Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California Table of Contents

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5 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California Table of Contents 1.0 INTRODUCTION BACKGROUND SUPPLEMENTAL GEOTECHNICAL INVESTIGATION REVIEW OF EXISTING INFORMATION FIELD EXPLORATION PROGRAM Subsurface Investigation for This Study LABORATORY TESTING GROUNDWATER CONDITIONS DAM MATERIAL PROPERTIES GENERAL CLASSIFICATION AND INDEX TESTS LIQUEFACTION POTENTIAL/LOSS OF SHEAR STRENGTH SHEAR STRENGTH Effective Shear Strength Failure Envelope Total Shear Strength Failure Envelope Anisotropic Shear Strength Failure Envelope SLOPE STABILITY ANALYSES CONCLUSION REFERENCES Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam - i - November 2008

6 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California Tables Table 1 Laboratory Testing Schedule Table 2 Recorded Piezometer Readings from January 2005 to July 2008 Table 3 Average Values of the Index Properties Table 4 Summary of Shear Strength Parameters Recommended for Slope Stability Analyses Table 5 Summary of Isotropic and Anisotropic Consolidated Undrained Triaxial Compression (TXCU-I and TXCU-A) Test Results Plate 1 Plate 2 Plate 3 Plate 4a Plate 4b Plate 4c Plate 4d Plate 5a Plate 5b Plate 5c Plate 5d Plate 6a Plate 6b Plate 6c Plate 6d Plate 7a Plate 7b Plate 7c Plate 7d Plate 8a Plate 8b Plate 8c Plate 8d Plate 9a Plate 9b Plate 9c Plate 9d Plate 10 Plates Site Location Map & Plan CPT and Boring Locations Plan Generalized Soil Profile across the Center of the Embankment Dam (Cross-Section A-A ) Moisture Content versus Elevation: Zone 1 Core Total Density versus Elevation: Zone 1 Core Void Ratio versus Elevation: Zone 1 Core Saturation versus Elevation: Zone 1 Core Moisture Content versus Elevation: Zone 2 D/S Shell Total Density versus Elevation: Zone 2 D/S Shell Void Ratio versus Elevation: Zone 2 D/S Shell Saturation versus Elevation: Zone 2 D/S Shell Moisture Content versus Elevation: Zone 3 U/S Shell Total Density versus Elevation: Zone 3 U/S Shell Void Ratio versus Elevation: Zone 3 U/S Shell Saturation versus Elevation: Zone 3 U/S Shell Moisture Content versus Elevation: Zone 4 Alluvium below D/S Shell Total Density versus Elevation: Zone 4 Alluvium below D/S Shell Void Ratio versus Elevation: Zone 4 Alluvium below D/S Shell Saturation versus Elevation: Zone 4 Alluvium below D/S Shell Moisture Content versus Elevation: Zone 4.5 Alluvium at D/S Toe Total Density versus Elevation: Zone 4.5 Alluvium at D/S Toe Void Ratio versus Elevation: Zone 4.5 Alluvium at D/S Toe Saturation versus Elevation: Zone 4.5 Alluvium at D/S Toe Moisture Content versus Elevation: Zone 5 Alluvium below U/S Shell and Core Total Density versus Elevation: Zone 5 Alluvium below U/S Shell and Core Void Ratio versus Elevation: Zone 5 Alluvium below U/S Shell and Core Saturation versus Elevation: Zone 5 Alluvium below U/S Shell and Core Atterberg Limits of Cohesive Soils Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam - ii - November 2008

7 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California Plates Plate 11 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 1 Core Plate 12 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 2 D/S Shell Plate 13 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 3 U/S Shell Plate 14 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 4 Alluvium below D/S Shell Plate 15 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 4.5 Alluvium at D/S Toe Plate 16 Total and Effective p-q Plots of All Combined TXCU-I Tests: Zone 5 Alluvium below U/S Shell and Core Plate 17 Effective Stress Strength Envelopes: Zone 1 Core Plate 18 Effective Stress Strength Envelopes: Zone 2 D/S Shell Plate 19 Effective Stress Strength Envelopes: Zone 3 U/S Shell Plate 20 Effective Stress Strength Envelopes: Zone 4 Alluvium below D/S Shell Plate 21 Effective Stress Strength Envelopes: Zone 4.5 Alluvium at D/S Toe Plate 22 Effective Stress Strength Envelopes: Zone 5 Alluvium below U/S Shell and Core Plate 23 Total Stress Strength Envelopes: Zone 1 Core Plate 24 Total Stress Strength Envelopes: Zone 2 D/S Shell Plate 25 Total Stress Strength Envelopes: Zone 3 U/S Shell Plate 26 Total Stress Strength Envelopes: Zone 4 Alluvium below D/S Shell Plate 27 Total Stress Strength Envelopes: Zone 4.5 Alluvium at D/S Toe Plate 28 Total Stress Strength Envelopes: Zone 5 Alluvium below U/S Shell and Core Plate 29 Comparison of TXCU-I and TXCU-A Test Results from Soil Sample SS- 40-PT-24 Plate 30 Comparison of TXCU-I and TXCU-A Test Results from Soil Sample SS- 40-PT-26 Plate 31 Comparison of TXCU-I and TXCU-A Test Results from Soil Sample SS- 41-SH-10 and SS-41-SH-12 Plate 32 Comparison of TXCU-I and TXCU-A Test Results for Alluvial Deposits Zone 4 Plate 33 Comparison of TXCU-I and TXCU-A Test Results for Alluvial Deposits Zone 4.5 Plate 34 Comparison of TXCU-I and TXCU-A Test Results for Alluvial Deposits Zone 5 Plate 35 Long-Term Static Stability of the Downstream Slope Plate 36 Short-Term Seismic Stability of the Downstream Slope Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam - iii - November 2008

