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HomeMy WebLinkAboutStockdale Ranch Sump #5 Slope Stability Analyses 08-02-214400 YEAGER WAY Phone: 661 831 5100 info@soilsengineering.com BAKERSFIELD, CA 93313-2019 Fax: 661 831 2111 www.soilsengineering.com August 2, 2021 SEI File No. 21-17961 Bolthouse Properties, LLC 11601 Bolthouse Drive, Suite 200 Bakersfield, CA 93311 Attention: Mr. Jeff Eittreim Subject: Addendum 1 - Sump Slope Stability Reference: GEOTECHNICAL INVESTIGATION REPORT Stockdale Ranch Sumps and Street Improvements Corner of Stockdale Hwy & S. Heath Rd, Bakersfield, CA. In accordance with your request and authorization, we have performed slope-stability analyses for proposed sumps on the above referenced project. Our analyses shows that the minimum factor-of-safety for a sump with 13 feet high slopes ranges from 2.87 to 2.89 for the case of saturated slopes and a phreatic surface that parallels the ground surface at a depth of one foot below between the sump toe and at least 50 feet beyond the top of the slope. Figures 1 through 3 show the ten failure-surfaces, each yielding the lowest safety factors. Three methods: Bishops Modified; Spencer; and Janbu; were used to determine three independent safety- factors. We hope this provides the information you require. Please contact us if you have any questions or if you need any further geotechnical services. Respectfully submitted, Soils Engineering, Inc. L Thomas Bayne CE 26106 GE 125 SOILS ENGINEERING, INC. Geotechnical Engineering Services SEI File No. 21-17961 Project: Stockdale Ranch Sumps and Street Improvements August 2, 2021 Location: Southwest Corner of Stockdale Hwy & S. Heath Rd, Bakersfield, CA Page 2 4400 YEAGER WAY Phone: 661 831 5100 info@soilsengineering.com BAKERSFIELD, CA 93313-2019 Fax: 661 831 2111 www.soilsengineering.com Slope Stability The proposed sump-embankment cross-section was analyzed using the computer program, WinStable stability for the subject site was assessed using the Spencer, Janbu, and Bishops method. Strength values were obtained from the results of Direct Shear tests on saturated samples of earth materials obtained from the upper twelve feet below the existing ground surface. Slip-surfaces for the Spencer, Janbu, and Bishops methods are shown on Figures 1 through 3. These methods yielded safety-factors that were comfortably-above the minimum allowed, ranging from 2.87 to 2.89. Slopes analyzed were selected as representing the most-critical slope locations and configurations based on our engineering judgment tempered by our experience performing slope-stability-analyses in the Southwest area of the city of Bakersfield, in Kern County, California. Since soils were relatively uniform and generally increased in density and strength with depth, critical circles passing through of the toe of the slope were selected as most representative of the critical failure mode. The Entry and Exit method was used to analyze failure surfaces assumed to pass through the toe of the slope while entering at the top of the slope usually within one slope- height from the top-edge of the embankment. Soil shear strengths were assumed to be anisotropic although direct shear tests on existing, undisturbed native soils promote failure to occur along an essentially horizontal failure plane owing to the design of the test equipment and the orientation of the test sample. Slope Stability in Stratified Soils Natural soils on the site are stratified with stratum thicknesses and sequencing varying extensively between test boring locations. Little or no lateral continuity exists between test boring locations owing to the mode of alluvial deposition. Slope stability analyses were performed using layer sequencing that would produce the lowest safety factor, i.e. by placing the soils layer with the lowest friction and cohesion at the lowest elevations on the slope profiles analyzed. Slope Stability Safety Factors Computed Safety factors obtained using the Entry and Exit method, the Grid-Radius method, and the Block method are given in table below. SLOPE STABILITY FACTORS OF SAFETY Janbu’s Method 2.86 Spencer’s Method 2.86 Bishop’s Method 2.87 SOILS ENGINEERING, INC. Geotechnical Engineering Services SEI File No. 21-17961 Project: Stockdale Ranch Sumps and Street Improvements August 2, 2021 Location: Southwest Corner of Stockdale Hwy & S. Heath Rd, Bakersfield, CA Page 3 4400 YEAGER WAY Phone: 661 831 5100 info@soilsengineering.com BAKERSFIELD, CA 93313-2019 Fax: 661 831 2111 www.soilsengineering.com Figures 1 through 3 show the shape and location of the most critical failure surfaces for each of the cases described above. Existing and proposed