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Home My WebLink AboutPD 2021-0042; 2940 CACATUA ADU; RESPONSE TO CITY COMMENTS; 2022-09-13 GEOTECHNICAL | ENVIRONMENTAL | MATERIAL September 13, 2022 Project No. 3766-SD Kris Alberts 2940 Cacatua Street Carlsbad, California Subject: Response to City Review Comments for Proposed Auxiliary Dwelling Unit (ADU) 2940 Cacatua Carlsbad, California Reference: GeoTek, Inc. 2022, “Proposed Auxiliary Dwelling Unit (ADU), 2940 Cacatua, Carlsbad, California,” Project No. 3766-SD, dated March 4, 2022. Latitude 33, Inc. “Minor Grading Plans for 2940 Cacatua Street, Carlsbad, California,” Project No. PD2021-0042, Grading Plan 2022-0010, dated September 7, 2022. Paul Christenson San Diego Engineering, “Alberts ADU, 2940 Cacatua Street, Carlsbad, CA 92009,” Job No. 21-121, sheets SN1, S1, and SD1, revised June, 21, 2021. Dear Mr. Alberts: As requested, GeoTek, Inc., (GeoTek) has prepared this letter to provide a supplemental response to City of Carlsbad review comments of GeoTek’s 2022 referenced report. A copy of the review comments addressed herein are included in Appendix A. Review Comment No. 1 The consultant should review the project grading plan 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. Response to Review Comment No. 1 A geotechnical review of the referenced grading plan and foundation plan for the subject site was performed. Based on this review, it is GeoTek’s opinion that the provided and reviewed plans have been prepared utilizing the soil engineering design parameters and recommendations provided in GeoTek, Inc. 1384 Poinsettia Avenue, Suite A Vista, CA 92081-8505 (760) 599-0509 Office (760) 599-0593 Fa www.geotekusa.com KRIS ALBERTS Project No. 3766-SD Response to City Review Comments September 13, 2022 2940 Cacatua, Carlsbad, California Page 2 GeoTek’s 2022 report. Where there were differences, (seismic design parameters), the designed values are more conservative than were provided in GeoTek’s 2022 report. The referenced plans are considered to be geotechnically suitable. GeoTek makes no representation as to the accuracy of dimensions, calculations, or structural/civil design provided on the referenced plans. Review Comment No. 2 The consultant should address impacts to adjacent property and improvements as a result of site grading and construction. (repeat comment - no response to comment report was provided with this submittal; please provide. With respect to this comment, please also provide geotechnical recommendations (allowed height of vertical cuts, inclination of backcuts, setbacks from property boundaries, etc.) for the temporary slopes that will apparently be necessary to excavate into the existing hillside and construct the retaining walls associated with the proposed ADU. Recommendations should be provided as necessary to assure worker safety and protect adjacent off-site property from any adverse impacts from the proposed construction.) Response to Review Comment No. 2 In preparation of this response a site visit was performed by the signing engineering geologist to observe site conditions. Graded topography of the site during rough grading consisted of a 2:1 (horizontal:vertical) slope from the site, ascending to the adjacent lot to the east. The site was excavated in anticipation of the ADU. Current site topography consists of an approximate 1/3:1 (near vertical) sloped excavation. Based on site conditions, the actual vertical excavation is approximately seven feet tall. The minor grading plan appears to indicate a taller cut condition, however, this only appears as such due to the vertical exaggeration of the section. This condition appears on the topographic map from field survey performed on September 20, 2021. During GeoTek’s field observations on January 20, 2022, the backcut exposed bedrock material consisting of Santiago Formation. Empirical data, (known dates between the exaction and today) suggest the excavation is stable in a temporary condition. The nearest structure from the excavation is approximately 48 feet east and at a higher elevation relative to the excavation. Considering the temporary excavation is approximately a year old, the excavation is waning on what is considered to be a temporary excavation. In general, depending on several conditions, peer industry considers a temporary excavation to be limited to six or twelve months from date of exaction. Temporary excavations have been open for longer, but are considered less frequent. The excavation for footings along the eastern retaining wall is approximately four feet west (or in front) of the temporary excavation backcut. In an effort to provide more commentary on GEOTEK KRIS ALBERTS Project No. 3766-SD Response to City Review Comments September 13, 2022 2940 Cacatua, Carlsbad, California Page 3 temporary slope stability, the mode of failure should be considered. In this case, if a failure of the temporary excavation occurs, the anticipated failure of the backcut is by topple. A topple failure occurs primarily in near vertical slopes that develop desiccation cracks or tension cracks near the tops of slopes