8 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California APPENDIX A APPENDIX B APPENDIX C Appendices Logs of Cone Penetration Tests Logs of Borings and Piezometer Installation Reports Laboratory Testing Results Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam - iv - November 2008

9 1.0 Introduction 1.1 Background Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam Lafayette, California This report presents the results of supplemental geotechnical exploration and laboratory testing performed for the Lafayette Reservoir Dam located in Lafayette, California (see Plate 1, Site Location Map & Plan). The objective of this study is to confirm the material properties assumed in the previous slope stability analyses by GEI Consultants (2005). The results of the investigation will be used to evaluate the performance of the reservoir dam during the Maximum Credible Earthquake (MCE) on the controlling seismic source, and compare the results with that by GEI Consultants (2005). 2.0 Supplemental Geotechnical Investigation The supplemental subsurface investigation program consisted of Cone Penetration Tests (CPTs), exploratory borings, and installation of 3 new piezometers (East Bay Municipal Utility District (EBMUD), 2006). The laboratory testing program consisted of index and strength tests of the subsurface materials. 2.1 Review of Existing Information Prior to the commencement of the field exploration program, we reviewed available geotechnical data from the following reports and documents: Review of Stability of Lafayette Dam, Report submitted to East Bay Municipal Utility District, Oakland, California, Prepared by Shannon and Wilson, Inc., Burlingame, California, January Seismic Stability Evaluation, Lafayette Dam, Contra Costa County, California, Report submitted to East Bay Municipal Utility District, Oakland, California, Prepared by W.A. Wahler and Associates, Inc., Palo Alto, California, May Dynamic Stability Review of Lafayette Dam, Report and Appendices (two volumes) submitted to East Bay Municipal Utility District, Oakland, California, Prepared by GEI Consultants, Oakland, California, August 16, Field Exploration Program Subsurface Investigation for This Study Six CPTs (SS-34 through SS-39) were performed on October 17 and 18, 2006; by Fugro Geosciences, Inc. of Santa Fe Springs, California. All CPTs were performed, using a truck- Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

10 mounted CPT unit with pushing equipment, by pushing the cone to depth until high refusal was encountered. SS-34, which is located near the center of the dam crest next to the edge on the downstream side, was pushed to a depth of feet. SS-35, SS-36, SS-37 and SS-39, which are located a few feet downstream of and parallel to the drainage ditch between the 4H:1V and 8H:1V downstream slopes, were pushed to depths of 100.5, 97, 111 and 73 feet, respectively. SS-38, which is located near the downstream embankment toe, was pushed to a depth of 83 feet. Locations of the CPTs are shown in Plate 2 and the results are included in Appendix A. As shown in Plate 2, SS-34 and SS-37, which are located along the center line of the embankment dam, and SS-35, which is located next to the drainage ditch and between the embankment center line and the left abutment, were performed to correlate the materials encountered in the borings to be located adjacent to these CPT locations. CPTs SS-36, SS-38 and SS-39 are located close to the estimated boundary of the construction failure area in 1928, and were performed to verify or better estimate the extent of the failure area and the depth of the slip zone where soft materials are suspected to exist. In addition, the CPTs were performed before the drilling program started and the results were used as a reference for planning the sampling type and frequency for each borehole to be drilled. Four borings were drilled in December 2006 for this study. All borings, SS-40, SS-41, SS-42 and SS-43 were drilled on the downstream side of the embankment dam to a depth of feet, feet, feet and 86.4 feet, respectively. Locations of the borings are shown in Plate 2 and the logs of the borings are included in Appendix B. Boring SS-40, SS-41 and SS-42 were drilled along the center line of the embankment dam with SS-40 on the dam crest next to the edge on the downstream side, SS-41 in the middle of the 4H:1V middle slope and SS-42 a few feet downstream of the drainage ditch at the top of the 8H:1V bottom slope. Boring SS-43 was drilled next to the drainage ditch at the top of the 8H:1V bottom slope and between the embankment center line and the left abutment. Each hole was augered to about 5 feet without drilling fluid and completed using a 5-inch rotary drill bit with bentonite drilling fluid. Boring SS-40 was drilled by Pitcher Drilling Company of Palo Alto, California using a truck-mounted Failing 1500 drill rig, whereas SS-41 through SS-43 were drilled by Gregg Drilling and Testing, Inc. of Martinez, California using a track-mounted CME-850 drill rig. The subsurface materials were logged in the field by our engineer and were reviewed by the project manager. Samples for visual classification and laboratory index testing were obtained using a Modified California (MC) split-barrel sampler (3-inch outside diameter and 2.44-inch inside diameter) and a Standard Penetration Test (SPT) (2-inch outside diameter and inch inside diameter) split-barrel sampler. The split-barrel samplers were used in accordance with the American Society for Testing Materials (ASTM) Standard D The split-barrel sampler was driven 18 inches into the subsurface materials with a 140-pound automatic hammer falling 30 inches. The penetration resistance (blow counts), measured in number of hammer blows to advance the sampler the final 12 inches (or a fraction thereof) of the 18-inch drive were added and shown on the boring logs at the appropriate sample depth. Relatively undisturbed samples were obtained by using 30-inch long thin-walled Shelby (SH) tubes and 36-inch long thin-walled Pitcher (PT) tubes. Both thin-walled tubes, which are made of steel and are 3-inch outside diameter with inch wall thickness, were used in accordance with the ASTM Standard D Depending upon the consistency of the subsurface materials, Shelby tube samples were obtained by pushing a 30-inch long thin-walled tube with steadily Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

