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HomeMy WebLinkAboutPD 2020-0050; COLINAS DE ORO SLOPE REPAIR; SLOPE REPAIR INVESTIGATION & RECOMMENDATIONS; 2020-08-0322725 Old Canal Road, Yorba Linda, CA 92887 - (714) 685-3900 - FAX (714) 685-3909 4901 Morena Boulevard, Suite 1110, San Diego, CA 92117 - (858) 450-4040 - FAX (858) 457-0814 3100 Fite Circle, Suite 103, Sacramento, CA 95827 - (916) 368-2088 - FAX (916) 368-2188 5600 Spring Mountain Road, Suite 201, Las Vegas, NV 89146 - (702) 562-5046 - FAX (702) 562-2457 August 3, 2020 File No. 24187-01 COLINAS DE ORO HOA C/O: Ms. Cameron Gonzales THE PRESCOTT COMPANIES 5950 La Place Court, Suite 200 Carlsbad, California 92008 Project: SLOPE REPAIR INVESTIGATION & RECOMMENDATIONS COLINAS DE ORO HOA Slope Failure across from 7927 Calle Madrid Carlsbad, California 92009 Dear Ms. Gonzales: American Geotechnical has performed a geotechnical investigation of the slope failure that occurred on the common area slope located approximately across from 7927 Calle Madrid within the Colinas De Oro HOA community in Carlsbad, California. The purpose for the investigation was to identify the geologic and geotechnical conditions of the failure and provide engineering repair recommendations to restore the slope back to its approximate original configuration with geogrid reinforcement. Our findings and recommendations are presented in this report. We appreciate the opportunity to be of service. If you should have any questions, please do not hesitate to contact our office. Sincerely, AMERICAN GEOTECHNICAL, INC. Kevin Rogers Fei-Chiu (Jerry) Huang, Ph.D. Chief Geologist Principal Engineer C.E.G. 2425 G.E. 2601 KR/JH:bp Distribution: Ms. Cameron Gonzales – (1) Via Email ~ • American Geotechnical, Inc. GEOTECHNICAL/CIVIL ENGINEERING, TESTING& INSPECTION PD 2020-0050 File No. 24187-01 August 3, 2020 Page 1 1.0 INTRODUCTION American Geotechnical has investigated the slope failure located approximately across the street from 7927 Calle Madrid within the Colinas De Oro HOA community in Carlsbad, California (see Figure 1 – Site Location Map). The purpose for the investigation was to evaluate the geologic and geotechnical conditions of the slope failure and provide engineering recommendations to restore the slope to its approximate original configuration with geogrid reinforcement. The investigation was conducted between April and July of 2020. 1.1 SCOPE OF SERVICES The scope of work performed for this project consisted of the following tasks: • Review of available background documents including the Rough Grading and Geotechnical plans for the site prepared by Leighton Engineering, dated 8/23/1984, and available published topographic and geologic maps of the area, which are listed on the References page. • Site review, mapping, and photographic documentation of the existing condition of the slope failure. • Subsurface exploration and soil sampling of the slope failure utilizing two hand-excavated test pits. • Laboratory testing of samples collected from the exploratory excavations. • Engineering/geologic analyses and slope stability analyses. • Preparation of engineering recommendations for repair of the slope failure. • Preparation of this report summarizing our findings, conclusions and discussions, and presenting conceptual landslide repairs and grading recommendations. 1.2 SLOPE FAILURE DESCRIPTION The subject slope failure is located on a common area slope that is approximately 27 feet tall with an approximate 2:1 (horizontal:vertical) slope gradient. The failure reportedly occurred in late March of this year following a series of storms that dropped near record rainfall in the area. The failure occurred at about mid-height of the slope, with the head scarp located approximately 6.5 feet below the top of the slope and the toe of slope failure located approximately 7 feet above the base of the slope. 