dissipated in the exposed area because portions of the subgrade soil had been exposed for nearly a week and test holes had been excavated through the subgrade soil, both of which would permit gas to escape. The monotonic increase in gas pressure with increasing distance from the exposed area (i.e., from P3 to P1) is also consistent with the assumption of gas pressure relief within the exposed area. Based on these measurements, the gas pressure beneath the GM probably was at least 7 kPa when the slide occurred, and may have been higher.
C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
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the cover soils, and the hydraulic gradient was set at 10. Tap water was used as the permeant liquid for the hydraulic conductivity tests. 3.2. Conventional direct shear tests
Fig. 5. Gas bubbling in test hole in subgrade soil of exposed area. Arrows denote bubbles.
3. Laboratory testing methods 3.1. Geosynthetic clay liner properties Tests were conducted on the GCL samples to determine the water content, hydraulic conductivity to water, and air permeability. The hydraulic conductivity and air permeability tests were conducted to assess whether gas could be transmitted through the GCL to the GMeGCL interface, thereby providing gas pressure for instability. Hydraulic conductivity tests were conducted in accordance with ASTM D 6766 and the air permeability tests with ASTM D 6539. The effective confining pressure was set at 17 kPa, approximating the average vertical overburden stress imposed by
Fig. 6. Bulging surface of GDL immediately after puncturing GDL and GM with utility knife. Survey rod shows distance geosynthetics are bulging above subgrade soil soon after puncture. Units on rod are in feet. Tip of rod sits on GCL and is at 25 ft on scale. The surface of the geocomposite drainage layer is at 24.45 ft, indicating the GM has bulged 0.55 ft, or 170 mm.
Large-scale direct shear tests were conducted on the GMeGCL interface following the procedure described in ASTM D 5321 using a 305 mm 305 mm steel shear box in a shear machine employing a stepper motor to displace the interface. A schematic of the shear machine is shown in Fig. 8. Subgrade soil from directly below the GCL was compacted in the bottom portion of the shear box to the average water content (14.7%) and dry density (1.79 Mg/ m3) measured in the field. A GCL specimen (305 mm 305 mm) was placed on the compacted subgrade soil, affixed to the shear box, and then moistened with tap water using a spray bottle to achieve the average water content measured in the test areas (77%). The GCL specimen was cut from a larger sample obtained from excess GCL stored at the landfill after construction. The surface of the moistened GCL specimen was sealed with plastic and the bentonite was allowed to hydrate for 24 h. A GM specimen of the same size was cut from the GM sample obtained from the test areas. The GM specimen was placed over the GCL (after the 24-h hydration period) and fixed to the shear box. The GM and GCL were both oriented with the shearing direction aligned with the machine direction, which is the orientation in the field. A layer of outwash sand from the field was placed in the upper box at the average water content and dry density measured during in the test areas. Normal stress was applied to the outwash sand using a loading platen surcharged with dead load. The normal stress was applied for 24 h prior to shearing, during which compression ceased. Shearing was conducted at 0.1 mm/min, which is the lowest rate at which the shear machine could be operated. Triplett and Fox (2001) show that peak and large-displacement shear strengths of GMeGCL interfaces are independent of displacement rate provided the displacement rates is within 0.01e10 mm/min. Stark et al. (1996) also show that the shear strength of interfaces between geomembranes and non-woven geotextiles is independent of displacement rate when the rate is between 0.03 and 25 mm/min. Shearing loads were measured with a load cell and vertical and horizontal displacements were measured with LVDTs. Tests were performed at normal stresses of 4.8, 9.6, 14.4, and 19.2 kPa to bracket the normal stress existing in the field (16.2 kPa) and to define any non-linearity in the shear strength envelope.
Fig. 7. Separation of geosynthetics after being cut with utility knife.
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Fig. 8. Schematic of large-scale direct shear box used to test GMeGCL interface.
Typical shear stress vs. horizontal displacement curves from the conventional direct shear tests are shown in Fig. 9. Peak shear strength was reached within 6 mm of displacement and was followed by a gradual reduction in shear strength. Curves of similar shape were obtained at all normal stresses.
system to approximate the field condition (the actual field shear stress was 4.0 kPa, but a stress this low could not be applied reliably with the apparatus). After an additional 24 h, the dead load used for the normal stress was gradually reduced (simulating an increase in gas pressure) until shear failure occurred under the applied shear stress.
