Investigation of White Bluffs Landslides in Washington

GeoCongress 2012 © ASCE 2012

Investigation of White Bluffs Landslides in Washington State by Christopher A. Bareither1, Tuncer B. Edil2, and Craig H. Benson3 1

Research Associate, Geological Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA, 53706, [email protected] 2 Professor, Geological Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA, [email protected] 3 Wisconsin Distinguished Professor, Director of Sustainability Research and Education, and Chair, Civil & Environmental Engineering, Geological Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA, [email protected] ABSTRACT The objective of this study was to conduct experimental and numerical investigations to evaluate potential failure mechanisms of the White Bluffs landslides. Increased landslide activity in the 1970s and 1980s has been attributed to irrigation activities behind the bluffs, which altered moisture conditions and caused seepage and piping. Soil instability is believed to originate in weakly cemented clays and silts of the Ringold Formation. The experimental analysis included triaxial compression tests on undisturbed and remolded specimens from the Ringold Formation, to evaluate anisotropy, wetting, and remolding effects. Strength anisotropy manifested an effective cohesion intercept (c') that is approximately 2.7 times larger for the vertical orientation (46 kPa) compared to the horizontal; however, similar effective stress friction angles (φ' ≈ 20-21°) were determined in both orientations. Larger φ' and c' were determined for air-dried specimens compared to saturated specimens in both orientations. A larger φ' and smaller c' were determined for the remolded saturated specimens, indicating that remolding altered the soil fabric. The numerical slope stability analyses indicate that landslide activity may be attributed to a combination of factors, i.e., strength reduction and pore pressure development with soil wetting, coupled with anisotropic shear strength. INTRODUCTION The White Bluffs are a series of 50-170 m tall bluffs, located along the north and east banks of the Columbia River in south-central Washington State (Newcomb 1958; Schuster et al. 1987). The name applies to outcropping layers of the Ringold Formation, which consist predominantly of horizontal layers of weakly cemented fluvial and lacustrine claystones, siltstones, and sandstones (Newcomb 1958; Lindsey and Gaylord 1990). Ringold Formation outcrops comprising the White Bluffs are underlain by Columbia River basalts and overlain by Quaternary sands and silts deposited in fluvial or eolian environments (Schuster et al. 1987; Bennett 1999).

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In the 1970s and 1980s an increase in landslide activity in the White Bluffs was attributed to anthropogenic-induced changes to groundwater conditions. An increase in the groundwater table and occurrence of perched water tables and seepage at the Ringold-Quaternary contact was linked to irrigation activity behind the bluffs (Schuster et al. 1987). Enhanced irrigation management in response to the increase in landslide activity, caused activity to eventually decline. However, the landslide complex still contains areas of saturated soils, perched water tables, fluctuating water tables, and irrigation ponds that pose risks of future landslide activity (Triangle Associates 2003). Future landslides have the potential to damage farmland and alter riverbank conditions, which can redirect river flow patterns, accelerate erosion, and impact aquatic wildlife habitat (Chugh and Schuster 2003; Bjornstad 2006). Landslides in the White Bluffs commonly initiate at the front of steep faces as earth slumps or slides, and subsequently disaggregate into avalanches and chaotic debris flows. Fluvial and lacustrine sediments of the Ringold Formation that constitute the White Bluffs are described as having high strength when dry and considerable strength loss when wetted based on visual observations (Schuster et al. 1987). Chugh and Schuster (2003) identified the Ringold Formation as a potential source of instability that triggered landslide activity. They conducted numerical analyses on unfailed slope geometries with groundwater conditions representing current conditions and conditions during enhanced landslide activity. Historic groundwater conditions included a fully saturated Quaternary layer above the Ringold Formation with seepage at the ground surface; these historic conditions yielded factors of safety close to 1.0, whereas current groundwater conditions yielded factors of safety ≥ 1.4. The objective of this study was to conduct an experimental and numerical analysis of the White Bluffs landslides. The experimental program aimed at quantifying undisturbed and remolded strength properties of a weakly cemented soil from the Ringold Formation in both saturated and air-dried conditions. Multi-stage triaxial compression tests were conducted on undisturbed specimens and traditional singlepoint triaxial compression tests were conducted on remolded specimens. Numerical analyses were conducted in WinSTABL to investigate potential failure modes with emphasis on two triggering mechanisms: (i) soil wetting and (2) pore pressure development. The combination of the experimental and numerical analyses provides insight into soil moisture conditions that are critical to slope stability. MATERIALS AND METHODS Materials. An intact block sample from the Ringold Formation was collected to evaluate soil characteristics and strength properties. Borings and undisturbed sampling operations were not feasible due to limitations on equipment accessibility. Particle size analysis (ASTM D 422), specific gravity (ASTM D 854), and Atterberg limits (ASTM D 4318) were determined. The soil contained 100% fine-grained particles (< 0.075 mm), had a liquid limit = 39, plasticity index = 13, specific gravity = 2.72, and was classified as low plasticity silt (ML) in accordance with the Unified