7929Call e M a d r i d 7925 Calle San Felipe N Site Location Map Colinas De Oro HOA Carlsbad, CAF.N. 24187.01 Figure 1 AmericanGeotechnical,Inc. 7929 Calle Madrid No Scale: I ] I E!i Rastroln Jacarnrida Ave .... -+ OBvenh ai n Rd ..L .... ➔ 01ivenhain Rd '-- .... .... Cort, et.I' t.) i e, •• Avenida PlmeL @ra ~ ,,, El I s~o I Olivenha in Municipal Water Ost ,t' El " ca.lie Vallarta_ ,t' ;ff c.amir,, - - - - ,- ' o Coron11cfo -r· ----. -· ......... Corte T File No. 24187-01 August 3, 2020 Page 2 Residential homes are located along the top of the slope, but the slope failure did not cause any damage to the homes or lots and does not pose an immediate threat to them. Also, the failure debris is contained entirely on the slope face and did not runout onto the sidewalk or the street below. The slope failure is oval- shaped and measures approximately 55 feet wide at its widest point, and measures approximately 32 feet long in the down slope direction, but the debris apron extends another 15 feet down the slope face almost to the sidewalk below. The failure has a well-defined basal slip surface in the head scarp contained within a sheared brown clay layer that is highly plastic, soft and very moist, with numerous striations oriented in the downslope direction. 2.0 SUBSURFACE EXPLORATION Two exploratory test pits were excavated by hand within the approximate center of the slope failure on July 10, 2020 (see Figure 2 – Geotechnical Map). One test pit was excavated in the head scarp area (AGTP- 1), and a second test pit was excavated near where the slope failure exited the slope face (AGTP-2). The geologic conditions encountered were identified and logged by a licensed engineering geologist, and soil samples were collected and transported back to our laboratory for testing. As stated above, there is a well- defined basal slip surface in the head scarp that occurs as a sheared brown clay layer, which is highly plastic, soft and very moist, and contains numerous striations oriented in the downslope direction. This slip surface could be traced through the upper portion of the failure in our exploratory test pit AGTP-1, but could not be found in our lower test pit, AGTP-2, where the lower portion of the failure occurs to have failed within a broader zone of soft, plastic, and very moist clayey fill containing multiple shears and slips. The slope failure is a classic rotational failure that occurred entirely within fill material and is approximately 5-feet thick. No groundwater seepage was encountered in the test pits and no broken pipes that could have been leaking into the slope were found during the subsurface investigation. Hence, the failure appears to have occurred due to oversaturation of the fill material from the multiple rainstorms that fell in the area causing loss of shear strength of the clay contained within the fill material. Our test pit field logs and site photographs are presented in Appendix A. Data obtained from the field investigation and the original rough grading plans were used to construct a geologic cross section through the slope failure, which is presented on Figure 3 – Geologic Cross Section A-A’. 150 160 El. 168.4 El. 169.4El. 167.3 AA’AGTP-1 AGTP-2 7925 Calle San Felipe Calle Madrid Approx. scale: 10ft05 AGTP-2 LegendApproximate location of cross section Approximate limits of slope failure Approximate location of test pit A A’ Original topo per As-Graded Plan by Leighton & Assoc., Inc. (8/29/84) Geotechnical Map Colinas De Oro HOA Carlsbad, CAF.N. 24187.01 Figure 2 AmericanGeotechnical,Inc. 7929 Calle Madrid N 160 ? ? AGTP-2 AGTP-1 As-graded topo7925 Calle San Felipe Failure surface Slope failure debris Pre-development topo Sidewalk Pale yellow sand lifts Per As-Built Grading Plansll key = 5’ deep x 20’ wide Clayey Fill Clayey Fill Clayey Fill Del Mar Formation Mudstone with inter-bedded Sandstone Existing ground cracks A’A 155 160 165 170 135 140 145 150 155 160 165 170 135 140 145 150 N56W Approx. scale: 5ft0 LegendAGTP-2 Approximate location test pit GeologicCross Section A-A’ Colinas De Oro HOA Carlsbad, CAF.N. 24187.01 Figure 3 AmericanGeotechnical,Inc. 7929 Calle Madrid 1 l ~ ......... ......... __ __ ------------------- -- L 7 □ 7925 Calle San Felipe =--- )> -»- .. • American Geotechnical, Inc. F.N. 24187.01 --- - A L A' A 0 5 1 Oft I I I I Approx. scale: Approximate location of proposed Schedule 40 PVC back drain repair and direction of flow Approximate extent of proposed keyway Approximate extent of proposed repair zone, minimum 5 ft. beyond slope failure area. Re-grade slope surface to match the surrounding area slope Approximate location of proposed chimney drain Approximate location of test pit Approximate location of cross section Original topo per As-Graded Plan by Leighton & Assoc.,lnc. (8/29/84) Approximate limits of slope failure Conceptual Slope Repair Map Figure Colinas De Oro HOA 7929 Calle Madrid Carlsbad, CA 4 170 165 160 155 150 145 140 A Roll geog rid up if bench width is less than 3m (min. 1 0') wide I 7925 Calle San Felipe I ------ Head scarp Clayey Fill Bench height to be approx. 