3.3. Stress reduction direct shear tests 4. Results of laboratory testing Additional large-scale direct shear tests were conducted to evaluate the impact of a reduction in normal stress due to gas pressure on the GMeGCL interface. The specimen was set up as in the conventional large-scale direct shear test and consolidated to the field normal stress (16.2 kPa) using a dead weight for 24 h. A shear stress of 5.3 kPa was then applied using a pulley and hanger
20
4.1. GCL conductivity The hydraulic conductivities and air permeabilities are summarized in Table 1. The hydraulic conductivity is very low, indicating that elevated gas pressure had no detrimental impact on the hydraulic conductivity of the GCL. The air permeability is also low, but sufficient to permit gas pressures to develop at the interface between the GCL and geomembrane. 4.2. GMeGCL interface shearing behavior
Normal Stress
Shear stress (kPa)
15
Shear strength envelopes corresponding to peak and largedisplacement (50 mm) interface strengths are shown in Fig. 10. A duplicate test conducted at 10 kPa confirmed that the shear
19.2 kPa
10
9.6 kPa
Table 1 Hydraulic conductivity and air permeability of GCLs.
5
0
0
10
20
30
40
50
60
Displacement (mm) Fig. 9. Typical shear stress vs. displacement curves for conventional direct shear tests on the GMeGCL interface.
Sample
Test area
Water content (%)
Hydraulic conductivity (cm/s)
TP-1-M TP-1-E TP-1-N TP-2-M TP-2-E TP-2-S
1 1 1 2 2 2
91.7 83.6 98.6 59.1 64.9 63.8
1.5 3.2 1.5 1.1 1.3 1.6
Note: NM ¼ no measurable flow.
109 109 109 109 109 109
Air permeability (m2) 2.0 2.2 1.8 3.6 NM NM
1018 1017 1018 1018
C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
105
the method described in Thiel (1998). The equation for the factor of safety is:
FS ¼
strengths were reproducible. A peak interface friction angle (d) of 25.9 and adhesion (a) of 4.1 kPa were obtained by linear regression. For large-displacement conditions, d ¼ 23.2 and a ¼ 2.6 kPa were obtained. These strength parameters are similar to strength parameters reported by Triplett and Fox (2001) and McCartney et al. (2009) for GMeGCL interfaces, and the ratio of peak to large-displacement strength is within the range reported by Triplett and Fox (2001). Displacement of the GMeGCL interface measured in the stress reduction tests is shown in Fig. 11 (duplicate tests were conducted). Distinct shear failure at the interface occurred when the normal stress was reduced by approximately 11e13 kPa (i.e., the effective normal stress was reduced to approximately 3e5 kPa), and displacement was initiated when the normal stress was reduced by 8e9 kPa (i.e., gas pressure reduced the effective normal stress to 7e8 kPa). These reductions in normal stress are in reasonable agreement with gas pressures measured at the farthest gas probe (7 kPa at P1), especially given the pressure relief provided by the exposed area and the gradient in gas pressure evident from gas probe P3 to gas probe P1 (i.e., the gas pressure may have been higher than 7 kPa before the pressure was relieved by the slide). The normal and shear stresses at failure from the stress reduction tests are shown in Fig. 10 along with the data from the conventional direct shear tests. The stress reduction data agree with the shear strength measured in conventional direct shear at a normal stress of 4.8 kPa. Thus, an additional peak strength envelope was created by regressing through the peak strengths obtained from the conventional direct shear tests and the stress reduction tests. This envelope has a peak interface friction angle (d) of 28.0 and adhesion (a) of 3.4 kPa.
(1)
where sv is the vertical overburden pressure from the soil layers over the geomembrane, u is the gas pressure at the GMeGCL interface, and b is the slope angle from the horizontal. Eq. (1) was used in two ways. First, the FS was computed for u ¼ 0 to confirm that the slope should have been stable as constructed if the gas collection system was maintaining gas pressures at near zero gage. Second, Eq. (1) was solved for u using FS ¼ 1 to determine the gas pressure required to induce failure. Analyses were conducted with the strength parameters corresponding to peak and largedisplacement conditions and with and without adhesion assuming a linear failure envelope (any nonlinearities that may have existed at low normal stresses were not considered) Peak strength was defined using the envelope from the conventional direct shear test and the data pooled form conventional direct shear test and the stress reduction tests. Results of the analyses are summarized in Table 2. For conditions without gas pressure, the FS ranges from 1.71 to 2.98. Thus, regardless of the assumption regarding adhesion or the displacement condition used to define the shear strength, the GMeGCL interface should have been stable under static conditions if gas pressures were near zero gage. This conclusion is consistent with observations made in other cells at the landfill that have active gas systems designed to draw a vacuum, no gas collection issues, and employ the same cover profile. Some of these cells were constructed more than a decade ago and all have stable final covers. The gas pressures required to induced failure (FS ¼ 1) range from 6.5 to 16.0 kPa, with lower pressures associated with largedisplacement conditions or the absence of adhesion. These gas pressures are in the same range as the gas pressures required to induce failure indicated by the stress reduction tests (8e9 kPa to initiate failure, 11e13 kPa for appreciable displacement) and are comparable to, but slightly higher than the maximum gas pressures measured in the field (7 kPa). However, as noted previously, the gas pressures measured in the field may underestimate the gas
Simulated Gas Pressure (kPa) 60 Interface Displacement (mm)
Fig. 10. Shear strength envelopes for the GMeGCL interface along with combinations of shear and normal stress from the stress reduction tests.