11 applied hydraulic pressure when soft materials were encountered, whereas Picher tube samples were obtained by rotary drilling a pitcher-barrel sampler with a 36-inch long thin-walled tube clamped inside when stiff materials were encountered. After careful withdrawal from the ground, the samples were visually inspected and logged. Selected samples obtained with the SPT sampler were kept in labeled zip-lock plastic bags, and selected brass-tube samples obtained with the MC sampler were capped, taped, labeled, and transported in a padded box. The thin-walled tube samples were carefully placed upright, sealed with plastic caps, taped and cleaned of disturbed soil at the ends of each tube. Field data and laboratory test results were used to classify the subsurface materials and to prepare boring logs that are presented in Appendix B. Description and identification of soils in the boring logs are based on visual classification as specified in ASTM Standard D 2488 and on laboratory test results of the soils based on the Unified Soil Classification System as specified in ASTM Standards D The boring logs also include an estimate of the relative density of cohesionless soils or consistency of fine-grained soils based on the blow counts; color, plasticity and moisture content. The laboratory test results for density, moisture content, Atterberg limits and fines content are also included on the boring logs at the appropriate sample depth. At the completion of each borehole, boreholes SS-40 and SS-42 were backfilled with cement grout, whereas borehole SS-41 was installed with 2 piezometers and SS-43 was installed with 1 piezometer as shown in the Piezometer Installation Reports in Appendix B. 2.3 Laboratory Testing Soil and rock samples obtained from the subsurface investigation were carefully packaged and sealed to prevent disturbance and to reduce moisture loss. The soil and rock samples were carefully inspected and reviewed for verification of the field classification by an engineer from the Materials Engineering Section before representative samples from the various strata were selected for laboratory testing. Appropriate tests were selected to assist in subsequent evaluation of material properties for use in the slope stability evaluation. The tests performed are listed in Table 1 along with their ASTM designations. Index tests were routinely performed on samples to accurately classify the soil types and also on samples that were tested for shear strength determination. All of the index tests and most of the isotropically consolidated undrained (ICU) triaxial compression tests (with pore pressure measurements) (or TXCU-I) were performed by the District s Materials Testing Laboratory. Some ICU and all anisotropically consolidated undrained (ACU) triaxial compression tests (with pore pressure measurements) (or TXCU-A) were performed by URS Laboratory in Pleasant Hill, California. The ICU and ACU tests that were performed by URS Laboratory were for comparison and correlation of the results for the same material type. All triaxial compression tests were performed on undisturbed thin-walled soil samples. Consolidation or confining pressures for the tests were selected based on the estimated overburden pressure at each sample depth. ACU tests were performed based on anisotropic consolidation stress ratio range of 1.3 to 1.4. Failure criterion for all triaxial compression tests were based on the maximum ratio of effective horizontal stress to effective vertical stress, 1 '/ 3 '. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