2-3'and horizontal cut-back approx. 3-5' and should be tipped back at approx. 10% ,___ ~--------~ --~--=--- Mirafi BXG 120 (roll parallel to slope face) or N 5( Synteen SF35 (roll and cut perpendicular to slope) ---or other approved geog rid@ 18" max. vertical spacing, tip at 10% into slope. Overlap splices at min. of 6''. t ng ground crdc ks ] Fa:1. ·--c;urface ADS 00C2TT (Coconut fiber) Geosynthetics Erosion Control Blanket, ECB. Anchor with 3/8 x 1 0" bright spikes with pan washers at 48" max. spacing. Lap fabric 6" at splices. I Slope failure deibric, r-1~P_a_le_ye_l_lo_w_sa_n_d_lif_ts_~ 1-------___ 7_ Chimney drain system:::: 30' o.c., construct with 3/4" rock in geofabric bags I // ,---~~-~-_,,-i::>velopment topo Tightline to (e.g. Mirafi 140N or similar) Del Mar Formati Mudstone wiH inter-bedded Sandstone Backdrain construction: :::: 8'vertical spacing 4" diameter perforated Schedule 40 PVC drain pipe installed with perforations down and encased in 2 cu.ft. per linear foot of open graded gravel wrapped in geo-filter fabric (e.g. Mirafi 140N or similar) Maintain positive drainage pitch (4% min.) '--------------------' 7 suitable drainage outlet 19 ----I I 1." ,~ ~~ " ~~~~~ .. M i@;,lf¥ ~ ~m (m in. 1 0') Clayey Fill ~----------------___ _J A' 170 165 160 155 150 -- 145 Sidewalk 140 135 -;,---------------------....... -------------....... -----------------------------------------'!-135 0 5ft Approx. scale: • American Geotechnical, Inc. F.N. 24187.01 Cross Section A-A' Repair Concept Colinas De Oro HOA 7929 Calle Madrid Carlsbad, CA Figure 5 File No. 24187-01 August 3, 2020 Page 3 3.0 LABORATORY TESTING Soil samples collected from the exploratory excavations were transported in sealed containers to our laboratory for testing of their engineering properties. Tests performed included intact moisture and dry density, laboratory maximum dry density and optimum moisture content (proctor test), Expansion Index, and direct shear strength. Laboratory test summaries are presented in Appendix B. 4.0 SITE AND STABILITY ANALYSIS Laboratory testing of samples judged typical of the site soil encountered during exploration indicates somewhat nonlinear shear-strength characteristics with disproportionally low strength at lower confining pressures for the fill soil when remolded. As can be seen from the direct shear test plots in Appendix B, soil strength obtained by slow shearing of saturated samples at low confining pressure was found to be about zero cohesion and 45 to 47 degrees of friction angles. To be conservative and for the purpose of design and analysis, a zero cohesion and 45 degree of friction angle has been utilized in the subsequent surficial stability analysis as well as for the gross stability analysis for the shear strength of fill and/or imported soil materials. A 300 pounds per square feet (psf) cohesion and 45 degrees of friction angle representing overall shear strength of the Del Mar Formation Mudstone with Inter-bedded Sandstone were also utilized in the subsequent gross stability analysis. For surficial stability analysis, a theoretical minimum factor-of-safety, equal to 0.96 has been estimated for the slope with a five-foot deep slump. This very low factor-of-safety result indicates a condition of probable failure in response to saturation and seepage. Since the theoretical factor-of-safety is typical less than 1.0, failure would occur even before mobilizing the full seepage conditions assumed in the analysis. Result of our surficial stability analysis is presented in Appendix C. Gross (long-term) stability analysis was performed utilizing commercial software, GSTABL7 with STEDWin, Version 2.005.3. Gross stability analyses were conducted using the Spencer Method and Circular Search to determine factors-of-safety under both static and pseudo-static conditions for the proposed slope repair with geogrid reinforcement to rebuild the slope and restore the failed slope back to its original, pre-failure slope configurations. Results of the gross stability analyses indicated that the factors-of-safety to restore the failed slope back to its original, pre-failure slope configurations with geogrid reinforcement utilizing Mirafi BXG 120 geogrid is 2.347 and 1.643 respectively, for static and pseudo-static (seismic) conditions. File No. 24187-01 August 3, 2020 Page 4 These calculated factors-of-safety are higher than the minimum code requirements of 1.5 and 1.1, respectively, for the static and pseudo-static conditions. Results of our gross (long-term) stability analyses under static and pseudo-static conditions are presented in Appendix C. 5.0 CONCLUSIONS AND RECOMMENDATIONS 5.1 BASIS Conclusions and recommendations contained in this report are based upon information provided, information gathered, investigation conducted, engineering and geologic evaluations, as well as our experience, and professional judgment. Recommendations contained herein should be considered minimums consistent with industry practice. More rigorous criteria could be adopted if lower risk of future problems is desired. 5.2 CONCLUSIONS Based on our evaluation of the site, and our experience and judgment, it is our opinion that the slope failure that occurred in March 2020 at the subject site is a result of the failure of weak soil triggered by saturation of the near-surface soil. The near-surface soil likely absorbed a significant amount of rainwater from the accumulated record amount of rainfall that hit the area this past rainy season, which likely weakened it to the point that the slope failure occurred. The somewhat flatter angle of the failure surface is an indicator of both soil weakness and a significant groundwater influence to trigger the failure. 5.3 TREATMENT CONCEPTS In order to provide the slope with a substantially lower risk of future problems, the low soil strength must be addressed. Strength could be added to the soil in a number of ways. Slope soil could be excavated and replaced with a better quality imported soil, but American Geotechnical’s experience with southern California soil types suggests that not much practical improvement could be generated by importing better soil. The soil strength could be augmented by excavating the soil, blending in cement, and then re-compacting. This process is both expensive and time consuming to properly implement. Furthermore, the resulting finished slope surface is difficult to landscape. File No. 24187-01 August 3, 2020 Page 5 Another method for strengthening the soil involves layering with high-strength "geogrid" designed to reinforce soil in a manner analogous to how steel reinforces concrete. American Geotechnical introduced this method of slope repair to southern California in 1986. For various reasons and our past experience, we believe a geogrid repair is the preferred method for improving the surficial stability condition of the slope. As aforementioned, not much success can be experienced by importing the soil. Reinforcing the soil with high- strength geogrid can be typically done relatively quickly, with less expense and with no associated landscape problem, such as can be expected with soil cement. 