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Soil Classification System (ASTM D 2487). No visible moisture was present in the soil sample when collected. Undisturbed specimens for triaxial compression tests were carved from the intact block sample in two planes orthogonal to one another. These specimens are referred to as “vertical” and “horizontal” in this paper, with direction oriented with the long axis of the specimen. The vertical orientation was assumed coincident with a face of the block sample where small animal borings and root growth were parallel to the face. Specimen orientations are further described in regards to strength behavior. Methods. Consolidated-undrained (CU) and consolidated-drained (CD) triaxial compression tests were conducted following ASTM D 4767 and D 7181, respectively, in stress ranges relevant to field conditions. Cell pressure and backpressure for soil saturation were controlled via a pressure panel and measured with pressure transducers (1380 ± 1.4 kPa). Axial force was applied with a synchronous motor-driven screw jack and measured with a load cell (8.92 ± 0.004 kN). Vertical displacement was measured with a linear variable displacement transducer (76.2 ± 0.01 mm). All tests were conducted at an axial strain rate of 0.015 %/min, determined in accordance with ASTM D 4767, to allow pore pressure equilibration within the saturated specimens. All tests were controlled with a PC and commercially available software. Multi-stage triaxial compression tests were conducted on all undisturbed specimens. Three undisturbed specimens were obtained in both vertical and horizontal orientations; two specimens were tested saturated and one was tested air-dried. The air-dried specimens were tested at four effective confining pressures ranging between 60 and 500 kPa. Each saturated specimen was consolidated and sheared at three effective consolidation pressures ranging between 50 and 420 kPa. Specimens were saturated by circulating water from the bottom up, followed by backpressure saturation until a B-parameter ≥ 0.95 was achieved. Each stage of a given multi-stage triaxial compression test was conducted until the slope of the deviator stress (i.e., principal stress difference) vs. axial strain relationship reached zero. For air-dried specimens, axial displacement was paused at this point, the confining pressure increased, and then axial displacement was restarted. For saturated specimens, the experiment was stopped and the axial piston locked when the slope of deviator stress vs. axial strain reached zero. The confining pressure was increased and pore pressure was allowed to dissipate to consolidate the specimen. Following 24 hrs to allow for complete primary consolidation, the subsequent stage of the multi-stage test was initiated. All saturated and air-dried specimens were sheared past failure when the final confining pressure was applied. Traditional CU and CD triaxial compression tests were conducted on remolded specimens under saturated and air-dried conditions. Remolded specimens were saturated by circulating water from the bottom up, followed by backpressure saturation until a B-parameter ≥ 0.95 was achieved. Saturation required

