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HomeMy WebLinkAboutPD 2020-0036; PLAZA BONITA SLOPE REMEDIATION; RESPONSE TO COMMENTS TO THIRD PARTY GEOTECHNICAL REVIEW AND SUPPLEMENTAL GEOTECHNICAL INVESTIGATION OF SLOPE FAILURE 2210 PLAZA BONITA CARLSBAD CA 92009; 2020-11-18i1B Geotechnical Engineer 4596 Mount Hubbard Avenue, San Diego, CA 92117 Phone/Text: 619-813-8462 Email: martinowen@geotechenglneer.com Job No. 200506 November 18, 2020 To: Mr. David .Rick City of Carlsbad, Land Development Engineering 1635 Faraday Avenue Carlsbad, California 92008 Subject: Response to Third-Party Geotechnical Review and Supplemental Geotechnical Investigation of Slope Failure 2210 Plaza Bonita S Carlsbad, California 92009 Project JTh PD2020-0036 References Geotechnical Inspection of Slope Failure, 2210 Plaza Bonita, Carlsbad, California 92009, by Martin R. Owen PE, GE, dated June 1, 2020 Cut/Fill Quantity for Repair of Slope Failure, 2210 Plaza Bonita, Carlsbad, California 92009, by Martin R. Owen PE, GE, dated September 22, 2020 Grading Plans for Slope Repair, 2210 Plaza Bonita, by The Sea Bright Company, dated October 5, 2020 (Sheets 1 through 4) Third-Party Geotechnical Review (First), Slope Failure, 2210 Plaza Bonita, Carlsbad, California 92009, Project ID: PD2020-0036, by Hetherington Engineering, Inc., dated October 29, 2020, Project No. 9236.1, Log No. 21188 Dear Mr. Rick: Please find herein my response to the third-party geotechnical review issues listed in the review letter by Hetherington Engineering, Inc. referenced above. I have also conducted a supplemental geotechnical investigation, which is presented at the end of this response. My previous geotechnical inspection• report, referenced above has been modified where necessary to reflect the response. Stephen E. Jacobs, CEG, Engineering Geologist contributed to the supplemental investigation and report. Response to Third-Party Geotechnical Review and Supplemental Geotechnlcal Page 2 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 RESPONSE TO HETHERINGTON REVIEW ISSUES Issue #1 - The Consultant should review the project grading plan, provide any additional geotechnical recommendations considered necessary, and confirm that the plans have been prepared in accordance with the geotechnical recommendations provided in the referenced report. Response: I have reviewed the project grading plan prepared by Bob Sukup, PE and confirmed that the plan has been prepared in accordance with the geotechnical recommendations provided in the referenced report, dated June 1, 2020. Issue #2 - The Consultant should provide an updated geotechnical map/plot plan utilizing the latest grading planfor the project to clearly show (at a minimum), a) existing site topography, b) proposed finished grades, c) locations of the subsurface exploration, d) geologic contacts, and e) remedial grading limits, etc. Response: I have provided an updated plot plan and geotechnical map and a grading plan and geotechnical map presented herein as Figures 2A and 2B, respectively. These maps show the existing site topography, proposed finished grades, locations of exploratory test pits, geologic contacts, and limits of proposed remedial grading on Figure 2A. Issue #3 - The Consultant should provide geologic cross-sections utilizing the current grading plan to clearly show (at a minimum), a) existing site topography, b) proposed finished grades, c) geologic contacts, d) geologic structure, e) locations of the subsurface exploration, f) temporary construction slopes, and g) remedial grading, etc. Response: I have provided two geologic cross-sections utilizing the current grading plan presented herein as Figures 4 and 5. These cross sections show the existing site topography, proposed finished grades, geologic contacts, geologic structure, locations of the exploratory test pits, temporary construction slopes, and limits of proposed remedial grading, Issue #4 - The Consultant should perform field exploration, laboratory testing, and analyses to provide a basis for the slope repair recommendations. Response: I performed site observations of the slope failure area and four exploratory test pits, laboratory testing, and analyses to support the slope repair recommendations. Issue #5—The Consultant should provide recommendations for temporary excavations. Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnlcal Page 3 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 Response: Recommendations for temporary excavations shown on the two geologic cross sections A-A' and B-B' (Figures .4 and 5, respectively) are presented on Figure 7- Slope Repair. Issue #6—The Consultant should address the surficial stability of the proposed repair slope. Response: The proposed repair slope should be surficially stable (FS greater than 1.5) provided the recommendations presented herein are implemented. Issue #7 - The Consultant should provide a statement regarding the impact of the proposed grading on adjacent properties and Improvements. Response: The proposed remedial grading of the failed slope should have a minimal impact on the adjacent properties and improvements. Issue #8— The Consultant should provide a list of recommended observation and testing during site grading and construction. Response: The recommended observation and testing during site grading and construction will include observations and testing of the slope excavation, filling and compaction. Observation and testing services will be provided by East County Soil Consultation & Engineering, Inc., 619- 258-7901. Issue #9 - The Consultant should provide a list of published maps/reports used in the preparation of the report. Response: I have provided a list of published and unpublished maps/reports used in the preparation of this report. This list is presented at the end of this report. SUPPLEMENTAL GEOTECHNICAL INVESTIGATION In accordance with the letter of third-party geotechnical review dated October 29, 2020 prepared by Hetherington Engineering, Inc., I have conducted a supplemental geotechnical investigation for the proposed slope repair to be constructed on the west-facing slope adjacent to the lot at 2210 Plaza Bonita, Carlsbad, California. My preliminary findings indicate that the site has several geotechnical-related issues that will impact project development. I provide herein my assessment of site conditions and recommendations for the proposed remedial grading of the failed slope. This assessment is subject to change as new data is developed. The scope of work performed for this supplemental Investigation was as follows: Martin R. Owen PE, GE Geotechnlcal Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 4 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 Review of available grading and slope repair plans. Review of available geologic literature and geotechnical reports associated with the tract development. Subsurface soil exploration and laboratory soil testing. Geotechnical analysis and preparation of my response to the third-party review and supplemental geotechnical investigation report summarizing my findings and conclusions. SITE AND PROJECT DESCRIPTION The subject slope is a steep (approximately 136:1 H:V) west-facing, graded fill slope, about 15 feet high, that descends from the west side of the subject residence down to a lower level and swimming pool. The residence (labeled Building #8 on the grading plan, Figure 2B) is the westerly unit of a two- story, multi-unit townhome building constructed in 1971. The building is (mostly) supported on conventional shallow footings and has lower level slab-on-grade floors. However, you indicated that the subject residence previously experienced foundation settlement and was stabilized by underpinning with pipe piles or other deep foundations. The date and extent of the foundation repairs are not known. You also indicated you recently inspected the interior of the residence and no building damage was observed. In addition, your insurance company sent an engineer to inspect the building and no damage was reported. Slope vegetation is moderate to good and consists of several shrubs and ivy and other ground cover. Reportedly, there was a recent leak in a sprinkler line near the top of the slope. It is not known how long the pipe was leaking. The leak is believed to be factor in the slope failure. Reportedly, also, there was a rupture of the sewer line, currently exposed on the back cut near the toe of the slope; the sewer line was reportedly repaired. I have previously listed my observations regarding the slope failure in the above-referenced report dated June 1, 2020. SUBSURFACE SOIL EXPLORATION A subsurface soil exploration was conducted on November 12, 2020 and consisted of hand excavating four test pits on the slope. The approximate locations of the test pits are shown on the attached Figures 2A and 2B. The logs of the test pits are presented on the attached Figure Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geôtechnical Page 5 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 3. Two cross-sections through the subject slope are provided as Figures 4 and S. The test pits were excavated to a depth of approximately 2 feet. The test pits were excavated and logged by Stephen E. Jacobs, CEG, Engineering Geologist SUBSURFACE SOIL CONDITIONS The area of the slope failure Is underlain by a relatively thin layer of the recent landslide deposit, underlain by undocumented artificial fill, in turn underlain, at unknown depths, by sedimentary bedrock that is mapped by Tan and Kennedy (1996) and Kennedy and Tan (2007) as the Eocene age Santiago Formation (Figures 2A, 2B and 6). Tan and Kennedy (1996) described the Santiago Formation as "Light-colored, poorly-bedded, poorly-indurated, fine- to medium-grained sandstone interbedded with landslide-prone siltstone and claystone." Materials of the Santiago Formation were not encountered in our exploratory test pits. The attached test pit logs (Figure 3) should be referenced for a complete description of the materials encountered. A brief description of these units is described in order of increasing age. Landslide Deposit (Qal): Disturbed fill soils of the landslide deposit involved in the slope failure were observed In Test Pits 1 and 2, and in the temporary back cut near the toe of the slope failure. The landslide deposit extends from a thickness of approximately 1 foot just below the head scarp to approximately 2 feet at the top of the temporary back cut near the toe of the slope. The landslide deposit encountered consists of dark brown and olive gray, slightly moist to moist, soft, clayey sand and clayey silt with numerous angular clayey siltstone rock fragments. This material will require removal and recompaction in the area of the proposed remedial grading of the slope failure. Published geologic maps by Tan and Giffen (1995), Tan and Kennedy (1996) and Kennedy and Tan (2007) reported no ancient landslides in the vicinity of the subject site. Artificial Fill (Qaf): Undocumented fill was observed in all four exploratory excavations and in the temporary back cut near the toe of the failed slope. The fill soils encountered consist of medium to dark olive gray and brown, moist to very moist, firm to stiff, sandy clay, clayey silt and silty clay. The upper approximately 1 to 2 feet of fill soils exposed in the temporary back cut near the slope toe were observed to have numerous desiccation cracks. Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geótechnical Review and Supplemental Geotechnical Page 6 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 GEOLOGIC STRUCTURE AND BEDDING Regional bedding within the Santiago Formation is not mapped by Tan and Kennedy (1996) and Kennedy and Tan (2007) within the immediate vicinity of the subject site. The nearest mapped bedding attitudes from Tan and Kennedy (1996, Figure 6) show dips of 5 degrees to the west- northwest; this bedding dip angle is projected onto the geologic cross sections on Figures 4-5. GROUNDWATER Groundwater was not encountered in the exploratory test pits. However perched groundwater may develop on the subject slope due to landscape irrigation and excessively heavy rainfall. CONCLUSIONS Base on the findings of this supplemental geotechnical investigation, the following conclusions have been developed: The landslide deposit appears to range from approximately 1 to 2 feet in thickness in the area of the slope failure. The landslide occurred in undocumented artificial fill. Possible removal depths are provided on the attached Figure 4, Geologic Cross Section A-A', and Figure 5, Geologic Cross Section B-B'. The primary cause of the slope failure appears to be saturation of the slope soils and loss of shear strength resulting from a combination of recent heavy rainfall and a leaking sprinkler pipe and/or a leaking sewer pipe. The steep slope and poor slope fill compaction appear to be other factors in the failure. The slope failure is shallow and relatively minor and the subject residence does not appear to have been damaged or threatened by the failure. RECOMMENDATIONS The slope failure should be repaired and the slope restored to its original grade and configuration, as follows: The slope can be repaired using small grading equipment or hand equipment. The existing slope vegetation within the slope restoration area should be entirely removed. Some of the shrubs may be salvageable and can be reused. Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 7 Investigation of Slope Failure 2210 Plaza Bonita S Job No. 200506 Carlsbad, CA 92009 November 18, 2020 The accumulated soils at the bottom of the slope should be remàved and temporarily stockpiled on the nearby sidewalk or other area. In addition, prior to placing fill, a 5-feet wide minimum (measured into slope) fill key, at least 2 feet deep, should be excavated at the bottom of the slope (see attached Figure 7). As grading proceeds to the north, where there has been less slope displacement, it may be possible to excavate the fill key further up the slope (instead of at the bottom). The onsite soils may be reused as fill, provided they are free of organics and debris and rocks and cobbles over 6 inches in dimension. Any import fill soils should be approved *by the undersigned or other soil testing company. It is understood that East County Soils will be performing the fill compaction testing and grading observation. Fill soils should be placed and compacted in 6 to 8 inches thick lifts/layers starting at the bottom of the fill key and working upwards. The fill should be moistened or dried as necessary to slightly above optimum moisture conditions and compacted by mechanical means to at least 90 percent relative compaction based on ASTM D 1557. The 5-feet wide minimum fill width should be maintained as the fill is brought up. The fill should be benched into the original slopesoils in 2 feet minimum, vertical benches (see attached Figure 7). The upper portion of the slope should be overbuilt and cut back, or compacted by other means. Horizontal layers of geogrid should be installed at 4 feet maximum vertical increments to provide extra strength. The slope grading and compaction and placement of geogrid should be observed and tested, as necessary by the undersigned or other soil testing company. SLOPE LANDSCAPING The slope should be re-landscaped with shrubs and deep-rooted ground cover such as ivy. Moderate irrigation will be required to establish the landscaping Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 8 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 LIMITATIONS The conclusions and recommendations presented in this report may be modified as necessary during additional investigation and/or construction and are subject to field conditions. if significant modifications are made to the slope repair plans, revision of this report may also be necessary. This investigation is not to be considered a complete review of the geotechnical conditions, nor an investigation of latent conditions that have not manifested damage to date. The above conclusions and recommendations are based on my inspection, limited subsurface exploration, and experience with other, similar properties in San Diego County. This report provides no warranty, either expressed or implied, concerning future building or slopedperformance. Future damage from geotechnical or other causes is a possibility. This opportunity to be of service is appreciated. If you have any questions, please do not hesitate to call or contact me. Very truly yours, Martin R. Owen PE, GE Geotechnical Engineer Stephen E. Jacobs, PG, CEG Engineering Geologist C Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 9 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 ATTACHMENTS: References Figure 1—Site Location Map Figure 2A - Plot Plan and Geotechnical Map Figure 2B - Grading Plan and Geotechnical Map Figure 3—Test Pit Logs (TP-1 through TP-4) Figure 4— Cross Section A-A' Figure 5— Cross Section B-B' Figure 6— Regional Geologic Map Figure 7—Slope Repair LIST OF PUBLISHED MAPS/REPORTS USED IN THE PREPARATION OF. THIS REPORT: County of San Diego, 1960, Topographic Survey, Sheet 338-1689, scale: 1" = 200'. County of San Diego, 1969, Grading Plans for Subdivision of Lot No. 227, La Costa Valley Unit No. 4, Map No. 5781, 2 sheets. County of San Diego, 1976, Topographic Survey, Sheet 338-1689, scale: 1" = 200'. Kennedy, M.P., and Tan, S.S., 2007, Geologic Map of the Oceanside 30' x 60' Quadrangle, California: California Geologic Survey, Regional Geologic Map Series, Map 2, scale 1:100,000. Soil Engineering Construction, 1989, Geotechnical Consultation for Remedial Repairs and Stabilization Revised, La Costa Village Condominiums, La Costa, California, Exhibit G, dated August 14, 1989. Solid Rock Engineering, Inc., 2007, Limited Phase I Geotechnical Engineering Evaluation for 2208 and 2212 Plaza De Las Flores, Updated to Include 2202, 2204, 2206, 2210, and 2214 Plaza De Las Flores, La Costa Village Condominium Complex, Carlsbad, California 92009, Project No. 61000268-03, dated June 22, 2007. Solid Rock Engineering, Inc., 2007, Limited Geotechnical Engineering Evaluation Report for La Costa Village Phase II Evaluation Project, 2208 and 2212 Plaza De Las Flores, Carlsbad, California 92009, Project No. 61000268-02, dated August 31, 2007. Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 10 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 LIST OF PUBLISHED MAPS/REPORTS (CONTINUED): Tan, S.S. and Giffen, D.G, 1995, Landslide Hazards in the Northern Part of the San Diego Metropolitan Area, San Diego County, California, Landslide Hazard Identification Map No. 35: California Division Mines and Geology Open File Report 95-04, pp. 1-6, Plate 35D, Encinitas Quadrangle, scale 1:24,000. Tan, S.S., and Kennedy, M.P., 1996, Geologic Maps of the Northwestern Part of San Diego County, California: California Division of Mines and Geology Open File Report 96-02, Plate 2-Geologic Maps of the Encinitas and Rancho Santa Fe 7.5' Quadrangles, scale 1:24,000. The Sea Bright Company, 2020, Grading Plans for Plaza Bonita Slope Repair (2210 Plaza Bonita), City of Carlsbad, Sheet 4, scale: 1" = 20', dated October 5, 2020. Martin R. Owen PE, GE Geotechnical Engineer Dc'n`rc's ':: OC s :NrasLCc,. ! ra F...'..A Jga "o - b i 2210 Plaza Bonita, Carlsbad, CA 92009 Spa La Costa 7esorl Spa o,~t,~ Av 7 - Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 11 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 FIGURE 1 SITE LOCATION MAP Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 14 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 FIGURE 3 TEST PIT LOGS by Stephen E. Jacobs, CEG TEST PIT TP-1 Depth Feet Soil Description Lab Results 0 - 1 Landslide Deposit (Fill): Clayey Sand (SC), fine- to medium-grained, Moisture Content brown, and Clayey Silt (ML), sandy, olive gray, numerous angular clayey = 17.6% siltstone fragments, abundant roots and rootlets to W, soft, slightly moist to moist. 1-2 Fill: Sandy Clay (CL), mottled brown and medium to dark olive gray, fine Moisture Content sandy, firm to stiff, very moist. =25.3% Bottom of Test Pit = 2 feet No Groundwater or Seepage TEST PIT TP-2 Depth Feet Soil Description Lab Results 0-1% Landslide Deposit (Fill): Clayey Silt (ML), sandy, mottled dark brown and dark Moisture olive gray, numerous angular clayey siltstone fragments, numerous roots and Content = 15.8% rootlets to 1/8", few brick fragments, soft, slightly moist to moist 1% - Fill: Clayey Silt to Silty Clay (MLJCL), fine sandy, mottled brown and medium to Moisture 1% dark olive gray, firm to stiff, moist to very moist. Content = 18.4% Bottom of Test Pit = 134 feet No Groundwater or Seepage Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnical Review and Supplemental Geotechnical Page 15 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 . November 18, 2020 FIGURE 3 TEST PIT LOGS (Continued) by Stephen E. Jacobs, CEG TEST PIT TP-3 Depth Soil Lab Feet Description Results 0—% Fill: Clayey Silt to Silty Clay (MLJCL) fine sandy, mottled brown and medium to dark olive gray, some roots and rootlets to 1I8, numerous angular dark olive gray clayey siltstone fragments, soft to firm, moist to - very moist. @ Y4, firm to stiff, very moist. Bottom of Test Pit = '/2 foot No Groundwater or Seepage TEST PIT TPA Depth Soil Lab Feet Description Results 0—% Fill: Clayey Silt to Silty Clay (ML/CL), fine sandy, mottled brown and medium to dark olive gray, some roots and rootlets to 1I8, numerous angular dark olive gray clayey siltstone fragments, soft to firm, moist to very moist. © ½', firm to stiff, very moist. Bottom of Test Pit = 1/2 foot No Groundwater or Seepage Notes: Test pits were hand excavated, logged, and backfilled on November 12, 2020. All measurements were taken from existing grade at top of test pit. Martin R. Owen PE, GE Geotechnical Engineer Response to Third-Party Geotechnlcal Review and Supplemental Geotechnical Page 19 Investigation of Slope Failure 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 November 18, 2020 FIGURE 7 SLOPE REPAIR SCHEMATIC ONLY 2210 Plaza Bonita S. S. 2' High Appro Fill Benches EAST-WEST SECTION THROUGH SLOPE Restored Slope 1.5:1 (H:V) Approx. Geogrid Layers at 4' Max. Height Intervals 15' Approx. Lower Level 2' Deep Mm. Fill Key Martin R. Owen PE, GE Geotechniccil Engineer Min. -I