5.4 CONCEPTUAL REPAIR RECOMMENDATIONS To enhance future performance and improve the surficial slope stability of the slope, we recommend that the slope be repaired using geogrid reinforcement. The existing failed slope, on the order of about 2:1 (horizontal: vertical), can be improved by installing layered geogrid to enhance the soil strength, and the slope can be rebuilt and restored back to its original configurations. Schematics of the proposed slope repair concept with geogrid reinforcement are presented in Figures 4 and 5. The entire failed slope area plus about 5 feet beyond the actual slope failure boundary should be treated. Actual limits of the slope repair should be verified in the field by the project engineering geologist after the slope vegetation has been removed. Non-failed areas could have some risk of failure in the future, but absent any signs of imminent failure they need not be improved at this time. The details shown in Figure 5 illustrates the layering with a high-strength geogrid, Mirafi BXG120 manufactured by Tencate Geosynthetics America, at 18-inch maximum vertical spacing, as well as associated drainage recommendation details. Calculations indicate that the minimum width of geogrid material should be at least 3 meters (about 10 feet). Actual geogrid widths might need to be wider depending upon the conditions encountered and the actual widths of the bench cuts made for the repair. Geogrid design calculations are presented in Appendix D. A minimum 3 meter (about 10 feet) wide and 2- foot deep keyway should be excavated at the toe of the failure to support the overlying repair as shown on the slope repair details. The actual key location and dimensions could vary as field conditions dictate. The bottom of the keyway may need to be deepened and should be field verified by the project engineering geologist to confirm that the key bottom is firm and unyielding prior to receiving any fill placement. The key and all subsequent bench cuts, geogrid layers, and fill layers should ideally be tipped slightly back into the slope as shown on the slope. Under no circumstances should they be tipped forward in the out-of-slope direction. 150 160 El. 168.4 El. 169.4El. 167.3 AA’AGTP-1 AGTP-2 Keyway Slope failure Chimney drains Chimney drains Sch 40 PVC back drain Extent of repair Tightline to suitable drainage outlet H.P.3m (min.10’)30’ 4%4% min.5’ min.5’ H.P. H.P. H.P. H.P. H.P. 7925 Calle San Felipe Calle Madrid Approx. scale: 10ft0 5 AGTP-2Legend Approximate location of cross section Approximate limits of slope failure Approximate location of test pit A A’ Original topo per As-Graded Plan by Leighton & Assoc.,Inc. (8/29/84) Conceptual Slope Repair Map Colinas De Oro HOA Carlsbad, CAF.N. 24187.01 Figure 4 AmericanGeotechnical,Inc. 7929 Calle Madrid N 160 Approximate location of proposed Schedule 40 PVC back drain repairand direction of ow Approximate extent of proposed keyway Approximate extent of proposed repair zone, minimum 5 ft. beyondslope failure area. Re-grade slopesurface to match the surroundingarea slope Approximate location of proposed chimney drain C) -- A .. . .. ? ? AGTP-2 AGTP-1 As-graded topo7925 Calle San Felipe Failure surface Slope failure debris Pre-development topo Sidewalk Pale yellow sand lifts Per As-Built Grading Plansll key = 5’ deep x 20’ wide Clayey Fill Clayey Fill Clayey Fill Del Mar Formation Mudstone with inter-bedded Sandstone Existing ground cracks A’A 155 160 165 170 135 140 145 150 155 160 165 170 135 140 145 150 N56W 5’ Head scarp Backdrain construction: 8’ vertical spacing 4” diameter perforated Schedule 40 PVC drain pipe installed with perforations down and encased in 2 cu.ft. per linear foot of open graded gravel wrapped in geo-lter fabric (e.g. Mira 140N or similar) Maintain positive drainage pitch (4% min.) Chimney drain system 30’o.c., construct with 3/4”rock in geofabric bags (e.g. Mira 140N or similar) Bench height to be approx.2-3’ and horizontal cut-back approx. 3-5’ and should be tipped back at approx. 10% Roll geogrid up if benchwidth is less than 3m(min. 10’) wide Mira BXG 120 (roll parallel to slope face) orSynteen SF35 (roll and cut perpendicular to slope)or other approved geogrid @ 18” max. vertical spacing, tip at 10% into slope. Overlap splices at min. of 6”. ADS 00C2TT (Coconut ber) Geosynthetics Erosion Control Blanket, ECB. Anchor with3/8 x 10” bright spikes with pan washers at 48” max. spacing. Lap fabric 6” at splices. Min. 4% Min. 4% Min. key width 3m (min. 10’) Min. 2’ Tightline to suitable drainageoutlet Approx. scale: 5ft0 Cross Section A-A’Repair Concept Colinas De Oro HOA Carlsbad, CAF.N. 24187.01 Figure 5 AmericanGeotechnical,Inc. 7929 Calle Madrid r I __ ---------1 I ~ I~ --1-=-l~.::::'r'" --~,~ ~---+~=-:-::r~~~~ ~~~~~~ --~ I r ----, --------------------- -------- -- 7 _J File No. 24187-01 August 3, 2020 Page 6 Geogrid is a polymer-grid material which has very high tensile strength. This open grid structure allows it to be embedded within the compacted fill material so that it can grip the soil, thereby transferring the geogrid strength to the soil. When geogrid is utilized in a slope repair, the engineering concept is to physically tie the weak soil zone at the face of the slope to the material which is more competent at depth. Geogrid is available in rolls that are relatively easy to handle. To install geogrid, layers are rolled out and trimmed to fit the dimensions of the repair. Compacted fill is placed on top of the grid taking reasonable care not to damage the grid during soil placement and compaction operations. The initial soil layer placed on top of the geogrid layers must be placed carefully so that the grid is not damaged or torn by the earthmoving equipment. Experience has shown that this process does not produce any particular problems in the grading process. The grid has proven to be quite tough. Generally, in small slope repairs, relatively lightweight equipment can be utilized. The geogrid should be laid out with the lead edge at the proposed slope face. Horizontal fill placement should extend beyond the slope face (e.g., about 1 foot) to support the compaction effort. After fill placement, entirely or possibly at about 4-foot increments, the slope face should be trimmed back to compacted soil and the lead edge for the geogrid. The existing failure area plus about 5 feet on each side of the failure area should be removed and replaced with geogrid-reinforced compacted fill. Non-failed areas will have some risk of failure in the future, but that risk can be reduced if the water sources contributing to the current failure are mitigated or controlled. Tencate Mirafi BXG120 is a bi-axial geogrid product, the strong grid direction is across the roll, and as such, the product can be simply rolled out along the slope and cut-off where needed. Additionally, at least a 6- inch overlap should be provided. At the time of construction, the contractor should verify in the field that all slide debris needs to be removed and replaced. If the slide debris width is more than 3 meters wide, actual geogrid lengths should be increased beyond the actual slide debris removal. Because the Mirafi BXG120 products are strong in the across roll direction, less cutting/fitting of the geogrid will be required for the Mirafi BXG120 products. If the total width between the face of the slope and the back-cut benching is less than 3 meters, the excess grid should be laid up the back-cut then folded over the next geogrid layer. In addition to the geogrid, a subsurface drainage system (i.e., backdrains and chimney drains) is recommended to mitigate the potential of future seepage and thereby further improve the stability of the repaired area. Experience has shown that Mirafi BXG120 geogrid is readily available, but the manufacturer should be consulted for availability and delivery time. Similarly, the contractor should check availability for the geofabric for the gravel backdrains behind the compacted fill mass and the chimney drain material. File No. 24187-01 August 3, 2020 Page 7 The product is often more available in 4-meter width so inventory should be checked as soon as possible in case the product needs to be shipped from the factory to the local supplier. Among geogrid products, we recommend Mirafi BXG120 geogrid be utilized for the slope repair for this project. Any proposed alternative geogrid product should be equal or better than Mirafi BXG120 in all respects and may be considered subject to review and acceptance by American Geotechnical prior to purchase and delivery to the site. American Geotechnical has often used higher strength geogrid, such as Synteen SF35, manufactured by Synteen Technical Fabrics on various projects. This product is stronger along the roll so more cutting and arranging is necessary when Synteen SF35 geogrid is used. Excavated onsite soil can be reused as fill material for the repair so long as it is cleaned and free of all