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Home My WebLink AboutCT 02-28; La Costa Condominiums; Third Party Review of Soils Analysis after Powers/Banich slope repair; 2009-06-09VERITAS June 9, 2009 Contract No. 148513.31 Mr. Frank Jimeno, P.E. City of Carlsbad Public Works - Engineering Division 1635 Faraday Avenue Carlsbad, CA 92008 Subject: Review of Alternate Landslide Remedial Recommendations, Proposed La Costa Condominiums, Carlsbad, California References: 1,"Alternate Landslide Remedial Recommendations, Proposed La Costa condominiums, Lot 185, La Costa Avenue South - Unit 1, Carlsbad Tract No. 02-28, Carlsbad, California", prepared by American Geotechnical Inc., File No. 23280.01, dated March 11, 2009. 2. "As-Built Report, Landslide Stabilization, Banich, Powers, Calso Landslide, 2416 Sacada Circle, Carlsbad, California", prepared by American Geotechnical Inc., File No. 23280.01, dated September 29,2008. Dear Mr. Jimeno: Introduction As requested Bureau Veritas, Testing Engineers-San Diego has performed a geotechnical review of the subject project. The purpose of our review was to provide an opinion on whether the geotechnical aspects of the project have been identified and appropriately addressed in the project geotechnical documents. This letter presents the results of our third-party review. Our review is based on geotechnical data presented in the project geotechnical documents and our professional judgment. We have not performed subsurface investigation, laboratory testing or independent geotechnical analysis for the project. Testing Engineers San Diego Main: 858.715.5800 Fax: 858,715.5810 www.us.bureauveritas.coni City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Results of Review Based on the results of our review, it is our opinion that there are geotechnically-related issues that should be addressed by the project geotechnical consultant. Our enumerated comments are presented as follows: Alternate Landslide Remediation Plans Sheet 1 of 5; General Notes: Comment 1.1: Consultant should verify the recommended adhesion value between the grout and soil/formation for preliminary design purposes of the tie- backs. Comment 1.2: The consultant is requested to evaluate the need for pre- construction site surveys of the potentially MSffected top-of-slope dwellings in connection with this project. ^ Sheet 2 of 5; Alternate Landslide Remediation Plan: Comment 2.1: In general, the consultant is requested to perform a global review of this plan sheet as to completeness and understandability. For example, the legend and/or labeling appear to be incomplete; it's very difficult to discern between existing and proposed grades; the relevant top-of-slope dwellings aren't shown; existing and additional proposed inclinometers aren't shown; remedial grading slot boundaries for 1 and 2 are incomplete; proposed grading adjacent to La Costa Avenue is incomplete; etc. Comment 2.2: The consultant should provide relevant design details relating to maintaining the stability of the proposed in-slope detention basin (i.e., basin lining, overflow, redundant subdrainage, etc.). The slope stability analyses should include a section that passes thru the center of the proposed detention basin and the adjacent regraded slope; also, see comments under "Slope Stability Analysis". Comment 2.3: The proposed slope area below the proposed tie-back wall where Sections B-B' and K-K' pass thru doesn't indicate any remedial grading, however, the adequacy of the internal slope stability of this zone hasn't been substantiated. Also, see comments under "Slope Stability Analysis". Comment 2.4: With respect to Sections B-B', K-K', and M-M', see comments under "Slope Stability Analysis". City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Sheet 3 of 5; Cross Sections A,B,D,F: Comment 3.1: The consultant should consider adding trends to Sections A-A' and B-B' (similar to Sections D-D' and F-F'). Comment 3.2: With respect to Notes 1 & 3, clarify and/or provide a detail showing a profile of the recommended back-drain features. Comment 3.3: With respect to Note 2, the consultant should consider removing all metric unit call-outs and replacing with appropriate imperial units. Comment 3.4: With respect to Section B-B', see comments under "Slope Stability Analysis". Sheet 4 of 5; Cross Sections I,J,K,L,M: Comment 4.1: The consultant should consider adding a trend to Section M-M' (similar to the other cross sections on this sheet. Comment 4.2: See previous comments 3.2 and 3.3 herein. Comment 4.3: With respect to Sections K-K' and M-M', see comments under "Slope Stability Analysis". Sheet 5 of 5; Cross Sections I,J,K,L,M: Comment 5.1: The consultant should consider modifying the title of this sheet (cross sections I,J,K,L and M are not depicted. Comment 5.2: The consultant should cross-reference Detail 1 to Sections B-B' and K-K'. Comment 5.3: Regarding Detail 3, the consultant should make reference to a relative compaction standard; replace all metric units with the appropriate imperial units; replace " 3 Cu/Ft. gravel " with " .3 cubic feet/ foot gravel "; and show the 4" diameter solid PVC, Sch. 40 pipe being directed to a suitable tight-line. Slope Stability Analysis Comment 6.1: The consultant should provide all input and output data for the slope stability analyses and number all sheets. Comment 6.2: The consultant should provide a discussion regarding minimum safety factors for the various analyses; it is suggested that local City of Carlsbad - La Costa Condominiums Contract No. 148513.30 industry standards be utilized (i.e., Static Permanent; F.S. = 1.5 min.; Static Temporary; F.S. = 1.3 min.; and Pseudo-static Permanent; F.S. = 1.1 min. utilizing a seismic coefficient, k = 0.15). Also, when analyzing to develop the minimum safety factors, utilization of potential rectilinear surfaces should be considered. Comment 6.3: The soil and formation strength parameters utilized in the slope stability analyses are considered to be unconservatively high (excluding the landslide debris). It's acknowledged that accurate validation of the in-situ strength parameters of the formational materials, by means of field sampling and laboratory testing, is very difficult; however, given the recent landslides that have occurred and the amount of data that has been developed by the consultant during the remedial slope stabilization work to date, the consultant is requested to develop appropriate sections which would provide strength parameters required to have a factor of safety of 1 for the failed zones (i.e., "back-analysis"). This methodology would provide a more accurate characterization from which to evaluate the appropriateness of the alternative tie-back design. Comment 6.4: For Section B-B', the consultant should provide static and pseudo-static internal stability analyses below the proposed tie-backs for the permanent condition. Comment 6.5: For Section K-K', the consultant should provide static and pseudo-static internal stability analyses below the proposed tie-backs using a rectilinear surface for the permanent condition (note: excluding the condo basement wall and adjacent backfill). Although a circular failure surface for this section has been provided, it doesn't appear to be the critical condition; also, the factor of safety obtained is reported as 1.383 which is less than 1.5. Therefore, the lateral resistance from the condo basement wall needed to raise the factor of safety to 1.5 needs to be determined and this surcharge should be appropriately documented for future structural design purposes with respect to these basement walls. Comment 6.6: For Section M-M1, the consultants' analysis appears to have yielded a static factor of safety of 1.159 which is less than 1.3 and suggests the need for mitigating measures for this temporary condition; please verify. City of Carlsbad - La Costa Condominiums Contract No. 148513.30 The opportunity to be of service to the City of Carlsbad on this project is appreciated. If you have any questions regarding our review, please contact our office. Respectfully submitted, Testing Engineers - San Diego, Inc. A Bureau Veritas Company Gene Custenborder, CEG Van Olin, PE, GE Senior Engineering Geologist Principal Geotechnical Engineer GC/VO:cs Distribution: (2) Addressee (1) via email