a þ sv cos2 b u tan d sv cos bsin b
0
2
12
14
4 6 8 10 12 Normal Stress Reduction (kPa)
14
4
6
8
10
Original
50
Duplicate
40
Applied shear stress = 5.3 kPa Initial normal stress = 16.2 kPa
30 20 10 0
5. Stability analyses Because the displaced area consisted of a relatively long (61 m) and broad (120 m) section of slope, the analysis was conducted assuming an infinite slope with gas pressure at the interface using
0
2
Fig. 11. Displacement of GMeGCL interface as normal stress is reduced during stress reduction tests in large-scale direct shear machine. The reduction in normal stress simulates how increasing gas pressure causes a reduction in effective normal stress.
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C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
Table 2 Factors of safety (FS) without gas pressure and gas pressure required for FS ¼ 1. For peak strength, interface friction angle and adhesion are reported for the strength envelope obtained from the conventional direct shear tests alone (left) and for the combined data from the conventional direct shear tests and the stress reduction tests (right). Interface friction angle, d ( )
Adhesion, a (kPa)
25.9, 28.0 25.9, 28.0 23.2 23.2
0 4.1, 3.4 0 2.6
Gas pressure for FS ¼ 1 (kPa)
FS for no gas pressure Direct shear envelope
Direct shear & stress reduction
Direct shear envelope
Direct shear & stress reduction
1.94 2.98 1.71 2.38
2.12 2.99 e e
7.6 16.0 6.5 12.6
8.3 14.7 e e
pressures at the time of failure due to pressure relief provided by the exposed area. These findings indicate that excess gas pressure was a key contributor to the slide. The findings from this study are also very similar to those reported in the case study by Thiel (1999), where gas pressures in a MSW landfill of the same magnitude as those observed in the current study resulted in a final cover slide on a 4:1 slope due to a reduction in shear resistance at the GMeGCL interface. The similarity of both of these studies illustrates the importance of controlling gas pressures in MSW landfills. An extensive gas recovery system was installed directly beneath the composite barrier when the cover was restored in Fall 2011. The restoration included permanent gas pressure monitoring stations (Janusz et al., 2012). 6. Summary and conclusions The study described in this paper was conducted to determine the primary mechanism contributing to the slide of a landfill cover at a MSW landfill where leachate had been recirculated vigorously. Elevated gas pressure beneath the cover was a significant contributor to the slide. Inadequate gas collection due to watering out of gas collection wells and inoperative leachate extraction pumps installed in the gas extraction wells contributed to elevated gas pressures beneath the final cover. The laboratory experiments, field observations, and stability analyses conducted in this study illustrate how these elevated landfill gas pressures resulted in a reduction in effective normal stress and ultimately slope instability due to inadequate shearing resistance between the GM and GCL. The study also illustrates that the slope would
have been stable if gas pressures had been maintained near zero gage. This study illustrates the importance of maintaining adequate gas collection or deploying means to relieve gas pressure in MSW landfills, particularly those with high gas generation rates such as landfills that recirculate leachate or operate as bioreactors. Filling strategies and recirculation rates should be selected so that the waste remains sufficiently transmissive and gas may be collected efficiently. In additional, transmissive gas collection layers should be considered to maintain gas pressures acceptably low at the base of the final cover. Careful consideration of these issues will reduce the likelihood of final cover instability and reduce fugitive emissions of landfill gas to the atmosphere. References Janusz, A., Benson, C., Edil, T., Wang, X., 2012. Gas pressures and a final cover slide: investigation and remedial measures, in: Proc. Global Waste Management Symposium 2012. New York: Penton Media on CD. McCartney, J., Zornberg, J., Swan, R., 2009. Analysis of a large database of GCL-geomembrane interface shear strength results. J. Geotech. Geoenviron. Eng. 135 (2), 209e223. Scalia, J., Benson, C., 2011. Hydraulic conductivity of geosynthetic clay liners exhumed from landfill final covers with composite barriers. J. Geotech. Geoenviron. Eng. 137 (1), 1e13. Stark, T., Williamson, T., Eid, H., 1996. HDPE geomembrane/geotextile interface shear strength. J. Geotech. Eng. 122 (3), 197e203. Thiel, R., 1998. Design methodology for a gas pressure relief layer below a landfill geomembrane cover to improve slope stability. Geosynth. Int. 5 (6), 589e617. Thiel, R., 1999, Design of gas pressure relief layer below a geomembrane cover to improve slope stability, in: Proc. Geosynthetics ’99, Industrial Fabrics Association International, St. Paul, MN, pp. 235e252. Triplett, E., Fox, P., 2001. Shear strength of HDPE geomembrane/geosynthetic clay liner interfaces. J. Geotech. Geoenviron. Eng. 127 (6), 543e552.
