The URL can be used to link to this page

Home My WebLink AboutPD 2022-0001; ISBELL RESIDENCE; RESPONSE TO THIRD-PARTY GEOTECHNICAL REVIEW; 2022-03-25 7840 EL CAJON BLVD., SUITE 200 ♦ LA MESA, CALIFORNIA 91942 Phone: (619) 462-9861 ♦ Email: clamonte@flash.net ♦ Fax: (619) 462-9859 TO: SUBJECT: REFERENCES: This report has been prepared in response to the City of Carlsbad, Third-party Geotechnical Review, by Hetherington Engineering, Inc., dated February 22, 2022. In order to provide continuity of discussion, we are responding to the memorandum’s issues in the numerical order requested in the document. 1. The Consultant should review the project grading plan (Reference 2) and foundation plans, provide any additional geotechnical analyses/recommendations considered necessary, and confirm that the plans have been prepared in accordance with the geotechnical recommendations. Based on this review we have determined that the intent of the soils reports recommendations have, in general, been incorporated into the above referenced grading plans. The recommendations provided in the referenced report are still appropriate for the plans currently under review. Our comments regarding the plan review are listed below: Job No. 21 7424 March 25, 2022 RESPONSE TO THIRD-PARTY GEOTECHNICAL REVIEW Isbell Residence Vacant Lot Adjacent to 2936 Cacatua St. Carlsbad, CA APN 215-390-35 Project No. 9621.1 Log No. 21779 City of Carlsbad Land Development Engineering 1635 Faraday Avenue Carlsbad, California 92008-7314 Report of Limited Geotechnical Investigation, Proposed Single-family Residence, Vacant Lot Adjacent to 2936 Cacatua St. , Carlsbad, CA APN 215-390-35, by C. W. La Monte Company Inc., dated August 30, 2021 Grading Plan for: Isbell Residence, Cacatua Street, by dk Greene Consulting, Inc., dated December 17, 2021 C. W. la Monte Cmnmp•J Tuttt. Jo··1 a di f'oY1 dia\to rs Cacatua Street Carlsbad, CA March 25, 2022 Page 2  The grading plan describes the cut slope along the east property line as 1.5:1 (horizontal to vertical). According to the site topographic map the inclination of the cut slope appears to range from about 1.7:1 to 2:1.  The cut slope will be retained with a masonry retaining wall as detailed on Sheet 2 of the grading plan. The wall will range from about 2 to 4.5 feet in height and retain 2:1 sloping backfill. The wall detail design appears to be from regional standard drawings, which is appropriate for use at the subject site. The standard drawing is designed for walls up to 5.33 feet in height. 2. The Consultant should provide an updated geotechnical map utilizing the current grading plan for the project to clearly show (at minimum): a) existing site topography, b) proposed structures/improvements, c) proposed finished grades, d) geologic conditions, e) locations of the subsurface exploration, 1) temporary construction slopes, g) remedial grading, etc. An updated geotechnical map is attached as Figure No. 1 using the current grading plan as a base map. 3. The Consultant should discuss geologic structure of the bedrock and if the site is considered stable. The site is primarily a cut lot into stable bedrock, which appears neutrally bedded. Excavation refusal was reached in all borings at depths ranging from 1.5 to 3 feet below the existing grade. Prior to the subdivision grading, the topography of the site and surrounding areas consisted of gentle to moderate topographic conditions with about a 9 percent slope to the west. 4. The Consultant should provide the results of the described moisture/density testing. Since the site is underlain with stable bedrock moisture-density testing was not performed. Soil design values are conservatively based on Table 1806A.2 of the California Building Code. 5. The seismic design criteria provided is for a Site Class C. The Consultant states the Site Class is D. The Consultant should update the recommendations as needed. As stated previously the lot is underlain with hard bedrock. Therefore, a Site Class of “C” is appropriate for the project site. The Site Class D stated in the report is a typo. The actual seismic design values provided in Table I of the referenced geotechnical report are for Site Class “C”, not Site Class “D” 6. The Consultant should provide the site Seismic Design Category. Refer to Page 6 of the referenced geotechnical report. Cacatua Street Carlsbad, CA March 25, 2022 Page 3 7. The Consultant should address impacts to adjacent property and improvements as a result of site grading and construction. It is our professional opinion the proposed development will not destabilize or negatively impact adjacent properties due to the underlying stable bedrock conditions. 