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approximately 2-3 d. All remolded specimens were compacted in six layers to a target dry unit weight (γd) representative of the undisturbed specimens (γd = 14.2 kN/m3) and water content (w) = 31%. The average γd of the remolded saturated specimens was 14.8 kN/m3 and the average γd of the remolded air-dried specimens was 17.3 kN/m3. The higher γd for the air-dried specimens is due to shrinkage that occurred as specimens were allowed to air-dry. EXPERIMENTAL RESULTS Undisturbed Specimens. Relationships of deviator stress vs. axial strain from multistage triaxial compression tests conducted on undisturbed saturated and air-dried specimens in both vertical and horizontal orientations are shown in Fig. 1. Stressstrain behavior for the saturated specimens (Fig. 1a) indicate that for a given effective confining pressure (σ'c) the vertical specimens are stronger (i.e., support a larger axial load) than the horizontal specimens. A similar strength increase is also observed for the air-dried vertical specimen compared to the horizontal specimen at comparable σ'c (Fig. 1b). Additionally, both the saturated and air-dried vertical specimens display a larger post-peak stress reduction compared to the horizontal specimens. The stress-strain behavior displayed in Fig. 1 is in agreement with general anisotropic behavior of fine-grained soils (Mitchell and Soga 2005). In natural depositional environments, non-spherical particles are typically oriented with their long axis perpendicular to gravity. Soils tend to be stiffer, with greater dilation potential when compression is applied in this preferred direction (i.e., gravity-oriented). The vertically oriented specimens in this study displayed higher strengths (Fig. 1) and also a greater propensity to dilate (excess pore pressure data not shown). This behavior suggests that the assumed vertical and horizontal orientations are reasonable. 2000

400 (a)

Deviator Stress (kPa)

Deviator Stress (kPa)

σ'c3

200 σ'c2 σ'c1

σ'c 1 2 3

100

0

Solid Line - Vertical Orientation Dashed Line - Horizontal Orientation

0

2

Vertical 152 269 422

4 6 Axial Strain (%)

Horizontal 148 270 389 8

σ'c3 σ'c2 1000

Solid Line - Vertical Orientation Dashed Line - Horizontal Orientation

σ'c1

500

10

Air-Dried

σ'c3

1500

300 σ'c2

σ'c4

(b)

Saturated

0

0

2

σ'c 1 2 3 4

Vertical 59 150 271 353

4 6 Axial Strain (%)

Horizontal 149 271 360 501 8

10

Fig. 1. Relationships of deviator stress vs. axial strain for multi-stage triaxial compression tests on undisturbed (a) saturated and (b) air-dried specimens. Note: σ'c = effective confining pressure.


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Strength envelopes for undisturbed saturated and air-dried specimens in both vertical and horizontal orientations are shown in Fig. 2. q-p' failure points for the two sets of saturated specimens, representative of vertical and horizontal orientations, overlap to form unique failure envelopes (Fig. 2a). Analysis of covariance conducted at the 5% significance level to evaluate similarity of the regression lines (i.e, tanψ) for each set of saturated specimens yielded p-statistics ≥ 0.77. This analysis indicates failure envelopes for a given set of specimens were not statistically different and suggests the multi-stage triaxial testing methodology was repeatable. Comparing failure envelopes of the vertical and horizontal specimens in both saturated and air-dried conditions suggests that anisotropic effects are dependent on moisture conditions. 1200

200 Vertical - No. 1 Vertical - No. 2 Horizontal - No. 1 Horizontal - No. 2

150

Vertical Horizontal 900

(b)

q (kPa)

q (kPa)

(a) 100

50

600

300 Air-Dried

Saturated 0 0

100

200 p' (kPa)

300

400

0 0

300

600 900 p (kPa)

1200

1500

Fig. 2. Strength envelopes for (a) saturated and (b) air-dried undisturbed specimens. Two saturated specimens (No. 1 and No. 2) were tested for both vertical and horizontal specimen orientations. A summary of the strength parameters for all undisturbed specimens tested as part of this study is given in Table 1. The effective friction angle (φ') is nearly identical for both vertical (19.8°) and horizontal (21.0°) saturated specimens. However, as evident by the offset between the failure envelopes (Fig. 2a), a larger cohesion intercept (c') was measured for the vertical (46.0 kPa) compared to horizontal (16.7 kPa) specimens. Larger φ' and c' were determined for air-dried specimens compared to saturated specimens (Table 1) in both vertical and horizontal orientations. These strength properties support the observation in Schuster et al. (1987) that soils comprising the Ringold Formation have considerable strength loss when wetted. The larger c' but lower φ' for the air-dried horizontal specimen compared to the vertical specimen is perhaps an artifact of effective stress effects on anisotropic behavior. Both undisturbed air-dried specimens contained approximately 3% moisture (determined after testing). This residual moisture translates to a high matric suction and is capable of inducing additional effective stress on the specimens. Further development into this uncertainty is beyond the scope of this paper. The