vegetation and organic debris, trash, and oversized rocks greater than 6-inches in diameter. Some import soil might be required to supplement the onsite soil for fill material to complete the repair. All fill soil should be moisture conditioned, placed, and compacted to in accordance with good construction practice, agency requirements, and recommendations from the geotechnical consultant at the time of grading. Fill should be placed in thin, level lifts no greater than 8-inches in loose thickness before being compacted. Each lift should be compacted to the minimum acceptable relative compaction of 90 percent of the laboratory maximum dry density for each soil type used, as determined in accordance with the latest ASTM Method D1557. General geotechnical guidelines for grading projects are attached hereto as Appendix E. Imported soils, if utilized, should consist of silty sand or clayey sand with the following criteria: 1.No particles larger than 6 inches in largest dimension; 2.Free of perishable material; 3.Plasticity Index of 20 or less and Liquid Limit of 40 or less; 4.Expansion Index of 20 or less. Compacted fill placement should be in accordance with good construction practice, agency requirements, and recommendations for the geotechnical consultant at the time of grading. Minimum acceptable relative compaction1 is 90 percent of the laboratory maximum density, as determined in accordance with the latest edition of ASTM Method D1557. The contractor should be responsible for protection of improvements in the adjacent areas and to have all utilities marked out before the earthwork begins. 1 Relative compaction refers to the ratio of the in-place dry density of soil to the maximum dry density of the same material as obtained by the "modified proctor" (ASTM D1557-12e) test procedure. File No. 24187-01 August 3, 2020 Page 8 Upon completion of the slope construction, the slope face should be covered with erosion control material. We recommend ADS OOC2TT 100% coconut fiber erosion control blanket (ECB) material. The blanket material should be overlapped at splices at least 6 inches and anchored with 3/8-inch by 10-inches bright spikes fitted with 3/8-inch by 2-inch fender washers. These anchor pins should be spaced at 4 feet maximum each direction. 6.0 CONSTRUCTION CONSIDERATIONS 6.1 PERMITTING & PLANNING The contractor selected for the repair work should obtain all necessary permits required by the City/County authority. The contractor should include the cost of obtaining these permits as part of the scope of work. The repair should be planned in advance to determine the best access and staging areas for equipment and materials and to secure any necessary permissions that may be needed to access the site. 6.2 UTILITY LOCATION All utilities should be located prior to the start of the repair work. Any utilities located should be marked and protected during the repair operation. 6.3 PROTECTION OF EXISTING APPURTENANCES AND LANDSCAPING Any appurtenances and landscaping located adjacent the slope repair area should be protected from damage during the slope repair. Some existing landscaping will likely be affected by the repair and will need to be repaired or replaced after the repair is completed. Following grading but before landscaping, erosion control material should be anchored to the slope surface in a water-management, overlapping style. Specifications for the erosion control material and installation are presented in Figure 5. 6.4 DEBRIS AND VEGETATION REMOVAL All deleterious debris and vegetation located within the area of slope repair should be removed from the site and disposed of in a municipal landfill. File No. 24187-01 August 3, 2020 Page 9 6.5 SLOPE FAILURE REMOVAL The entire slide mass must be excavated and removed to below the basal slide plane. A licensed engineering geologist should observe and guide the removal process to ensure that the entire slide mass is removed before the slope is re-built. Eroded soil that is not contaminated with debris or vegetation can be temporarily stockpiled for re-use as fill material for the slope repair. If needed, import soil can also be used for the repair provided that it is either non-expansive or very low expansive (EI <20) and is free of organics and other deleterious material. Laboratory testing of the engineering properties of the import fill material should be performed to verify that it is acceptable for use in the repair. 