8. The Consultant should provide recommendations for temporary excavations The proposed retaining wall construction will require temporary cuts up to 6 feet in maximum height. The foundation for the proposed wall will extend laterally 1.25 feet beyond the back of the wall (per the plans). The minimum horizontal distance between the proposed wall and the property line is about 8 feet, which will result in a minimum of 6.75 feet between the toe of the temporary cut and the property line. Cuts will be excavated into very dense, stable bedrock, which will stand well in temporary cuts. Therefore, temporary cuts may be excavated at a 0.33:1 (horizontal to vertical) inclination (or gentler) with a short vertical cut at the base of the cut to accommodate the foundation. 9. Foundation and slab design criteria for expansive soils should be consistent with Section 1808.6 of the 2019 California Building Code. The Consultant should update foundation recommendations, as necessary. The soils underlying the site are low to “borderline” moderately expansive. The recommendations for floor slabs and foundations provided in the geotechnical report are appropriate for such conditions. 10. The Consultant should address minimum slab-on-grade thickness and reinforcement recommendations. Interior concrete slabs should have a minimum thickness of four inches (actual) and should be reinforced with at least No. 3 reinforcing bars placed at 18 inches on center each way. Slab reinforcement should be placed approximately at mid-height of the slab. 11. The Consultant should address expected total and differential settlement due to grading and foundation loads. Based on our experience with the soil types on the subject site, the soils supporting the structure should experience settlement in the magnitude of less than 0.5 inch under proposed structural loads. Cacatua Street Carlsbad, CA March 25, 2022 Page 4 12. The Consultant should specify the sulfate exposure category (AC! 318) based on soluble sulfate testing and provide recommendations for sulfate resistant concrete, if necessary, or default to a severe exposure category, if testing is not available. A sample has been collected and shipped to a certified testing laboratory, which will take about two weeks to turnover. Until the test results prove otherwise, the contractor will assume sulfate resistant concrete is required. Based on the actual laboratory results an addendum letter will be issued regarding the sulfate content of the soil with appropriate recommendations. 13. The Consultant should provide geotechnical recommendations for the proposed pool. The pool will embedded in stable bedrock. No special recommendations are required. 14. The Consultant should provide a list of recommended observation and testing during site grading and construction. Soil special inspections shall be performed periodically in compliance with Table 1705.6 (Required Verification and Inspection of Soils) from the current California Building Code. We anticipate soil special inspections will include the following:  Verification of the compaction of the reprocessed lot surface after scarification.  Observation of foundation excavations for the residence and retaining walls.  Compaction testing and or verification of retaining wall backfill.  Observation of the pool excavation. 15. The Consultant should provide a list of codes and published maps/reports used in the preparation of the report. References to prepare the geotechnical report include the following:  2019 California Building Code  Geologic Map of the Oceanside 30' X 60' Quadrangle, Southern California, by Kennedy – Tan, (2005)  DMG Open-File Report 95-03, Landslide Hazards In The Northern Part Of The San Diego Metropolitan Area, San Diego County, by the California, California Department of Conservation, Division of Mines and Geology (1995)  Seismic Design Web Services, U.S. Geological Survey Cacatua Street Carlsbad, CA March 25, 2022 Page 5  The National Map Website, U.S. Geological Survey  Fault Activity Map of California (Website), California Geological Survey  USGS Topographic Maps, Rancho Santa Fe Quadrangle, 7.5’, 1968 and 1949  Google Earth (Website), photo imagery 1985 to 2021  Historic Aerials (Website), photo imagery 1947 to 2018 If you should have any questions after reviewing this document, please do not hesitate to contact our office. This opportunity to be of professional service is sincerely appreciated. Respectfully submitted, C.W. La Monte Company Inc. _____________________________ Clifford W. La Monte, R.C.E. 25241, G.E. 0495 12/31/2023 12/31/2023 FIGURE NO. 1 Cacatua St., Carlsbad, CA APN 215-390-35 PLOT PLAN AND GEOTECHNICAL MAP Approximate Test Boring Location LEGEND B-2 Geologic Unit Tsa = Santiago Formation B-3 B-5 B-2 B-1 B-4 PROPOSED S.F.R. FF=498.11 PAD=497.44 I I i 0 10 20 i-----~-- 30 I