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Home My WebLink AboutCDP 2017-0074; STAINBACK RESIDENCE; REVIEW OF GEOTECHNICAL REPORT; 2019-03-28C City of Carlsbad 1635 Faraday Avenue Carlsbad, California 92008 Attention: Mr. David Rick, PE Subject: Project: Review of Geotechnical Report Proposed Stainback Residence 5360 Los Robles Drive Carlsbad, California City Project ID: CDP2017-0074 GR2019-0008 City GR No.: NV5 ~el.J'Jrf' 4~..JRdJJ t:f!,P]I'-, flA/1J. i'.:5)1-1h2,~ 1 w31tC t->t>-r tit J ;, /JZ J J .:i-J) WI 'd-4 _t,.-4-1 f .> "r,r J'.l"'J ~ fl Ir L, -0, fl I~ n.lL- f<,t:tc=-I r,,, ~J.I JJ March 28, 2019 Project No.: 226816-00 IO 1.53 RECEIVED LAND DEVELOPMENT ENGINEER/NG References: 1) "Pre/imina,y Geotechnical Investigation, Eaton-Stainback Residence, 5360 Los Robles Drive, Carlsbad, California", prepared by Geotechnical Exploration, Inc., Project No. 17- 11606, dated September 28, 2017. 2) "Technical Guidelines for Geotechnical Reports", issued by the City of Carlsbad, dated January 1993. 3) "CGS Note 41, Guidelines for Reviewing Geologic Reports", prepared by the State of California, Department of Conservation, California Geological Survey, dated 2017. Dear Mr. David Rick: As requested, NV5, West Inc. (NV5) has conducted ageotechnical review of the above-referenced geotechnical report for the proposed residence located at 5369 Los Robles Drive in Carlsbad, California. The purpose of the review was to provide an opinion on whether the geotechnical aspects of the project have been identified and appropriately addressed in the project geotechnical report. NV S's geotechnical review is based on geotechnical information presented in the referenced geotechnical report and experience with the geotechnical conditions in the general site area. NVS has not perfonned an independent geotechnical investigation at the project site and therefore does not offer or imply any guarantee or warranty as to future site performance. The opinions presented below are limited. Other consultants could arrive at different conclusions. This report presents a summary of the review. Review Summary Based on our review, the referenced geotechnical report for the proposed project is !!Q! approved. The report does not meet the minimum standards as outlined in the City of Carlsbad, "Technical Guidelines for Geotechnical Reports", {reference 2 above). Additional infonnation is requested. The applicant should submit a revised or addendum report that meets the minimum criteria as outlined in the City's technical guidelines. For reference, the attached City of Carlsbad Geotechnical Report Review Checklist outlines item that are typically required to be addressed in a project geotechnical report prior to approval by the City of Carlsbad. It is noted that not all of the items on the Checklist may be applicable or pertinent to every project, but the Checklist does present list of items that are checked by City reviewers. OF'FICES NATIOIIWICE 1 :1092 AVENUE OF sc1ENcE, SUITE 200 I SAN D1e:00, CA li>2 12a I www.Nve.coM I o..-..-,cE 858,38l5.o5oo I FAll 858. 385.0400 CousTRUCTTOH ou .. LlfY AssuA.:.tlCI: • IIIF'IUSTRUCTUAI: • EuCRCl • PAOCF!Af.f MAll&CCMEtlT • tm,1A011MEtiTAL C Review of Geo technical Report Proposed Stainback Residence 5360 Los Robles Drive Carlsbad, California Project No:: 226816-00101.53 In addition to the Checklist, the following present NV5's comments which should be addressed by the project geotechnical consultant. Comment 1 -The site description on page 2 of the geotechnical report is incomplete. The site description should provide approximate site elevations and include a description of overall topographic relief or gradient even if it is measured with simple hand tools. Please refer to the City of Carlsbad Geotechnical Report Review Checklist for items that should be included in the project description. Comment 2 -The report indicates that the onsite soils have a very low expansion potential with an expansion index ofless than 20. It is not clear how the report concludes that the onsite soils have very low expansion index since no laboratory expansion test results are reported. The consultant should provide the results of the laboratory expansion index test(s). Comment 4 -On page 2, the report that the site is underlain by variable depths of fill and formational materials. Based on the logs of the handpits and the report descriptions, the geologic conditions underlying the site cannot be surmised. Logs also provide general assessments of "apparent density" but it is unclear how this was assessed since there is no SPT "blow counts" information provided. The geotechnical report should include a geologic map, and since the property has significant fill, should include a scaled cross section depicting the subsurface geology. These are essential to the understanding of the report and site geology. Comment 5 -On page 2 the report recommends that the loose to medium dense fill soils in the proposed building pad area should be removed and recompacted. The report should be more specific. Define what is meant by "the building pad area". Does the recommendation intend that all of the existing fill needs to be removed? If not, the limits and required depths of removal need to be specified. In addition, if not all fill is to be removed, the report should describe the type of evaluation that will be performed by the geotechnical consultant during grading to verify the suitability of the structural fill subgrade. The consultant should provide criteria for the establishing suitability of soils to be left in place. Comment 6 -On page 2 the report indicates that the new foundations for the proposed residence can be founded in properly compacted fill or the existing formational materials. It appears that site grading may result in the proposed residence being underlain by a natural/fill transition condition. Experience has shown that structures that are underlain partially by natural soils and partially by fill soils may be subject to distress due to differential settlement across the transition. The report does not address the potential for the proposed residence to be underlain by a natural/fill transition. For typical projects, if site grading will result in a natural/fill transition underlying a proposed structure, the natural area is undercut to a specified depth and replaced as compacted fill, which mitigates the potential for differential settlement and associated structural distress. The project report should discuss the potential for site grading to result in a transition condition underlying the proposed residence, and provide appropriate recommendations to mitigate the potential for differential settlement. Comment 7 -The referenced report does not address groundwater conditions at the site. On page 4 the report discusses liquefaction and mentions "the lack of a shallow static groundwater surface under the site". It is not clear how the report came to that conclusion, since the deepest exploration was 8 feet below the existing ground surface. The report should provide information regarding the anticipated groundwater level at the site based on field exploration or at minimum based on known historic site and regional groundwater NIVl5 OFFlCES HATIO~J'.VIOE CO!-ISTRUCTIOU 0UALITl' ASSUR.UICE. -h-'F"RASTRUCTURE EUGIUECRUIG • MUUICIPAL 0UTSOURCIUO • ASSET MANACEHENT -EUVIA:OIH•tE:UTAL 5ERVtCE!. / . ,, . .. C Review of Geotechnical Report Proposed Stainback Residence S360 Los Robles Drive Carlsbad, California Project No.: 226816-00101.53 measurements. The Geotracker and Water Data Library web pages of the California State Water Resources Control Board website are typical resources for such information. It is recommended that the project geotechnical consultant review and approve the grading, shoring and building foundation plans prior to construction. NVS appreciates the opportunity to be of service to you on this project. If you have any questions regarding this report, please do not hesitate to contact our office. Respectfully submitted, NV5 West, Inc. Altuchment: City of Carlsbad Geotechnical Report Review Checklist Distribution: ( I ) Addressee, via email NIVl5 OFFICES NATIOHWIDE Carl Henderson, PhD, GE 2886 CQA Group Director (San Diego) COHSTFIUCTIOtl OUAUTr AssunAt,ICE • lttFP.ASTRUCTURE ENGIHEERIUC • MUHICIPAL OUTSOURCIUG -AS~ET MAHAOEl-1£.N.T • EuvlROto•tENTAL SERVICE$