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effective stress parameters determined from the saturated specimens and the total stress parameters for the air-dried specimens, both in the vertical orientation, are used subsequently in the numerical analysis. Table 1. Strength parameters for undisturbed and remolded specimens. Cohesion Failure Specimen Moisture Stress Friction Intercept Envelope Description Conditions Parameters Angle (°) (kPa) R2 Total 11.3 23.0 0.973 Undisturbed Saturated vertical Effective 19.8 46.0 0.983 orientation Air-dry Total 32.9 274 0.989 a Total 9.5 0.0 0.971 Undisturbed Saturated Effective 21.0 16.7 0.999 horizontal orientation Air-dry Total 22.7 368 0.999 Totala 15.0 0.0 0.911 Saturated Remolded Effective 31.8 2.5 0.998 Air-dry Total 35.5 597 0.977 a

Failure envelope forced through origin

Remolded Specimens. Effective stress paths for the remolded saturated specimens are shown in Fig. 3. The effective stress failure envelope regressed through all q-p' data falling along the failure envelope was used to determine strength parameters in Table 1. The effective stress failure envelope in Fig. 3 corresponds to φ' = 31.8° and c' = 2.5 kPa. Thus, remolding the silt obtained from the Ringold Formation reduces c' to essentially zero, but increases φ' perhaps due to the modestly higher dry unit weight and altered fabric of the remolded specimens. The smaller c' is likely caused by destroying the lightly cemented particle fabric of the undisturbed specimens. 300 Effective stress failure envelope

q (kPa)

200

53 kPa 138 kPa 257 kPa 346 kPa

100

0 0

Remolded & Saturated Specimens 100

200

300 p' (kPa)

400

500

600

Fig. 3. Effective stress paths for remolded saturated specimens.


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The remolded air-dried specimens displayed a considerable strength gain when tested, resulting in φ' = 35.5° and c' = 597 kPa. This pronounced increase in c' is attributed to shrinkage and possible unintended cementation that occurred during air-drying. Shrinkage caused a 22% increase in γd relative to the undisturbed specimens, whereas cementation may have transpired during the week-long drying period. φ' for the airdried and saturated remolded specimens are comparable and likely representative of the frictional strength of the remolded silt. The larger φ' for the air-dried specimen is attributed to the higher γd. However, c' of the air-dried remolded specimen is believed not representative. NUMERICAL ANALYSIS Geologic and Hydrologic Conditions. A cross-section of a slope in the White Bluffs that failed during the heightened landslide activity in the 1980s is shown in Fig. 4. The pre-failed slope profile and inferred slope failure surface are reproduced from geologic field reconnaissance reported in Schuster et al. (1989). Stratigraphy and groundwater locations are extrapolated from nearby boreholes and geologic cross-sections reported in Bennett (1999). A 7° slope for the Columbia River bed was assumed to extend the ground surface into the river channel. The Columbia River Basalts underlie sediments from the Ringold Formation (Fig. 4); however, the location of the basalts in this area was not reported.

Elevation above mean sea level (m)

220 Inferred Failure Surface

200

Unit 1

Perched GWT

180 160 Unit 2 140 120

Columbia River Unit 3

100 80 -100

Main GWT 0

100 200 Distance from river edge (m)

300

400

Fig. 4. Cross-section of a slope in the White Bluffs that failed from irrigation activity in the 1980s (Schuster et al. 1989). Stratigraphy and groundwater tables (GWT) are extrapolated from Bennett (1999). Notes: Unit 1 = Quaternary glaciofluvial sediments, Unit 2 = Ringold Formation clay and silt, and Unit 3 = Ringold Formation differentially cemented sands.


Stability Analysis. Numerical analysis for identifying potential triggering mechanisms of the White Bluffs landslides was executed in WinSTABL (University of Wisconsin-Madison, Madison, WI). All analyses were conducted via limit equilibrium using a Modified-Bishop method. The triggering mechanisms analyzed were (1) reduced strength of the Ringold Formation clays and silts upon wetting (Unit 2 in Fig. 4) and (2) pore pressure development due to a changing groundwater table (GWT). The following assumptions were incorporated in the analyses: (i) drained strength properties are appropriate for all layers and are not stress dependent and (ii) air-dried and saturated strength properties measured on undisturbed vertical specimens (Table 1) are applicable to Ringold Formation clays and silts (Unit 2 in Fig. 4). Location of the main and perched GWTs (Fig. 4) were extrapolated from groundwater levels reported in Bennett (1999). Elevation of the perched GWT was increased 5 m to reflect the conditions during intensive irrigation compared to the pizometric potential in the glaciofluvial layer (Unit 1 in Fig. 4) following mitigation of irrigation activity in the 1980s (Bennett 1999). A summary of the stability cases analyzed is presented in Table 2. Factors of safety (FS) in Table 2 are representative of failure surfaces approximately coinciding with the inferred failure surface in Fig. 4. Case 1 represents slope conditions prior to irrigation activity; no perched GWT was assumed present and strength was related to air-dried parameters. Case 2 represents the onset of irrigation activities; a perched GWT forms within the more permeable glaciofluvial sediments (Unit 1) overlying less permeable silts and clays of the Ringold Formation (Unit 2), but air-dried strength parameters were assumed still applicable. FS ≈ 2.1 for Case 1 and Case 2, indicating that the presence of a perched GWT does not affect slope stability prior to groundwater permeation into Unit 2 and subsequent strength reduction with wetting. Table 2. Summary of slope stability cases analyzed in the numerical analysis. Case Groundwater Conditions Unit 2 Strength Conditions Factor of Safety 1 Only main GWT present Air-dried parameters ≈ 2.1 2 Main & perched GWT Air-dried parameters ≈ 2.1 Saturated & air-dried 3 Main & perched GWT parameters; interface at ≈ 1.8 elevation = 140 m 4 Main & perched GWT Saturated parameters ≈ 1.1 5 Single full GWT Saturated parameters ≈ 0.7 6 Single full GWT Air-dried parameters ≈ 1.1 Cases 3 represents a conservative wetting scenario as water permeates into Unit 2 from the perched GWT. As the wetting front permeates downward, strength properties of Unit 2 behind the wetting front are assumed reduced to saturated conditions with pore pressures developed. Soil in front of the wetting front is assumed to retain air-dried strength properties, a conservative estimate since the soil will likely contain higher water content. As the wetting front progresses downward through Unit 2, full saturation will eventually be achieved, reducing strength and

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increasing pore pressures throughout the layer (Case 4 in Table 2). FS for Case 3 ≈ 1.8 and demonstrates the decrease in slope stability as the wetting front propagates through the clays and silts of the Ringold Formation. FS ≈ 1.1 for Case 4 indicates that the slope is near critical conditions and can potentially fail when Unit 2 is fully saturated. However, the wetting scenario described for these cases can potentially lead to a slope failure prior to complete saturation (FS ≈ 0.9 for Case 3), if a failure surface is allowed to daylight within the saturated zone. The effect of anisotropy was investigated for Cases 3 and 4 (Table 2) by incorporating saturated strength properties of the horizontal-oriented specimen (Table 1). Vertical strength parameters were assumed applicable for failure surfaces oriented from 90° to 45° and horizontal strength parameters applicable for failure surfaces oriented from 45° to 0° (0° = horizontal plane). Accounting for anisotropy, saturated strength for Unit 2 is reduced in the horizontal compared to vertical orientation, which leads to FS for Case 3 ≈ 1.7 and FS for Case 4 ≈ 0.9. Thus, factoring anisotropy into the stability analysis reduces FS and should be included to yield an appropriate FS. The final two cases in Table 3 (Cases 5 and 6) represent hypothetical scenarios incorporating a fully saturated slope (i.e., single full GWT connecting the river to the perched GWT). Using saturated strength parameters for Unit 2 in this analysis yields FS ≈ 0.7. However, incorporating air-dried strength parameters (Case 6) increases FS to 1.1. Case 5 indicates that with continued downward movement of the wetting front the slope will ultimately fail. Case 6 indicates that if Unit 2 were to retain air-dried strength after wetting, the presence of pore pressures alone are not enough to induce failure. Thus, landslides in the White Bluffs likely occur due to a combination of reduced shear strength upon wetting and increased pore pressures with soil saturation. CONCLUSIONS The following conclusions are drawn from the study: • Shear strength parameters of lightly cemented silt from the Ringold Formation are reduced when wetted (i.e., from air-dried to saturated states). • Anisotropic strength behavior was identified in both undisturbed air-dried and saturated specimens. φ' was similar (≈ 20-21°) in both vertical and horizontal orientations, but c' was approximately 2.7 times larger for the vertical orientation (46 kPa) compared to the horizontal (17 kPa). • Although a larger φ' (32°) was determined for remolded saturated specimens, c' = 2.5 kPa, indicating that remolding negated cementation effects on c'. • Numerical analyses of a failed-slope in the White Bluffs suggest that a combination of strength reduction due to soil wetting and pore pressure development resulting from irrigation, coupled with anisotropic shear strength, contributed to slope failure. • A stability analysis assuming a completely saturated slope with full pore pressure development and either air-dried or saturated strength parameters for the Ringold Formation silts and clays yielded a stable slope with air-dried