Contents lists available at SciVerse ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Evaluation of a final cover slide at a landfill with recirculating leachate Craig H. Benson*, Tuncer B. Edil, Xiaodong Wang Geological Engineering, University of Wisconsin-Madison, 2205 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706-1691, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2012 Received in revised form 19 July 2012 Accepted 26 July 2012 Available online 27 August 2012
A forensic investigation was conducted to determine the mechanism causing the slide of a final cover on a 4:1 (horizontal: vertical) slope at a municipal solid waste (MSW) landfill where leachate had been recirculated. The slide occurred as a veneer displacement along the interface between the geomembrane (GM) and geosynthetic clay liner (GCL). Site observations suggested that elevated gas pressures were a significant contributor to the failure. Laboratory tests were conducted to determine the shear strength parameters of the GMeGCL interface and the reduction in normal stress required to cause displacement of the GMeGCL interface. Hydraulic conductivity and gas permeability of the GCL were also determined. Slope stability analyses were conducted to determine the gas pressure required to cause a slide and the factor of safety that would have existed if the gas pressures was at zero gage. Good agreement was obtained between gas pressures measured in the field, measurements of the reduction in normal stress required to cause sliding on the GMeGCL interface in a large-scale direct shear test, and the gas pressures corresponding to FS ¼ 1 (imminent sliding) from the slope stability analysis. The findings from this study, and a similar case history, illustrate the importance of managing gas at an acceptable level beneath the cover at MSW landfills. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Interface shear Geomembrane Geosynthetic clay liner Landfill gas Slide Leachate recirculation Bioreactor landfill
1. Introduction In early July 2011, a 0.6-ha section of final cover slid unexpectedly on the eastern slope of a recently completed cell at a municipal solid waste (MSW) landfill in northern Wisconsin, USA. The slide area was within a 4.25-ha section of final cover constructed in Fall 2010, and a post-slide survey indicated that the slope was oriented at 4:1 (horizontal: vertical), 0.1. The cover profile consists of a composite barrier overlain by a geocomposite drainage layer (GDL), 760 mm of outwash sand as a drainage/protective layer, and 150 mm of topsoil (Fig. 1). The composite barrier consists of a needle-punched geosynthetic clay liner (GCL) overlain by a textured 1-mm-thick linear low-density polyethylene (LLDPE) geomembrane (GM). Both geotextiles in the GCL were non-woven. The geomembrane was manufactured as a blown film with nitrogen blowing agent coextruded to create texturing. The GDL was comprised of a geonet (GN) with non-woven needle-punched geotextiles (GT) heat-bonded on both sizes. The final cover was constructed on a 600-mm-thick layer of silty clay placed directly on the waste. A schematic of the area where the slide occurred is shown in Fig. 2. The slide occurred as a veneer displacement along the interface between the GM and GCL, which exposed an area
* Corresponding author. Tel.: þ1 608 262 7242; fax: þ1 608 890 3718l. E-mail address: [email protected] (C.H. Benson). 0266-1144/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.geotexmem.2012.07.006
approximately 14 m long (slope direction) and 37 m wide (cross slope direction) (Fig. 3a). The surface soils (top soil and drainage/ protection layers) were bunching downslope (Fig. 3b), and cracked upslope of the exposed area (Fig. 3c). The total area where the slope was deformed is shown in Fig. 2 along with the area where the GCL was exposed. Tensile failure of the GM and the GDL were evident near the upslope edge of the exposed area, where the GM was exceptionally thin and distorted (Fig. 4a) and the ribs of the GN and the GT were torn (Fig. 4b). Test holes excavated in the subgrade soil of the exposed area showed evidence of perched water (leachate) as well as gas bubbling (Fig. 5). These observations were consistent with recent operational conditions. The landfill had been recirculating leachate vigorously in the cell prior to installation of the final cover, which had “watered out” the gas wells. Pumps for extracting leachate concurrent with gas collection were installed in the gas wells, but had been shutdown prior to the slide for repair. Consequently, gas extraction in the region was limited prior to the slide. Moreover, gas was problematic when this section of cover was constructed. During cover construction, eleven gas relief holes were intentionally placed in the GM to ensure it remained in contact with the ground surface during construction (these holes were sealed prior to placement of overlying geosynthetics and soils). Based on these observations, the failure mechanism was initially hypothesized to be a reduction in shear resistance at the interface between the GM and GCL due to elevated gas pressure. Water
C.H. Benson et al. / Geotextiles and Geomembranes 35 (2012) 100e106
Fig. 1. Schematic cross-section of final cover with composite barrier.
pressure (from perched leachate) acting on the lower side of the composite barrier was considered a potential secondary cause. This paper describes the field observations, sampling and laboratory testing, and stability analyses conducted to evaluate the hypothesized failure mechanism. 2. Sampling and inspection 2.1. Test areas Two test areas were opened south of the area where the slide occurred and near the edge of any visible distress of the cover soils.
