September 16, 2011 Project No Ms. Christine Arbogast BAS, A Tetra Tech Company 1360 Valley Vista Drive Diamond Bar, CA 91765

Transcription

1 Geologists, Hydrogeologists and Engineers September 6, 0 Project No. 0-0 Ms. Christine Arbogast BAS, A Tetra Tech Company Valley Vista Drive Diamond Bar, CA 976 STOCKPILE SLOPE STABILITY PRIMA PHASE B PRIMA DESHECHA LANDFILL ORANGE COUNTY, CA In accordance with your request and authorization, Geo-Logic Associates (GLA) has evaluated the slope stability of the proposed Phase B Stockpile at the Prima Deshecha Landfill, County of Orange, CA. In preparation of this evaluation, we have used the topo and stockpile maps provided by BAS in August, 0 (Figure ). Background After construction of the toe buttress for Landslide B in, the stockpile fill on the north side of the biomitigation channel was placed relatively rapidly during the month of April. On May 6,, linear cracking was noted in the contractor s equipment yard on the deck area of the temporary fill stockpile. The deck was at an approximate elevation of feet. The height of the stockpile fill was approximately 60 feet above the pre-fill soil grade. The cracking was approximately 0 feet in length and roughly parallel to the trend of the biomitigation channel (extending from Station 0+0 to +00 and approximately feet northwest of the channel). After this cracking was noted, inclinometer SI- was installed to establish the failure geometry. The stockpile fill soils were subsequently removed to the grades shown on Figure. Piezometers NPZ- and NPZ- were installed in 7 to evaluate the piezometric conditions in the area of the stockpile. Piezometers PZ-, PZ-, and PZ- were installed in June 00 for additional piezometric control in the stockpile area. The approximate locations of the piezometers are shown on Figure. Slope Stability Analyses Shear Strength Parameters Previous stability evaluations for this project (GLA, ) were based on a combination of shear strengths estimated from laboratory tests and from back analysis of pre-existing West Bernardo Court, Suite, San Diego, California 97; Phone: (88) -6 FAX: (88) -087

2 Phase B Stockpile Evaluation, Prima Deshecha Landfill, Orange County, CA failures. Since previous back analyses of sheared failure surfaces along two-dimensioned sections resulted in minor variations in the back-calculated slip surface strengths, initial - dimensional analyses used these section-specific strength characteristics. More recent and updated - and -dimensional analyses used a multi-section average strength of cohesion (c) = psf and a friction angle (φ) of 9. degrees. This multiple section average strength characteristics for the south slope failure is expected to reasonably represent operational strengths and is considered appropriate for the conditions observed at the site. A summary of strength parameters used in the stability calculations is provided in Table. Table Shear Strength Parameters Soil Or Formation Unit weight (pcf) Cohesion (psf) Friction Angle (degrees) Uncompacted/Stockpile Fill 0 Compacted Fill 0 0 Landslide 0 00 Oxidized Capistrano 0 00 Formation (treated as impenetrable bedrock) Clay Seam/Slide Plane 0 9. Pore Pressures Pore pressures were obtained from the recent water level reading from July 0 for wells NPZ-, NPZ-, PZ-, PZ-, PZ-D, and PZ-S (Figure ). The water levels were slightly increased to account for seasonal variations. In addition, the subdrains installed at the base of the toe stabilization buttress provide a known elevation for draw-down. These data were used to establish the piezometric conditions assumed in the stability calculations. The current conditions of groundwater as shown on Plate are generally slightly higher near the east end of the biomitigation channel than those previously assumed (GLA ). Method of Analysis Several cross sections were evaluated for this analysis. The cross sections analyzed included the -dimensional cross sections: F-F, F-F, F7-F7, F8-F8, and F-F. In addition, to insure that the most critical cross section was used for the stockpile analysis, an additional section, Cross Section SP-SP was prepared and analyzed. Nine cross sections comprising the -dimensional array D through D9 were also analyzed. The approximate location of these -dimensional and -dimentional cross sections are presented on Figure. Based on the results of the review, Cross Section F-F was chosen as the most critical sections and was used to perform the parametric analysis described below. First, the static analysis of the current stability was performed under conventional static conditions using the Morgenstern-Price Method with the limit equilibrium slope stability software, SLOPE/W (GEO-SLOPE International, 7). A variety of search procedures were utilized to determine the critical potential failure surface. All of the individual /BAS_Prima Phase B Stockpile Stability Analysis_rev.docx/9/6/0 --