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parameters and failed slope with saturated parameters. Thus, pore pressures alone were not likely to fail the slope and a reduction in strength parameters has likely occurred. ACKNOWLEDGEMENT This material is based on work supported by the U. S. Department of Energy, under Cooperative Agreement Number DE-FC01-06EW07053 entitled ‘The Consortium for Risk Evaluation with Stakeholder Participation III’ awarded to Vanderbilt University. The opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily represent the views of the Department of Energy or Vanderbilt University. REFERENCES Bennett, D. J. (1999). Locke Island Landslide Study, Phase I, White Bluffs Area, Columbia Basin Project, Washington, U.S. Bureau of Reclamation, Pacific Northwest Regional Office, Boise, Idaho. Bjornstad, B. N. (2006). Past, present, future erosion at Locke Island, PNNL-15941, Pacific Northwest National Laboratory, Richland, Washington. Chugh, A. K. and Schuster, R. L. (2003). Numerical assessment of Locke Island landslide, Columbia River Valley, Washington State, USA, in Culligan, P. J., Einstein, H. H., and Whittle, A. J., eds., Soil and Rock America 2003, Proc. 12th Panamerican Conf. on Soil Mechanics and Geotechnical Engineering, 24892496. Lindsey, K. A. and Gaylord, D. R. (1990). Lithofacies and sedimentology of the Miocene-Pliocene Ringold Formation, Hanford Site, south-central Washington, Northwest Science, 64(3), 165-180. Mitchell, J. K. and Soga, K. (2005). Fundamentals of Soil Behavior, 3rd Edition, John Wiley & Sons, Inc., Hoboken, New Jersey. Newcomb, R. C. (1958). Ringold Formation of Pleistocene age in type locality, the White Bluffs, Washington, American Journal of Science, 256, 328-340. Schuster, R. L., Chleborad, A. F., and Hays, W. H., (1987). Irrigation-induced landslides in fluvial-lacustrine sediments, south central Washington State, 5th Int. Conf. and Field Workshop on Landslides, Christchurch, New Zealand, 147-156. Schuster R. L., Chleborad, A. F., and Hays, W. H. (1989). The White Bluff landslides, south-central Washington, in Glaster, R. W. ed., Engineering Geology of Washington, Washington Division of Geology and Earth Resources, Olympia, Bulletin 78, Vol. II, 911-920. Triangle Associates, Inc. (2003). White Bluffs Landslides: Assessment Report, prepared under contract to the U.S. Institute for Environmental Conflict Resolution and the U.S. Fish and Wildlife Service for the Hanford Reach National Monument, Seattle, Washington.

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GEOCONGRESS 2012 STATE OF THE ART AND PRACTICE IN GEOTECHNICAL ENGINEERING March 25-29, 2012 Oakland, California

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Published by the American Society of Civil Engineers