slope configurations were analyzed for static stability. Pseudo-seismic (pseudo-static) was not assessed. Chart solutions were used to estimate approximate locations for circle-centers for failure modes simulating two-dimensional circular failure surfaces. Slopes were analyzed in accordance with the following criteria: A.Circular failure surfaces were used in our analyses. Because existing stratigraphy is random and no predominant orientation for bedding exists, circular failure surfaces having the lowest safety factors were those that were both the thinnest and mostly oriented parallel to the slope surface. B.Using Slope/W possible centers for circular failure surfaces were positioned in a random pattern. The approximate centers for circular failure surfaces most likely to yield the lowest Factors-of-Safety were usually found at a locations in which failure surfaces enter near the toe of the slope and exist near the top of the slope and in the height range of two to two and one-half times (2 x - 2 ½ x ) the height of the slope as measured from the toe elevation. C.Plots showing the shape of the failure surface, centers for moment equilibrium, centers of rotation for failure surfaces, and ranges for entry and exit of slip-circles are shown on Figures 1, 2, and 3, below. D.Mohr-Coulomb equation was used to define soil strength for use in stability analyses. Because most of the materials located within the existing embankments and specified in recommendations for embankment construction are free-draining, effective-strength parameters as determined from direct-shear strength tests were used in the analyses. E.Soil strength values were obtained from our laboratory test data consisting of grain- size analyses, drained-direct-shear tests. Angles of effective internal friction (∅’) and effective cohesion (strength-intercept) (c’) were obtained by plotting effective shear stress (τ) in kips per square foot (ksf) on the ordinate and effective normal stress (s’) in kips per square foot (ksf) on the abscissa. The lower-bound value line was used to obtain ∅’ and c’. The plot of shear stress (τ) in kips per square foot (ksf) vs. effective normal stress (s’) in kips per square foot (ksf). F.Slopes analyzed were selected to be representative of the most-critical slope locations and configurations based on the slope height. Slopes are assumed to be saturated with phreatic surfaces paralleling the ground surfaces near the top, from the top to the bottom of the slope, and at the toe of the slope and beyond. H.Analyses that were performed using ranges of entry and exit points for slip-circles were set-up whereby circles were assumed to pass within a few feet up-slope and down- slope from the toe of the slope; while exiting near at the top of the slope within the range SOILS ENGINEERING, INC. Geotechnical Engineering Services SEI File No. 21-17961 Project: Stockdale Ranch Sumps and Street Improvements August 2, 2021 Location: Southwest Corner of Stockdale Hwy & S. Heath Rd, Bakersfield, CA Page 4 4400 YEAGER WAY Phone: 661 831 5100 info@soilsengineering.com BAKERSFIELD, CA 93313-2019 Fax: 661 831 2111 www.soilsengineering.com bounded by points within one slope-height offset away from the edge of the embankment and a few feet down-slope on the embankment face. Limits for entry and exit of failure circles are shown on stability analyses plots. I.Soil shear strengths were assumed to be isotropic although direct shear tests on existing, undisturbed native soils promote failure to occur along an essentially horizontal failure plane. This is due to the design of the test equipment and the orientation of the test sample as taken in the field. Orientation of samples as taken and tested in the laboratory result in the shear-strength analyses skewed slightly toward the conservative side. This conservatism occurs because soil bedding of alluvium tends to be horizontal and parallel to the direction that samples are sheared in the laboratory. Were samples to be sheared in a direction transverse to the natural soil bedding, shear strength results would be slightly higher than their horizontally sheared counterparts. J.Slope Stability in Stratified Soils Natural soils on the site are layered. Layer thicknesses and sequencing vary randomly between test boring locations. Little or no lateral continuity exists between test boring locations owing to the mode of alluvial deposition. Figure 1 Minimum Safety Factor – Spencer’s Method Phreatic Surface SOILS ENGINEERING, INC. Figure 2 - Minimum Safety Factor Janbu's Method Phreatic Surface SOILS ENGINEERING, INC. Figure 3 - Minimum Safety Factor Bishop’s Method Phreatic Surface SOILS ENGINEERING, INC.