3 Phase B Stockpile Evaluation, Prima Deshecha Landfill, Orange County, CA analyses took advantage of a slip surface optimization procedure within SLOPE/W wherein the lowest factor-of-safety potential slip surface at the end of standard limit equilibrium iterations is further iterated on a segment-wise basis to find potentially lower factor-ofsafety (and often non-circular) slip surfaces. Use of this procedure will always result in a factor-of-safety that is as low or lower than if it had not been used (i.e. it is conservative). The cross section was then analyzed for various heights of stockpile material assuming a : (horizontal to vertical) slope up from the limits of the stockpile (Figure ) with a deck elevation ranging from to feet. The result of the analysis is presented in Table. Deck Elevation Table Slope Stability Analysis Results Figure No. Static Factor of Safety without Increased PWP Static Factor of Safety with Increased PWP Figure No. 7 (no additional (no additional stockpile) stockpile or PWP) Notes: All significant units of the factors of safety (above) are presented to associate the result to the computer print-out (Figures -6) and do not represent the degree of accuracy of the resultant. PWP = pore water pressure. The result of the static analysis without an increased pore water pressure (PWP) indicates that the stockpile may rise up to an elevation of approximately feet with a resulting factor of safety of.. However, with increasing stockpile height, the pore water pressure at the base of the landslide will increase, thus decreasing the overall factor of safety. After the static factor of safety was calculated for the above conditions without additional pore water pressures, the stockpile was increased to various deck heights with an associated pore water pressure increase. The pore water pressure at the base of the failure surface was analyzed by using the pore water pressure from the potentiometric water surface (as shown on the computer print-outs) plus an increased pore water pressure equal to the weight of the stockpile fill soils (plus a slight increase to account for localized rises in the groundwater table due to seasonal effects). The results indicate that the stockpile cannot be raised greater than an elevation of approximately 0 to feet without allowing the pore water pressures to dissipate. A minimum factor of safety of. was chosen to account for uncertainties /BAS_Prima Phase B Stockpile Stability Analysis_rev.docx/9/6/0 --

4 Phase B Stockpile Evaluation, Prima Deshecha Landfill, Orange County, CA An analysis of the dissipation of the pore water pressure was then performed. Using relationships between the plasticity of the clayey landslide failure surface and the coefficient of consolidation (NAVFAC, 98), a coefficient of consolidation ranging from 0.0 to 0. square feet/day was chosen. Based on an effective fracture spacing at the base of the landslide mass ranging from to 0 feet, the time for 60% pore pressure dissipation is calculated to be approximately 00 to days. The interconnectivity of the fractures at the base of the landslide is extremely difficult to predict and the dissipation of pore pressures over time can only be roughly estimated by conventional soil mechanics principles. The actual pore pressure dissipation has to be verified by actual piezometric reading during stockpile filling. An additional slope stability analysis was performed at 60% pore water pressure dissipation with a stockpile elevation of feet and yielded a static factor of safety of. (Figure 7). This factor of safety is deemed adequate if substantiated by piezometric readings. The results of the recommended maximum stockpile elevation versus time after the initial stockpile fill soils are placed to an elevation of 0 feet is shown on Table. Table Stockpile Deck Elevation vs. Time Stockpile Deck Elevation, ft Months Seismic Deformation Analyses Since the Phase B stockpile is considered to be an interim condition, long-term seismic deformation analysis was deemed to not be necessary /BAS_Prima Phase B Stockpile Stability Analysis_rev.docx/9/6/0 --