12 Test Description In-situ moisture-density Sieve analysis Atterberg limits Specific gravity Isotropically consolidated undrained (ICU) triaxial compression test with pore pressure measurements Anisotropically consolidated undrained (ACU) triaxial compression test with pore pressure measurements Table 1: Laboratory Testing Schedule Test Type Index Strength ASTM Designation ASTM D 2216, D 2937 ASTM D 422 ASTM D 4328 ASTM D 854 ASTM D 4767 ASTM D 4767 (Modified) Results on Table C-1, C-2 Appendix C Table C-1, C-2 Appendix C Table C-1, C-2 Appendix C Table C-3 Appendix C Table C-4 thru C-7 Appendix C Table C-7 Appendix C The laboratory tests were performed in general accordance with the ASTM (2003) standards as listed in Table 1 above. The overall index test results together with that from previous studies (Shannon & Wilson, 1966 and Wahler & Associates, 1976) are summarized in Table C-1, and the index properties for different soil types (or soil zones as originally defined by Shannon & Wilson, 1966 and later refined by DSOD as discussed in GEI, 2005 report) are summarized in Table C-2 in Appendix C. Specific gravities for different soil types are summarized in Table C- 3. It should be noted that the void ratios and degrees of saturation in Tables C-1 and C-2 were computed based on the specific gravity for the soil type as summarized in Table C-3. Atterberg limits of all cohesive soils are plotted on Plate C-1 in Appendix C. The results of ICU Triaxial compression tests performed for the Shannon and Wilson (1966) study and for Wahler and Associates (1976) study are summarized in Table C-4 and C-5, respectively. The results of triaxial compression tests performed by the District and URS Laboratory for this study are summarized in Table C-6 and C-7, respectively. And results of all ICU tests combined are summarized in Table C-8 and for different soil types, as defined in GEI (2005) study, in Table C-9. Total and effective p-q plots of ICU tests for Shannon & Wilson (1966) and Wahler & Associates (1976) studies for different soil types are shown in Plates C-2a through C-2e for Zone 1, 2, 3, 4.5 and 5, respective; and for this study for Zone 2, 4, 4.5 and 5 in Plates C-3a through C-3d, respectively, and for all combined for Zone 1, 2, 3, 4, 4.5 and 5 in Plates C-4a through C-4f, respectively. More meaningful and representative total shear strength of each material in terms of a and values and effective shear strength in terms of a and values were estimated based on the best-fit line regression of the p-q and p -q data, respectively. It should be noted that cyclic triaxial compression tests were performed for the Wahler and Associates (1976) study, but the test results are not included in this studies because the test is not considered to be the state-of-practice for assessment of dynamic soil properties anymore. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

13 Particle size distribution plots and Atterberg Limits plots together with the original test data sheets of each test are also presented in Appendix C. Mohr s Circles from the triaxial compression tests together with the original data sheets and the associate total and effective p-q plots of each test are also presented in Appendix C. All these plots and data sheets are organized in groups and placed after the tables in Appendix C. 3.0 Groundwater Conditions Groundwater was not recorded during drilling for this study since the rotary wash drilling method was used and the borings were grouted at the end of drilling, except for Boring SS-41 and SS-43 in which standpipe piezometers were installed two with different tip depths in SS-41 and one in SS-43. Based on the review of reservoir water level data from January 2005 to July 2008, the reservoir water level ranges from Elevation 443 feet to 449 feet with an average at Elevation 446 feet, about 20 feet below the dam crest. In order to define a representative phreatic surface in the embankment dam for slope stability analysis, historic piezometer data from piezometers located near the center line of the dam area were reviewed. Based on the piezometer data from January 2005 to July 2008, the piezometer readings (in terms of Elevations) that can be used to defining the phreatic surface for slope stability analysis are summarized in Table 2 below. Table 2: Recorded Piezometer Readings from January 2005 to July 2008 Phreatic Piezometer ID (1) Min. Elevation (feet) Max. Elevation (feet) Avg. Elevation (feet) Surface Elev. (feet) (2) Reservoir Water Level SS-6A SS-8A SS-8B SS-30B SS-41A SS NOTES: (1) See Plate 2 for piezometer locations. (2) Phreatic surface elevations at similar locations used in slope stability analysis in GEI (2005) study. 4.0 Dam Material Properties 4.1 General As shown in Plate 3, the dam and its foundation soils consist primarily of three types of materials: 1. Compacted dam fill materials which in turn consist of Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

14 1.1 Core (or Zone 1) 1.2 Downstream Shell (or Zone 2) 1.3 Upstream Shell (or Zone 3) 2. Alluvial deposits which in turn consist of 2.1 Zone 4 below downstream shell 2.2 Zone 4.5 at downstream toe 2.3 Zone 5 below core and upstream shell 3. Orinda Formation. 4.2 Classification and Index Tests Grain size distribution and Atterberg limits determinations were performed on numerous samples of the dam fill and alluvial deposits in both previous and present studies. These results indicate that the dam fill materials and the underlying alluvial deposits predominantly consist of stiff to hard sandy clay materials with low to medium plasticity, whereas the alluvial deposits directly underneath the downstream toe area where the postulated zone of mobilized materials exists (Zone 4.5) predominantly consist of medium stiff to very stiff clay with medium to high plasticity and with small amounts of sand. Table C-2 in Appendix C summarizes the classification and index test results and with the average index values for each zone of materials. Plates 4a through 4d provide the distribution of in-situ moisture contents, total densities, void ratios and degrees of saturation, respectively, at different elevations for the dam core (Zone 1). Likewise, Plates 5a through 5d for the downstream shell (Zone 2), Plates 6a through 6d for the upstream shell (Zone 2), Plates 7a through 7d for the alluvial deposit Zone 4, Plates 8a through 8d for the alluvial deposit Zone 4.5 and Plates 9a through 9d for the alluvial deposit Zone 5. Table 3 below summarizes the average index properties for each zone of materials. Zone Table 3: Average Values of the Index Properties Moisture Content w (%) Total Density t (pcf) Specific Gravity Gs Void Ratio e Degree of Saturation S (%) Liquefaction Potential/Loss of Shear Strength Most of the cohesionless materials encountered in the dam fill material and alluvial deposits are occasional lenses of silty, clayey sand with high blow counts, therefore, significant loss of strength in these materials due to liquefaction is considered very unlikely. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

15 For cohesive soils, Plate 10 shows a plasticity chart with the results of all the Atterberg limits tests performed on the cohesive soils collected from the dam fill and alluvial deposits. Data points shown on this plate indicate that the materials fall under the CL and CH classification. Table C-1 in appendix C includes results of Atterberg limits and the in-situ moisture content of the sample tested. As the table indicates of all the samples tested, none are susceptible to liquefaction based on Seed, et. al. (2003) method for assessment of liquefaction potential of clayey soils, except sample SS-42-MC-13 at the depth 101 feet in Zone 4.5, and sample SS-24- O-8 at the depth of 85 feet in Zone 5 are estimated to be liquefiable. The materials in these samples are lean clay and sandy lean clay, respectively, and both have Plasticity Index of 12. These two samples are estimated to be liquefiable due to their relatively high in-situ moisture content. However, because of the in-situ high confining pressure on these materials due to their depths and thickness, liquefaction is considered very unlikely. Hence, it is in concurrence with the conclusion addressed in GEI (2005) study that the embankment dam and foundation materials are not susceptible to liquefaction by sudden loss of strength during the design earthquake. 4.4 Shear Strength In order to obtain more representative shear strengths for the dam fill material and alluvial deposits, all the ICU test data from Shannon and Wilson (1966) and Wahler and Associates (1976) studies were combined with those performed for this study. To make the previous test data consistent with that of this study, all horizontal and vertical stresses from the previous test data were selected based on the failure criterion of maximum effective stress ratio and were converted into p-q and p -q values. As discussed in Paragraph 2.3, representative total shear strength of each material in terms of a and values and effective shear strength in terms of a and values were estimated based on the best-fit line regression of the p-q and p -q data, respectively. Total and effective p-q plots of all combined ICU tests for Zone 1, 2, 3, 4, 4.5 and 5 are shown in Plates 11, 12, 13, 14, 15 and 16, respectively. Representative total and effective shear strengths of each material, which are to be used for slope stability analyses, were then obtained using the following equations: Effective friction angle: ø' = sin -1 (tan ), Effective cohesion: c = a / cos(ø'); and Total friction angle: ø = sin -1 (tan ), Total cohesion: c = a / cos(ø). Table C-10 in Appendix C summarizes the shear strength parameters from different studies for comparison Effective Shear Strength Failure Envelope For static slope stability analysis, effective shear strengths are used. The effective shear strength of the dam fill material and alluvial deposits were estimated using ICU tests with pore water Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

16 pressure measurements. Plates 17, 18, 19, 20, 21 and 22 show the effective strength failure envelopes from different studies for Zone 1, 2, 3, 4, 4.5 and 5, respectively Total Shear Strength Failure Envelope For seismic (pseudo-static) slope stability analysis, undrained shear strength are used for this study. The undrained shear strength of the dam fill material and alluvial deposits were estimated using the static total shear strength of ICU tests with pore water pressure measurements. Plates 23, 24, 25, 26, 27 and 28 show the total strength failure envelope from different studies for Zone 1, 2, 3, 4, 4.5 and 5, respectively. Table 4 summarizes the unit weights and shear strength parameters recommended for slope stability analyses. Table 4: Summary of Shear Strength Parameters Recommended for Slope Stability Analyses Effective Shear Strength Total Shear Strength Zone Material Total Unit Weight m (pcf) Sat. Unit Weight sat (pcf) Cohesion Intercept c (psf) Friction Angle (degree) Cohesion Intercept c (psf) Friction Angle degree) 1 Dam Core [128] (1) 700 [835] 21 [17] 850 [1200] 12 [8] 2 Downstream Shell [132] 530 [300] 30 [30] 730 [1400] 20 [16] 3 Upstream Shell [131] 440 [660] 26 [20] 640 [1400] 16 [8] 4 Alluvium below D/S Shell 4.5 Alluvium at D/S Toe 5 Alluvium below U/S Shell & Core [125] [125] [128] NOTE: (1) Values in brackets were used by GEI (2005). 0 [350] 220 [350] 360 [300] 31 [28] 28 [28] 29 [30] 990 [760] 450 [760] 1160 [1900] It should be noted that the soil parameters used in the GEI (2005) study are slightly different from those recommended in this study as shown in Table 4 above. The soil parameters used in the GEI (2005) study were based on the laboratory testing results in Shannon and Wilson (1966) and Wahler and Associates (1976) studies, whereas the soil parameters recommended in this study are the overall averaged values of both the previous laboratory testing results as well as those performed for this study. These recommended parameters of each material are up-to-date and considered to be more representative values to be used in the slope stability analyses. 14 [12] 15 [12] 15 [13] Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