101
Test Area 1 was opened near the top of the slope and Test Area 2 around mid slope (Fig. 2). Both test areas were approximately 4 m 4 m in cross-section. Cover soils were removed with a backhoe until the excavation was within approximately 300e400 mm of the GDL. The remaining soils were removed using hand tools. When the cover soils were removed, the thickness of each layer was measured with a steel tape, density and water content were measured with a nuclear densometer, and grab samples of the cover soils were collected for use in the laboratory. The cover soils were moist but not saturated when they were excavated, with water contents ranging between 9.7 and 13.2% for the top soil and 2.7e6.4% for the drainage/protection layer. Only films of free water were present in the GDL when it was removed. The total density of the topsoil was 1.58 Mg/m3; for the outwash sand used for the protective/drainage layer, the total density was 1.92e1.94 Mg/m3. Additional clues regarding the failure mechanism became evident as the soil remaining over the GDL was removed using hand tools. The surface of the soil began to bulge and when the surface soils were completely removed, the GM bulged 170 mm off the underlying surface due to gas pressure, despite being anchored tightly around the periphery of the test area by the cover soils (Fig. 6). A small puncture through the GDL and GM made using a utility knife released the pressure, and the odor of the emanating gas was indicative of landfill gas. The geomembrane and GCL were cut with a razor knife around the periphery of the test area so that samples could be retrieved for laboratory testing. As soon as the geosynthetics were cut, they immediately separated by approximately 30 mm (Fig. 7), indicating that the geomembrane and GCL were in tension. After the geomembrane was removed from the test area, three GCL samples were collected using the procedure described in ASTM D 6072, with PVC plates (300 mm 300 mm 5 mm) used as the supporting layer. The GCL samples were sealed in plastic sheet in essentially undisturbed condition using the procedure described in Scalia and Benson (2011). Grab samples (2 kg) of the subgrade soil were collected from the area beneath each GCL sample and stored
Fig. 2. Schematic of final cover showing area where the cover displaced (dark shading), area where GCL and subgrade soil were exposed (light shading), test areas, and gas pressure monitoring locations (circled P1eP5). Elevations shown are in feet. Scale is in meters.
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Fig. 4. Photographs from slide: (a) torn edge of geomembrane and (b) torn geonet and geotextile from the geosynthetic drainage layer.
Fig. 3. Photographs from slide: (a) exposed GCL, (b) bulging of cover soils downslope, and (c) cracks in cover soil upslope.
in sealable plastic bags. The remaining GCL was removed from the excavation. The subgrade soil was inspected after the GCL was removed. There was no evidence of saturation, free water, or seeps indicative of elevated pore water pressures. However, gas was present and was emanating from fine cracks and voids in the subgrade soil. For example, a whistling sound was audible in the test area and gas flow could be felt when a hand was placed near the cracks or voids in the subgrade soil. 2.2. Gas pressure monitoring The bulging of the GM when the cover soil was removed in the test areas, the presence of gas, and the absence of free water were strong indications that excess gas pressure was a primary factor responsible for the slide. The condition in the slide area was also consistent with this inference. Gas was bubbling vigorously in test holes, indicating that elevated gas pressure was present. Water was observed in test holes excavated in the subgrade soil, but free water
and seeps were not present at the surface of the subgrade soil. Perched or other sources of free water were not observed during the forensic examination or shortly thereafter when the cover was removed from the side slope during restoration, further confirming that water pressure was an unlikely cause of the failure. Based on these observations, excess gas pressure was considered to be the primary factor contributing to the slide. Thus, five gas probes were installed to measure gas pressures (P1eP5) so that analyses could be conducted to assess the impact of gas pressure on slope stability. Probes were driven into the waste directly beneath the subgrade soil, and the pressure was observed until a steady condition was achieved. P1 and P2 were located near the test areas, P4 and P5 were located within the area where the GCL and subgrade soil were exposed, and P3 was located at a transition area near the southern edge of the exposed area (Fig. 2). Gas pressures were nil (<1 kPa) at P4 and P5 (within exposed area). At P1 (farthest from open area, Fig. 2), the gas pressure was highest (7 kPa). At P2, the gas pressure was 4 kPa and at P3 (closest to exposed area, Fig. 2) the gas pressure was 3 kPa. The absence of measurable gas pressure in the exposed area was not unexpected. Gas pressures almost certainly dissipated in the exposed area because portions of the subgrade soil had been exposed for nearly a week and test holes had been excavated through the subgrade soil, both of which would permit gas to escape. The monotonic increase in gas pressure with increasing distance from the exposed area (i.e., from P3 to P1) is also consistent with the assumption of gas pressure relief within the exposed area. Based on these measurements, the gas pressure beneath the GM probably was at least 7 kPa when the slide occurred, and may have been higher.
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the cover soils, and the hydraulic gradient was set at 10. Tap water was used as the permeant liquid for the hydraulic conductivity tests. 3.2. Conventional direct shear tests
Fig. 5. Gas bubbling in test hole in subgrade soil of exposed area. Arrows denote bubbles.
3. Laboratory testing methods 3.1. Geosynthetic clay liner properties Tests were conducted on the GCL samples to determine the water content, hydraulic conductivity to water, and air permeability. The hydraulic conductivity and air permeability tests were conducted to assess whether gas could be transmitted through the GCL to the GMeGCL interface, thereby providing gas pressure for instability. Hydraulic conductivity tests were conducted in accordance with ASTM D 6766 and the air permeability tests with ASTM D 6539. The effective confining pressure was set at 17 kPa, approximating the average vertical overburden stress imposed by
Fig. 6. Bulging surface of GDL immediately after puncturing GDL and GM with utility knife. Survey rod shows distance geosynthetics are bulging above subgrade soil soon after puncture. Units on rod are in feet. Tip of rod sits on GCL and is at 25 ft on scale. The surface of the geocomposite drainage layer is at 24.45 ft, indicating the GM has bulged 0.55 ft, or 170 mm.
Large-scale direct shear tests were conducted on the GMeGCL interface following the procedure described in ASTM D 5321 using a 305 mm 305 mm steel shear box in a shear machine employing a stepper motor to displace the interface. A schematic of the shear machine is shown in Fig. 8. Subgrade soil from directly below the GCL was compacted in the bottom portion of the shear box to the average water content (14.7%) and dry density (1.79 Mg/ m3) measured in the field. A GCL specimen (305 mm 305 mm) was placed on the compacted subgrade soil, affixed to the shear box, and then moistened with tap water using a spray bottle to achieve the average water content measured in the test areas (77%). The GCL specimen was cut from a larger sample obtained from excess GCL stored at the landfill after construction. The surface of the moistened GCL specimen was sealed with plastic and the bentonite was allowed to hydrate for 24 h. A GM specimen of the same size was cut from the GM sample obtained from the test areas. The GM specimen was placed over the GCL (after the 24-h hydration period) and fixed to the shear box. The GM and GCL were both oriented with the shearing direction aligned with the machine direction, which is the orientation in the field. A layer of outwash sand from the field was placed in the upper box at the average water content and dry density measured during in the test areas. Normal stress was applied to the outwash sand using a loading platen surcharged with dead load. The normal stress was applied for 24 h prior to shearing, during which compression ceased. Shearing was conducted at 0.1 mm/min, which is the lowest rate at which the shear machine could be operated. Triplett and Fox (2001) show that peak and large-displacement shear strengths of GMeGCL interfaces are independent of displacement rate provided the displacement rates is within 0.01e10 mm/min. Stark et al. (1996) also show that the shear strength of interfaces between geomembranes and non-woven geotextiles is independent of displacement rate when the rate is between 0.03 and 25 mm/min. Shearing loads were measured with a load cell and vertical and horizontal displacements were measured with LVDTs. Tests were performed at normal stresses of 4.8, 9.6, 14.4, and 19.2 kPa to bracket the normal stress existing in the field (16.2 kPa) and to define any non-linearity in the shear strength envelope.
Fig. 7. Separation of geosynthetics after being cut with utility knife.
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Fig. 8. Schematic of large-scale direct shear box used to test GMeGCL interface.
Typical shear stress vs. horizontal displacement curves from the conventional direct shear tests are shown in Fig. 9. Peak shear strength was reached within 6 mm of displacement and was followed by a gradual reduction in shear strength. Curves of similar shape were obtained at all normal stresses.
system to approximate the field condition (the actual field shear stress was 4.0 kPa, but a stress this low could not be applied reliably with the apparatus). After an additional 24 h, the dead load used for the normal stress was gradually reduced (simulating an increase in gas pressure) until shear failure occurred under the applied shear stress.
3.3. Stress reduction direct shear tests 4. Results of laboratory testing Additional large-scale direct shear tests were conducted to evaluate the impact of a reduction in normal stress due to gas pressure on the GMeGCL interface. The specimen was set up as in the conventional large-scale direct shear test and consolidated to the field normal stress (16.2 kPa) using a dead weight for 24 h. A shear stress of 5.3 kPa was then applied using a pulley and hanger
20
4.1. GCL conductivity The hydraulic conductivities and air permeabilities are summarized in Table 1. The hydraulic conductivity is very low, indicating that elevated gas pressure had no detrimental impact on the hydraulic conductivity of the GCL. The air permeability is also low, but sufficient to permit gas pressures to develop at the interface between the GCL and geomembrane. 4.2. GMeGCL interface shearing behavior
Normal Stress
Shear stress (kPa)
15
Shear strength envelopes corresponding to peak and largedisplacement (50 mm) interface strengths are shown in Fig. 10. A duplicate test conducted at 10 kPa confirmed that the shear
19.2 kPa
10
9.6 kPa
Table 1 Hydraulic conductivity and air permeability of GCLs.