that run parallel along the slope. The tension crack allows a large piece or several smaller pieces to fall down and away from the near vertical slope. Sometimes this occurs in a pealing motion away from the slope. Lateral displacement varies based on slope height and tension crack location. It should be noted that the upslope lots appear to be irrigated and mature vegetation is established along the top of slope (offsite). The irrigated landscape and established trees provide surficial consistency of moisture and root structure that reduces the potential for tension or desiccation cracks to form behind the temporary excavation. If a topple failure were to develop, considering the height of the slope, a topple failure should anticipate the material running out three to four feet from the face or base of the excavation. Along the north and south sides of the proposed retaining wall for the ADU, there appears to be adequate room for the existing topography or excavation to be laid back at a 1:1 with a maximum five foot vertical cut at the base of the excavation to facilitate a temporary excavation for building construction. The vertical cut should include the depth of the retaining wall footing. Considering the site conditions, the proposed improvements should be expedited in a diligent manner (build sooner, rather than incur additional delay) to provide longterm stability to site and offsite conditions. If compaction equipment is required, vibratory equipment should not be utilized, as the vibration will increase the potential for a slope failure to occur. Excavation means and methods, including employee safety is the responsibility of the employer, not GeoTek. The employer of the workers should follow CalOSHA guidelines, which include designating a competent person to evaluate the safety environment of the site. GEOTEK KRIS ALBERTS Project No. 3766-SD Response to City Review Comments September 13, 2022 2940 Cacatua, Carlsbad, California Page 4 The opportunity to be of service is sincerely appreciated. If you should have any questions, please do not hesitate to call GeoTek. Respectfully submitted, GeoTek, Inc. Attachments: Appendix A – City of Carlsbad Review Comments Distribution: (1) Addressee via email Edwin R. Cunningham RCE 81687 Exp. 3/31/24 Project Engineer Christopher D. Livesey CEG, 2733 Exp. 05/31/23 Associate Vice President GEOTEK Appendix A City of Carlsbad Review Comments GEOTEK GEOTECHNICAL REPORT REVIEW __________________________________________________________ DATE: August 11, 2022 TO: City of Carlsbad Land Development Engineering 1635 Faraday Avenue Carlsbad, CA 92008 Attention: Nichole Fine PROJECT ID: PD2021-0042 GRADING PERMIT NO.: GR2022-0010 SUBJECT: 2940 Cacatua Street (2nd review) Items Submitted by Applicant Items Being Returned to Applicant  “Clarification of Limited Geotechnical Evaluation for Proposed Auxiliary Dwelling Unit (ADU), 2940 Cacatua, Carlsbad, California,” by GeoTek, Inc., dated May 11, 2022.   Third-Party Geotechnical Review (First), 2940 Cacatua Street, Carlsbad, California,” by Hetherington Engineering, Inc., dated April 6, 2022.   “Limited Geotechnical Evaluation, Proposed Auxiliary Dwelling Unit (ADU), 2940 Cacatua Street, Carlsbad, California,” by GeoTek, Inc., dated March 4, 2022.  Written report review comments. Based on our review of the submitted geotechnical reports, we are providing the following comments that should be addressed prior to the next submittal. Please provide complete and thorough written responses to all comments. Please note that the basis for this review consists solely of the review of the referenced geotechnical reports prepared by GeoTek, Inc. as they relate to the “Third-Party Geotechnical Review (First), 2940 Cacatua Street…,” by Hetherington Engineering, Inc. referenced above. Please note that this submittal did not include a geotechnical response to comment report, consequently the comments from the previous review by are repeated below. GEOTECHNICAL COMMENTS: 1. The consultant should review the project grading plan and foundation plans, provide any additional geotechnical analyses/recommendations considered necessary, and confirm GR2022-0010 August 11, 2022 Page 2 of 2 that the plans have been prepared in accordance with the geotechnical recommendations. (repeat comment – no response to comment report was provided with this submittal; please provide.) 2. The consultant should address impacts to adjacent property and improvements as a result of site grading and construction. (repeat comment - no response to comment report was provided with this submittal; please provide. With respect to this comment, please also provide geotechnical recommendations (allowed height of vertical cuts, inclination of backcuts, setbacks from property boundaries, etc.) for the temporary slopes that will apparently be necessary to excavate into the existing hillside and construct the retaining walls associated with the proposed ADU. Recommendations should be provided as necessary to assure worker safety and protect adjacent off-site property from any adverse impacts from the proposed construction.)