17 It should also be noted that the overall shear strength of the alluvial deposits at the downstream toe (or Zone 4.5 as defined by DSOD) is just slightly lower than that of the other alluvial deposits (Zones 4 and 5) Anisotropic Shear Strength Failure Envelope Per State Division of Safety of Dams (DSOD, 2006) request, some ACU tests were performed to assess the anisotropic shear strength properties of the alluvial deposits for slope stability analysis, and are summarized in Table C-6 in Appendix C. All ACU tests were performed based on anisotropic consolidation stress ratio range of 1.3 to 1.4, as requested by DSOD (2006). ACU tests were performed for soil samples which were also tested for ICU for comparison and correlation of the isotropy and anisotropy properties of the same material type. However, due to limitation of soil sample recovery and change of soil type within a soil sample, only three pairs of ICU and ACU tests were performed for soil samples with good recovery and uniform material type within the soil samples; results of other ACU tests were then grouped into different material groups, namely Zone 4, 4.5 and 5, and compared with that of ICU tests for the same material group. Table 5 summarizes the results of the ICU and ACU tests. Comparison of the test results from both tests from Soil Samples SS-40-PT-40, SS-40-PT-26, and from SS-41-SH-10 and SS- 41-SH-12 are shown in Plates 29, 30 and 31, respectively. And comparison of the test results from both tests for Zone 4, 4.5 and 5 are shown in Plates 32, 33 and 34, respectively. Table 5: Summary of Isotropic and Anisotropic Consolidated Undrained Triaxial Compression (ICU and ACU) Test Results Sample No. / Zone No. Test Type c (psf) ø' (degrees) c (psf) ø (degrees) d (1) (psf) (1) (degrees) SS-40-PT-24 TXCU-I K c = K c =1.44 SS-40-PT-24 TXCU-A, K c =1.37 SS-40-PT-26 TXCU-I K c =1.44 SS-40-PT-26 TXCU-A, K c =1.36 SS-40-SH-10 TXCU-I K c =1.49 SS-40-SH-12 TXCU-A, K c =1.36 Zone 4 TXCU-I K c =1.49 Zone 4 TXCU-A, K c =1.36 Zone 4.5 TXCU-I K c =1.40 Zone 4.5 TXCU-A, K c =1.36 Zone 5 TXCU-I K c =1.44 Zone 5 TXCU-A, K c = K c = K c = K c = K c = K c =1.44 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

18 NOTE: (1) See description in paragraph below. As shown in Table 5, all total and effective cohesion, c and friction angle, ø values are computed from the total and effective a and values which are derived from the best-fit line regression of the p-q and p -q data. To compare the isotropy and anisotropy shear stresses of a soil, failure stress envelopes from both ICU and ACU tests are plotted as shown in Plates 29 through 34. As shown in these plots, the K c = 1 line, which corresponds to the isotropic consolidation with 1 '/ 3 ' = 1, is obtained based on the d value (y-axis intercept) from Equation 9.6 and value (slope of the line) from Equation 9.7 in Duncan and Wright (2005) using both total and effective shear stress parameters from ICU test, and the K f line (i.e., K c = K failure ), which corresponds to anisotropic consolidation with the maximum effective principal stress ratio possible, is essentially the effective stress envelope from ICU test based on the maximum ratio of effective horizontal stress to effective vertical stress, 1 '/ 3 ' as K f. According to Duncan and Wright (2005), values of undrained shear strength for a given anisotropic consolidation stress ratio can be obtained from linear interpolation between the K c = 1 and K c = K f envelopes using Equation Specific K c values of different materials based on the average stress ratios along the failure plane from a slope stability analysis for a long-term static case were obtained to represent the anisotropic stress condition of different materials during slope failure. Undrained shear strength envelope of these specific K c values for different materials were then developed by interpolating the K c = 1 and K c = K f envelopes as discussed above. Both ICU and the corresponding ACU test results, together with the K c = 1, K c = K f and the specific K c envelopes are plotted in Plates 29 through 34 for comparison purposes. Based on the comparison of the isotropic and anisotropic consolidated undrained triaxial compression test results (ICU vs. ACU) as shown in Plates 29 through 34, there is no definitive correlation between the two testing results as suggested by Duncan and Wright (2005). However, based on the test results in this study, using the ICU total shear strength parameters to model the short-term seismic case will give the most conservative results for the short-term seismic slope stability. If an anisotropic strength envelope for undrained shear strength under a given anisotropic consolidation stress ratio is used by linear interpolation between the K c = 1 and K c = K f envelopes from ICU test to model the instantaneous stress conditions during earthquake, as recommended by Duncan and Wright (2005), seismic slope stability result will be less conservative than using the total shear strength parameters from ICU test results, but it is considered to be closer to the real stress situation during earthquake. On the other hand, although it is found to be the least conservative, the shear strength parameters derived from the ACU test results for a given anisotropic consolidation stress ratio, or 1.3 to 1.4 in this case, are considered to be more direct parameters to be used to model the instantaneous stress conditions during earthquake. 5.0 Slope Stability Analyses Tentative slope stability analyses were performed for the downstream embankment using a representative cross-section across the center of the dam as shown in Plate 3. Two loading conditions were performed: 1) a long-term static case using the overall representative effective shear strength parameters from ICU test results and a phreatic surface as discussed in Section 3.0 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

19 above, and 2) a short-term seismic case using the overall representative total shear strength parameters from ICU test results as the undrained shear strength without a phreatic surface. Slope stability analyses were performed using the computer program SLOPE/W, Version 5 (GEO-SLOPE International, 2002). The program uses two-dimensional limit equilibrium technique and the method of slices, and allows the user to choose various methods to calculate the minimum factors of safety for both circular and non-circular failure modes. The slope stability analyses performed for the above two cases used the Spencer Method with a circulartype failure surface. In the Spencer Method, compatibility of both moment and force equilibrium is enforced, which provides a higher level of reliability than methods which compute moment or force equilibrium separately. For the long-term static stability of the downstream slope, the lowest factor of safety in this study is 2.19, as shown in Plate 35. Although it is lower than 2.4 as presented in the GEI (2005) report, it is higher than 1.5 as the minimum required factor of safety. For the short-term seismic stability of the downstream slope, a pseudo-static approach was utilized. In the pseudo-static analysis, the result is the yield acceleration, K y, an effective horizontal ground acceleration at which a potential sliding surface would develop a factor of safety equal to unity. This yield acceleration is the minimum pseudo-static acceleration required to produce slope instability and any earthquake-induced deformation of the slope. In this study, the yield acceleration is g, as shown in Plate 36, a value higher than 0.14 g as presented in the GEI (2005) report. This is considered an improvement from the previous study; hence, the earthquake-induced deformations on the downstream slope during the design earthquake would be smaller than that estimated in the GEI (2005) study. 6.0 Conclusion As recommended in Paragraph 10.2 in the GEI (2005) report, this study has implemented supplemental geotechnical investigation for the downstream embankment of Lafayette Dam and the alluvial deposits underneath it. Supplemental investigation includes: Five Cone Penetration Tests (CPTs) with 1 performed on the dam crest and 4 on the downstream embankment slope, all to depths of high refusal where Orinda Formation is expected. Four exploratory borings with 1 on the dam crest and 3 on the downstream embankment slope. Three standpipe piezometers installed, with 1 in 1 borehole and 2 with different tip depths in another on the downstream embankment slope, to supplement more phreatic surface information. A laboratory testing program consisting of index and strength tests for both the downstream dam shell material and the alluvial deposits underneath it. Isotropic and anisotropic consolidated undrained (ICU and ACU) triaxial compression tests were performed to supplement the effective and undrained as well as anisotropic shear strengths of the dam and foundation materials. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

20 It should be noted that field vane shear tests to obtain the in-situ residual shear strengths of the alluvial deposits at the downstream dam toe, as suggested in GEI (2005) study, were not performed in this study after carefully reviewing the CPT results. The results of this supplemental geotechnical investigation confirm the findings of the GEI (2005) study. The shear strength data from this study were combined with the data from the previous investigations and the analyses show that the dam is stable for both long-term static and short-term seismic conditions. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

21 References 1. ASTM (2003). Annual Book of ASTM Standards, Section 4 Construction, Volume 04.08: Soil and Rock (I): D 420 D 5611, American Society for Testing and Materials, West Conshohocken, Pennsylvania. 2. Duncan, J.M. and Wright, S.G. (2005). Soil Strength and Slope Stability, John Wiley & Sons, Inc., Hoboken, New Jersey. 3. East Bay Municipal Utility District, Proposed Field Exploration Plan for Lafayette Dam, Letter to the State Department of Water Resources, Division of Safety of Dams, August GEI Consultants, Dynamic Stability Review of Lafayette Dam, Report and Appendices. Report for East Bay Municipal Utility District, August GEO-SLOPE International (2002). User s Guide SLOPE/W for Slope Stability Analysis, Calgary, Alberta, Canada. 6. Seed, R. B., et. al., (2003), Recent Advances in Soil Liquefaction Engineering: a unified and consistent framework, Report No. EERC , EERI, University of California, Berkeley, June. 7. Shannon and Wilson, Inc., Review of Stability, Lafayette Dam. Report for East Bay Municipal Utility District, January. 8. State Division of Safety of Dams, Lafayette Dam, No. 31-2, Contra Costa County. Approval letter to Bay Municipal Utility District, September Wahler and Associates, Seismic Stability Evaluation, Lafayette Dam, Contra Costa County, California. Report for East Bay Municipal Utility District, May. Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