5 Phase B Stockpile Evaluation, Prima Deshecha Landfill, Orange County, CA Conclusions and Recommendations Based on the results of GLA s stability evaluation, the proposed Phase B Stockpile development has an acceptable static factor of safety provided the filling schedule presented in Table is followed during earthwork construction. This assumes that the stockpile will be brought up to an elevation of approximately 0 feet during roughly the first month after filling has started and that the additional stockpile fill will not be brought up in elevation more than to 6 feet per month, thereafter. It is imperative that the existing piezometers are preserved in operating condition and monitoring on a weekly basis during earthwork construction so that the pore water pressures do not rise above the levels assumed herein. If the installed piezometers do not indicate an increased pore pressure during stockpile placement up to an elevation of 0 feet, additional piezometers may be recommended prior to additional stockpile placement (above an elevation of 0 feet) so that the increase in pore water pressure can be monitored. Closure This report is based on the project as described, the geotechnical data obtained from literature review and laboratory tests of materials used previously at this and similar sites, published references, and plans prepared by BAS (0). This report was not prepared for use by parties and projects other than those named or described herein and may not contain sufficient information for other parties or other projects. This report was prepared in accordance with generally accepted geotechnical and geological practices and makes no other warranties either expressed or implied, as to the professional advice or data included. We appreciate this opportunity to be of service. If you have any questions regarding this report, please do not hesitate to contact the undersigned. Geo-Logic Associates Joseph G. Franzone, GE 89 Supervising Geotechnical Engineer Distribution: () Addressee-electronic submittal Attachments: References Figure Stockpile Location Map Figure Geologic Map Figures -7 Slope Stability Analysis Results /BAS_Prima Phase B Stockpile Stability Analysis_rev.docx/9/6/0 --

6 Phase B Stockpile Evaluation, Prima Deshecha Landfill, Orange County, CA References BAS, 0, Topo file (AutoCAD) and PDF file transmitted by in August 0. Bowles, 977, Foundation Analysis and Design, Second Edition. Geo-Logic Associates, 999, Prima Deshecha Landfill, Geotechnical Investigation Report Zone, dated October 999, PN 967. Geo-Logic Associates,, Interim Construction Report, Zone Phase B Expansion Project, Prima Deshecha Landfill, dated August, PN 979. Geo-Logic Associates,, Updated Slope Stability Analyses, Zone Development Plan, Prima Deshecha Landfill, dated November, PN 979. Geo-Slope, 7, SLOPE/W, Slope Stability Analysis for Windows, Version 7.7, revised 00. Kramer, 996, Geotechnical Earthquake Engineering, Prentice Hall, pp. 6. NAVFAC, 98, DM-7., Soil Mechanics. -6-

7 PZ- PZ- PZ-S & D Approximate Limits of Phase B Stockpile NPZ- and REFERENCE: BAS, 0, PDF drawing by . LEGEND Approximate location of existing piezometer For approximate scale, cross-hairs are 00' apart N FIGURE PHASE B STOCKPILE EVALUATION PRIMA DESHECHA LANDFILL ORANGE COUNTY, CALIFORNIA STOCKPILE LOCATION MAP Draft: JGF Date: 08- Project No. 0-0

8 PZ- PZ- SP' APPROXIMATE LIMITS OF STOCKPILE PZ-S&D SP PZ-S&D LEGEND APPROXIMATE STOCKPILE LIMITS APPROXIMATE PIEZOMETER LOCATION FIGURE GEOLOGIC MAP PHASE B STOCKPILE EVALUATION PRIMA DESHECHA LANDFILL ORANGE COUNTY, CALIFORNIA SP SP' APPROXIMATE LOCATION OF CROSS SECTION Draft JGF Date 9/0 Project No. 0-0

9 File Name: F-F'-Stockpile analysis wo stockpile.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

10 File Name: F-F'-Deck='.gsz Date: 9/7/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

11 File Name: F-F'-Deck=0'.gsz Date: 9/7/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

12 File Name: F-F'-Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

13 File Name: F-F'-Deck=0'.gsz Date: 9/7/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

14 File Name: F-F'-Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

15 File Name: F-F'-Deck=0'.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line Optimize Critical Slip Surface Location: No Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

16 File Name: F-F'-Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

17 File Name: F-F'-with PWP_Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

18 File Name: F-F'-with PWP_Deck=0'.gsz Date: 9/7/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

19 File Name: F-F'-with PWP_Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

20 File Name: F-F'-with PWP_Deck=0'.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

21 File Name: F-F'-with PWP_Deck='.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

22 File Name: F-F'-with PWP_Deck=0'.gsz Date: 8/9/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #:

23 File Name: F-F'-with 60% PWP_Deck='.gsz Date: 9/7/0 PWP Conditions Source: Piezometric Line with B-bar Unit Weight: Cohesion: 0 Cohesion: 00 Cohesion: 0 Cohesion: Phi: Distance (feet) (x 000) Material #: