HomeMy WebLinkAboutPD 2020-0027; 3481 SEACREST DRIVE SLOPE REHABILITATION; RESPONSES TO GEOTECHNICAL REVIEW CORRECTIONS LETTER; 2020-10-20
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Richard Feld October 20, 2020 3481 Seacrest Drive Carlsbad, CA 92010 dainesphoto@earthlink.net Subject: RESPONSES TO GEOTECHNICAL REVIEW CORRECTIONS LETTER Plan Check 1 Project No. PD2020-0027 (GR2020-0021/DWG526-4A) 3481 Seacrest Drive and 3470 Charter Oak Drive Carlsbad, California Reference: SLOPE STABILITY INVESTIGATION AND REPAIR RECOMMENDATIONS 3481 Seacrest Drive and 3470 Charter Oak Drive Carlsbad, California SGSI Project Number 3.31684; Dated August 17, 2020 Included herein is our response to the City of Carlsbad consultant NV-5 review comments dated September 24, 2020, for the project. Review comments are listed below, followed by our response. Comment 1: The geotechnical report provides earthwork recommendations for re-grading of the slope failure area. The project plans include re-grading of the failure and construction of several retaining walls. The report does not include geotechnical parameters or specific recommendations pertaining to the proposed retaining walls (lateral pressures, bearing capacities, drainage, backfill, etc.). The geotechnical consultant should provide a revised or addendum report that addresses the proposed retaining walls. Response: Our above referenced report was issued prior to review of the project Grading and Drainage plan prepared by Triad-Holmes Associates, dated August 24, 2020. At that time, a grading repair without Keystone walls was assumed, and as a result soil design values were not included. However, subsequent analysis by the project Civil Engineer (CE) determined that landscape/garden walls were needed to achieve a 2:1 slope. SGSI has since reviewed the project Plans and Segmented Retaining Wall with Geogrid Calculations. Our findings are noted below: Wall Design Values Used Comment Allowable Soil Bearing = 1500 psf Acceptable Soil Phi Used = 28˚ Acceptable Soil Unit Weight = 110 pcf 129 pcf recommended Wall Soil Friction = 19˚ 2/3rd phi; Acceptable Ka (horiz) = .59 Acceptable
Wall backfill shall be accomplished in accordance with the Section 6 and Appendix D of our report. In addition, site soils and any import used for backfilling should have an expansion index (EI) of no greater than 50 and a sand equivalent (SE) greater than 15. The backfill soils should be tested by the soils engineer prior to backfill operations starting for the retaining wall structures. Wall drainage shall be per manufacturer and project CE. Comment 2: The geotechnical report indicates that the slope stability calculations are in Appendix C of the report. It appears that the consultant used Janbu global stability factor of safety charts (NAVFACDM7.01), but it is not clear how factors of safety were determined or what equations were used. The consultant should clarify. In addition, the stability analyses should reflect the proposed retaining walls as shown on the project plan, and pseudo static (seismic) stability should also be addressed. Response: The slope failure was due to shallow topsoil sloughing/translation over a relatively homogeneous native soils originally cut at roughly 1.6:1 some 50 years ago. An analysis of the stability of the slope including pseudo static (seismic) stability and reflecting the proposed retaining walls was performed with the Slope/W by Geoslope computer program. The factor of safety in each under was over 1.5. Calculations and charts used are included herein.
Comment 3: It is recommended that the project geotechnical consultant review and approve the grading and retaining wall plans prior to construction. Response: SGSI has reviewed the project grading and drainage plans for conformance with our report. With exceptions noted above, the plans have been prepared in general accordance with our recommendations. We appreciate the opportunity to be of service to you. Should you have any questions regarding this report, please do not hesitate to contact us. Respectfully, SIERRA GEOTECHNICAL SERVICES, INC.
Joseph A. Adler Thomas A. Platz Principal Geologist Principal Engineer
CEG 2198 (exp 3/31/2021) PE C41039 (exp 3/31/2021)
2.31FillQopAA'Project: 3480 Seacrest DriveMethod: SpencerSlip Surface Option: Entry and ExitHorz Seismic Load: 0Name: Fill Unit Weight: 120 pcf Cohesion: 320 psf Phi: 28 ° Name: Qop Unit Weight: 125 pcf Cohesion: 350 psf Phi: 31 ° Name: Ret. Wall Unit Weight: 150 pcf Cohesion: 1500 psf Phi: 45 ° FillLot 90Lot 83Name: Fabric reinforcement- Tensar miragrid 3XT LTLDS 1220 lbs/ftPLDistance0 10 20 30 40 50 60 70270280290300310320Elevation270280290300310320
1.73FillQopAA'Project: 3480 Seacrest DriveMethod: SpencerSlip Surface Option: Entry and ExitHorz Seismic Load: 0.15Name: Fill Unit Weight: 120 pcf Cohesion: 320 psf Phi: 28 ° Name: Qop Unit Weight: 125 pcf Cohesion: 350 psf Phi: 31 ° Name: Ret. Wall Unit Weight: 150 pcf Cohesion: 1500 psf Phi: 45 ° FillLot 90Lot 83Name: Fabric reinforcement- Tensar miragrid 3XT LTLDS 1220 lbs/ftPLDistance0 10 20 30 40 50 60 70270280290300310320Elevation270280290300310320
0 105APPROX. SCALE IN FEETFigure 1DRAFT3480 Seacrest DriveOctober 2020NAA'
REFERENCE:3481 Seacrest Drive, Carlsbad, CAGrading & Drainage Plan, Sheet C2,dated 08/24/2020, prepared by THA0 105APPROX. SCALE IN FEETFigure 1Cross Section A-A'DRAFT3480 Seacrest DriveOctober 2020Elevation in Feet (MSL)320300310290280270A320300310290280270A'QopLOT 83LOT 90Fill