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97 Appendix B Logs of Borings LIST OF PLATES Plate B-1A Plate B-1B Plate B-2 Plate B-3 Plate B-4 Plate B-5 Plate B-6 Plate B-7 Soil Classification Properties Legend Formational Material Description, Legend for Report Boring Logs Log of Boring SS-40 Log of Boring SS-41 Log of Boring SS-42 Log of Boring SS-43 Piezometer Installation Report SS-41 Piezometer Installation Report SS-43 Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

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120 Appendix C Laboratory Test Results LIST OF TABLES Table No. Title C-1 Summary of Overall Laboratory Index Test Results C-2 Summary of Index Tests Results for Different Zones of Materials C-3 Summary of Specific Gravities for Different Zones of Materials C-4 Summary of Consolidated Undrained Triaxial Compression Test Results from Shannon and Wilson 1966 Study C-5 Summary of Consolidated Undrained Triaxial Compression Test Results from Wahler and Associates 1976 Study C-6 Summary of Consolidated Undrained Triaxial Compression Test Results by EBMUD C-7 Summary of Consolidated Undrained Triaxial Compression Test Results by URS C-8 Summary of Overall Isotropic Consolidated Undrained Triaxial Compression Test Results C-9 Summary of Isotropic Consolidated Undrained Triaxial Compression Test Results for Different Zones of Materials C-10 Comparison of Strength Parameters for Different Zones of Materials from Different Studies C-11 Summary of TXCU-I and TXCU-A Test Results LIST OF PLATES Plate No. Title C-1 Atterberg Limits of Cohesive Soils C-2a Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 1 Core C-2b Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 2 D/S Shell C-2c Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 3 U/S Shell C-2d Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 4.5 Alluvium at D/S Toe C-2e Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 5 Alluvium below U/S Shell and Core C-3a Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 2 D/S Shell C-3b Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 4 Alluvium below D/S Shell Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

121 C-3c C-3d C-4a C-4b C-4c C-4d C-4e C-4f Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 4.5 Alluvium at D/S Toe Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 5 Alluvium below U/S Shell and Core Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 1 Core Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 2 DS Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 3 U/S Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 4 Alluvium below D/S Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 4.5 Alluvium at D/S Toe Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 5 Alluvium below U/S Shell and Core GROUPS OF ORIGINAL LABORATORY TESTING DATA Group C-1 Group C-2 Group C-3 Group C-4 Group C-5 Group C-6 Moisture Contents, Unit Weights, Particle Size Distribution and Atterberg Limits Reports Particle Size Distribution and Atterberg Limits Reports for Triaxial Compression Test Samples by the District Materials Lab Specific Gravity, Particle Size Distribution and Atterberg Limits Reports for Triaxial Compression Test Samples by the URS Pleasant Hill Lab Triaxial Compression Test Reports and p-q Plots by the District Materials Lab Triaxial Compression Test Reports and p-q Plots by the URS Pleasant Hill Lab Triaxial Compression Test Reports and p-q Plots from Wahler and Associates 1976 Study Group C-7 Triaxial Compression Test Reports and p-q Plots from Shannon and Wilson 1966 Study Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

122 Appendix C Laboratory Test Results TABLES Table No. Title C-1 Summary of Overall Laboratory Index Test Results C-2 Summary of Index Tests Results for Different Zones of Materials C-3 Summary of Specific Gravities for Different Zones of Materials C-4 Summary of Consolidated Undrained Triaxial Compression Test Results from Shannon and Wilson 1966 Study C-5 Summary of Consolidated Undrained Triaxial Compression Test Results from Wahler and Associates 1976 Study C-6 Summary of Consolidated Undrained Triaxial Compression Test Results by EBMUD C-7 Summary of Consolidated Undrained Triaxial Compression Test Results by URS C-8 Summary of Overall Isotropic Consolidated Undrained Triaxial Compression Test Results C-9 Summary of Isotropic Consolidated Undrained Triaxial Compression Test Results for Different Zones of Materials C-10 Comparison of Strength Parameters for Different Zones of Materials from Different Studies C-11 Summary of TXCU-I and TXCU-A Test Results Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

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144 Appendix C Laboratory Test Results PLATES Plate No. Title C-1 Atterberg Limits of Cohesive Soils C-2a Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 1 Core C-2b Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 2 D/S Shell C-2c Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 3 U/S Shell C-2d Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 4.5 Alluvium at D/S Toe C-2e Total and Effective p-q Plots of TXCU-I Test Results from Previous Studies: Zone 5 Alluvium below U/S Shell and Core C-3a Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 2 D/S Shell C-3b Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 4 Alluvium below D/S Shell C-3c C-3d C-4a C-4b C-4c C-4d C-4e C-4f Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 4.5 Alluvium at D/S Toe Total and Effective p-q Plots of TXCU-I Test Results from This Study: Zone 5 Alluvium below U/S Shell and Core Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 1 Core Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 2 DS Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 3 U/S Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 4 Alluvium below D/S Shell Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 4.5 Alluvium at D/S Toe Total and Effective p-q Plots of All Combined TXCU-I Test Results: Zone 5 Alluvium below U/S Shell and Core Supplemental Geotechnical Investigation Report Lafayette Reservoir Dam November 2008

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