5
0
0
10
20
30
40
50
60
Displacement (mm) Fig. 9. Typical shear stress vs. displacement curves for conventional direct shear tests on the GMeGCL interface.
Sample
Test area
Water content (%)
Hydraulic conductivity (cm/s)
TP-1-M TP-1-E TP-1-N TP-2-M TP-2-E TP-2-S
1 1 1 2 2 2
91.7 83.6 98.6 59.1 64.9 63.8
1.5 3.2 1.5 1.1 1.3 1.6
Note: NM ¼ no measurable flow.
109 109 109 109 109 109
Air permeability (m2) 2.0 2.2 1.8 3.6 NM NM
1018 1017 1018 1018
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the method described in Thiel (1998). The equation for the factor of safety is:
FS ¼
strengths were reproducible. A peak interface friction angle (d) of 25.9 and adhesion (a) of 4.1 kPa were obtained by linear regression. For large-displacement conditions, d ¼ 23.2 and a ¼ 2.6 kPa were obtained. These strength parameters are similar to strength parameters reported by Triplett and Fox (2001) and McCartney et al. (2009) for GMeGCL interfaces, and the ratio of peak to large-displacement strength is within the range reported by Triplett and Fox (2001). Displacement of the GMeGCL interface measured in the stress reduction tests is shown in Fig. 11 (duplicate tests were conducted). Distinct shear failure at the interface occurred when the normal stress was reduced by approximately 11e13 kPa (i.e., the effective normal stress was reduced to approximately 3e5 kPa), and displacement was initiated when the normal stress was reduced by 8e9 kPa (i.e., gas pressure reduced the effective normal stress to 7e8 kPa). These reductions in normal stress are in reasonable agreement with gas pressures measured at the farthest gas probe (7 kPa at P1), especially given the pressure relief provided by the exposed area and the gradient in gas pressure evident from gas probe P3 to gas probe P1 (i.e., the gas pressure may have been higher than 7 kPa before the pressure was relieved by the slide). The normal and shear stresses at failure from the stress reduction tests are shown in Fig. 10 along with the data from the conventional direct shear tests. The stress reduction data agree with the shear strength measured in conventional direct shear at a normal stress of 4.8 kPa. Thus, an additional peak strength envelope was created by regressing through the peak strengths obtained from the conventional direct shear tests and the stress reduction tests. This envelope has a peak interface friction angle (d) of 28.0 and adhesion (a) of 3.4 kPa.
(1)
where sv is the vertical overburden pressure from the soil layers over the geomembrane, u is the gas pressure at the GMeGCL interface, and b is the slope angle from the horizontal. Eq. (1) was used in two ways. First, the FS was computed for u ¼ 0 to confirm that the slope should have been stable as constructed if the gas collection system was maintaining gas pressures at near zero gage. Second, Eq. (1) was solved for u using FS ¼ 1 to determine the gas pressure required to induce failure. Analyses were conducted with the strength parameters corresponding to peak and largedisplacement conditions and with and without adhesion assuming a linear failure envelope (any nonlinearities that may have existed at low normal stresses were not considered) Peak strength was defined using the envelope from the conventional direct shear test and the data pooled form conventional direct shear test and the stress reduction tests. Results of the analyses are summarized in Table 2. For conditions without gas pressure, the FS ranges from 1.71 to 2.98. Thus, regardless of the assumption regarding adhesion or the displacement condition used to define the shear strength, the GMeGCL interface should have been stable under static conditions if gas pressures were near zero gage. This conclusion is consistent with observations made in other cells at the landfill that have active gas systems designed to draw a vacuum, no gas collection issues, and employ the same cover profile. Some of these cells were constructed more than a decade ago and all have stable final covers. The gas pressures required to induced failure (FS ¼ 1) range from 6.5 to 16.0 kPa, with lower pressures associated with largedisplacement conditions or the absence of adhesion. These gas pressures are in the same range as the gas pressures required to induce failure indicated by the stress reduction tests (8e9 kPa to initiate failure, 11e13 kPa for appreciable displacement) and are comparable to, but slightly higher than the maximum gas pressures measured in the field (7 kPa). However, as noted previously, the gas pressures measured in the field may underestimate the gas
Simulated Gas Pressure (kPa) 60 Interface Displacement (mm)
Fig. 10. Shear strength envelopes for the GMeGCL interface along with combinations of shear and normal stress from the stress reduction tests.