7.0 AGENCY REVIEW All aspects of the proposed slope repair are subject to the review and approval of the governing agency(s). It should be recognized that the governing agency(s) can dictate the manner in which the project proceeds. 8.0 FIELD CONSTRUCTION REVIEW During implementation of any of the recommended slope repairs, representatives from American Geotechnical should be present at the site to perform field observation and testing. A separate contract will be provided to you for our observation and testing services during the slope repair. This process will ensure that repairs are in conformance with the recommendations presented in this report. Inspections should include the key, benching, geogrid, drain placement, and compaction. At least a 24-hour notice is required for site services. A 48-hour notice is preferred. 9.0 PROJECT SAFETY The contractor is the party responsible for providing a safe site including temporary excavation stability. This consultant will not direct the contractor's operations and cannot be responsible for the safety of personnel other than his own representatives on site. The contractor should notify the owner if he/she is aware of and/or anticipates unsafe conditions. If the geotechnical consultant at the time of slope repair considers conditions unsafe, the contractor and the owner's representative will be notified. File No. 24187-01 August 3, 2020 Page 10 Within this report the terminology, safe or safely, may have been utilized. The intent of such use is to imply low risk. Some risk will remain, however, as is always the case. 10.0 REMARKS This report has been prepared for the sole use and benefit of our client. The intent of this report is to advise our client on earth science matters involving the proposed slope repair. It should be understood that the geotechnical consulting provided and the contents of this report are not perfect. Any errors or omissions noted by any party reviewing this report and/or any other geotechnical aspect of the project should be reported to this office in a timely fashion. Conclusions and recommendations presented herein are based upon the evaluation of technical information gathered, experience, and professional judgment. Other consultants could arrive at different conclusions and recommendations. Final decisions on matters presented are the responsibility of the client and/or the governing agencies. This work was performed in accordance with generally accepted professional geotechnical engineering principles and practice in southern California at the present time. We make no other warranty either expressed or implied. File No. 24187-01 August 3, 2020 Page 11 REFERENCES Reports “Report of Geotechnical Investigation (Phase i) La costa Areas SE18 Thru SE21 for the Meister Company” prepared by Shepardson Engineering Associates, Inc., dated April 21, 1981. “Report of Geotechnical Investigation Phase II Proposed Vista Santa Fe Subdivision, Carlsbad, California” prepared by Shepardson Engineering Associates, Inc., dated June 8, 1983. “Grading Plan Review and supplemental Geotechnical Investigation for Phase A, Vista Santa Fe and Commercial Area SE-17, Vista Santa Fe, Portions of Carlsbad Tract 81-16, City of Carlsbad, California” prepared by Leighton and Associates, dated October 5, 1983. “Volume II Geotechnical Report of Rough for Parcel A, Units 1-2 and 4 Building Pads 1 Through 110 and 123 Through 131, A portion of Tract 81-16 Vista Santa Fe, City of Carlsbad, California” prepared by Leighton and Associates, dated August 23, 1984. Topographic & Geologic Maps “County of San Diego Topographic Survey Map Sheet 330-1695”, dated July 1960. “County of San Diego Topographic Survey Map Sheet 330-1695”, dated 9-18-1975. “DMG Open File Report 96-02 Geologic Map of the Encinitas and Rancho Santa Fe 7.5 Quadrangles Plate 2”, by Siang S. Tan and Michael P. Kennedy, 1996, California Division of Mines and Geology.