a þ sv cos2 b u tan d sv cos bsin b
0
2
12
14
4 6 8 10 12 Normal Stress Reduction (kPa)
14
4
6
8
10
Original
50
Duplicate
40
Applied shear stress = 5.3 kPa Initial normal stress = 16.2 kPa
30 20 10 0
5. Stability analyses Because the displaced area consisted of a relatively long (61 m) and broad (120 m) section of slope, the analysis was conducted assuming an infinite slope with gas pressure at the interface using
0
2
Fig. 11. Displacement of GMeGCL interface as normal stress is reduced during stress reduction tests in large-scale direct shear machine. The reduction in normal stress simulates how increasing gas pressure causes a reduction in effective normal stress.
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Table 2 Factors of safety (FS) without gas pressure and gas pressure required for FS ¼ 1. For peak strength, interface friction angle and adhesion are reported for the strength envelope obtained from the conventional direct shear tests alone (left) and for the combined data from the conventional direct shear tests and the stress reduction tests (right). Interface friction angle, d ( )
Adhesion, a (kPa)
25.9, 28.0 25.9, 28.0 23.2 23.2
0 4.1, 3.4 0 2.6
Gas pressure for FS ¼ 1 (kPa)
FS for no gas pressure Direct shear envelope
Direct shear & stress reduction
Direct shear envelope
Direct shear & stress reduction
1.94 2.98 1.71 2.38
2.12 2.99 e e
7.6 16.0 6.5 12.6
8.3 14.7 e e
pressures at the time of failure due to pressure relief provided by the exposed area. These findings indicate that excess gas pressure was a key contributor to the slide. The findings from this study are also very similar to those reported in the case study by Thiel (1999), where gas pressures in a MSW landfill of the same magnitude as those observed in the current study resulted in a final cover slide on a 4:1 slope due to a reduction in shear resistance at the GMeGCL interface. The similarity of both of these studies illustrates the importance of controlling gas pressures in MSW landfills. An extensive gas recovery system was installed directly beneath the composite barrier when the cover was restored in Fall 2011. The restoration included permanent gas pressure monitoring stations (Janusz et al., 2012). 6. Summary and conclusions The study described in this paper was conducted to determine the primary mechanism contributing to the slide of a landfill cover at a MSW landfill where leachate had been recirculated vigorously. Elevated gas pressure beneath the cover was a significant contributor to the slide. Inadequate gas collection due to watering out of gas collection wells and inoperative leachate extraction pumps installed in the gas extraction wells contributed to elevated gas pressures beneath the final cover. The laboratory experiments, field observations, and stability analyses conducted in this study illustrate how these elevated landfill gas pressures resulted in a reduction in effective normal stress and ultimately slope instability due to inadequate shearing resistance between the GM and GCL. The study also illustrates that the slope would
have been stable if gas pressures had been maintained near zero gage. This study illustrates the importance of maintaining adequate gas collection or deploying means to relieve gas pressure in MSW landfills, particularly those with high gas generation rates such as landfills that recirculate leachate or operate as bioreactors. Filling strategies and recirculation rates should be selected so that the waste remains sufficiently transmissive and gas may be collected efficiently. In additional, transmissive gas collection layers should be considered to maintain gas pressures acceptably low at the base of the final cover. Careful consideration of these issues will reduce the likelihood of final cover instability and reduce fugitive emissions of landfill gas to the atmosphere. References Janusz, A., Benson, C., Edil, T., Wang, X., 2012. Gas pressures and a final cover slide: investigation and remedial measures, in: Proc. Global Waste Management Symposium 2012. New York: Penton Media on CD. McCartney, J., Zornberg, J., Swan, R., 2009. Analysis of a large database of GCL-geomembrane interface shear strength results. J. Geotech. Geoenviron. Eng. 135 (2), 209e223. Scalia, J., Benson, C., 2011. Hydraulic conductivity of geosynthetic clay liners exhumed from landfill final covers with composite barriers. J. Geotech. Geoenviron. Eng. 137 (1), 1e13. Stark, T., Williamson, T., Eid, H., 1996. HDPE geomembrane/geotextile interface shear strength. J. Geotech. Eng. 122 (3), 197e203. Thiel, R., 1998. Design methodology for a gas pressure relief layer below a landfill geomembrane cover to improve slope stability. Geosynth. Int. 5 (6), 589e617. Thiel, R., 1999, Design of gas pressure relief layer below a geomembrane cover to improve slope stability, in: Proc. Geosynthetics ’99, Industrial Fabrics Association International, St. Paul, MN, pp. 235e252. Triplett, E., Fox, P., 2001. Shear strength of HDPE geomembrane/geosynthetic clay liner interfaces. J. Geotech. Geoenviron. Eng. 127 (6), 543e552.