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Home My WebLink AboutMS 08-03; NOLAN DEVELOPMENT; GEOTECHNICAL UPDATE AND PLAN REVIEW; 2014-01-21GeoTek, Inc. 1384 Poinsettia Avenue, Suite A Vista, CA 92081-8505 (760) 599-0509 Office (760) 599-0593 Fax www.geotekusa.com January 21, 2014 Project No.: 328 I-SD3 - Frank and Joann Nolan Family Trust 7331 Las Brisas Court I Carlsbad, California 92009 .RIECjJf1,b I Subject: Geotechnical Update and Plan Review MAR 04 Proposed Residential Lot LAN,., 14 Li 7331 Las Brisas Court fFNG1 _ VLL 1 Carlsbad, California r Reference: I) "Preliminary Geotechnical Evaluation, Proposed Residential Development, 7331 Las Brisas Court, Carlsbad, California", By GeoTek, Inc. dated February 25, 2008 (PN 3281 -SD3) I 2) "Grading and Erosion Control Plans for 7331 Las Brisas Court" By bHA, Inc., date printed January 22, 2014 Dear Mr. Nolan: In accordance with your request, this report is to update the 2008 "Preliminary Geotechnical Evaluation" (reference I) for the subject property and to review the reference Grading Plan prepared by bHA. In preparing this letter we reviewed the referenced report and plan, performed a brief site reconnaissance and reviewed aerial photographs available on Google Earth dated March 2008, June 2009, August 2010, September 2010 and October 2012. Discussion Site conditions are substantially similar to those at the time of our prior study. The only notable difference is a stone apron placed where the dirt road through the site meets Piragua Street. 0 Air photos indicate it was placed between September 2009 and before October 2012. Our review of the plan indicates that grading and additional development will be limited to that tp associated with Parcel 2. Parcel I has been previously developed with a single family residence and no additional development is planned at this time. Therefore Parcel I is not further 1!JJ discussed herein. GEOTECHNICAL I ENVIRONMENTAL I MATERIALS 5 ,n, oco'03 Frank and Joann Nolan Family Trust January 21, 2014 Carlsbad, California Project No.: 328 I-SD3 Geotechnical Update and Plan Review Page 2 of 4 Grading will be performed to create a residential building pad on Parcel 2 at an elevation of 409 feet with a drive access off Piragua Street. A bio-retention basin will be created in the western portion of the site. The planned fill area is primarily limited to the building pad and descending slope above an elevation of approximately 400 feet. Other graded areas are shown as cuts. All slopes shown are planned as cuts with the exception of the fill descending the building pad. Slopes are planned at gradients of 2:1 or flatter. The proposed highest slope is approximately 16 feet overall and is the fill over cut that descends from the building pad to Piragua Street. The site is indicated as an export site with 825 cubic yards (CY) cut and 528 CY fill. Remedial grading is indicated as 2500 CY. The two geotechnical conditions that are likely to most impact site grading and construction are the presence of clay and the potentially hard bedrock. Recommendation for both these were presented in our referenced report and are still considered applicable. Those recommendations included a five (5) foot thick cap of select material below finished grade on the building pad. Some select material may be generated from the bedrock although import should be anticipated. Recommendations In general, the recommendations in our referenced report are considered valid and remain applicable unless specifically modified herein. Based on the current plan and codes some modifications are warranted. These are offered below: Remedial Grading - Paving/Hardscape Areas I Section 5.3.3 on page 7 indicated: "Remedial grading as discussed above shall be performed to provide a minimum three I (3) foot thick uniformly compacted select fill mat for support of paving and hardscape. Remedial grading should extend a minimum of three feet beyond the outside edge of paving and hardscape." The intent of this recommendation was to remove the expansive materials (topsoil, residuum and highly weathered bedrock) sufficiently deep to limit their potential impact on surface improvements. Test pits indicate the problematic expansive materials very from three (3) to 4.5 feet thick. Review of the Grading Plan indicates that the majority of the driveway area will be in bedrock cut sections greater than 3 feet in depth. As such, over-excavation need not extend a uniform 3 feet below grade. It can be limited to highly expansive materials or approximately 12 inches below finished grade, whichever is lower. I I I I I I I I [ I I [ I I GE OT E IC I Frank and Joann Nolan Family Trust January 21, 2014 Carlsbad, California Project No.: 328 I-SD3 Geotechnical Update and Plan Review Page 3 of 4 In areas where vehicle traffic is planned the subgrade should be low enough to accommodate at least 6 inches of compacted base beneath the paving. Grading Procedures The presence of the very highly expansive materials that are not advisable to be placed in shallow fill occur across most of the site. Additionally, oversized rock may be produced from cuts in the bedrock. Given the limited areas that these could be used as fill, it is suggested that removals and key ways at the toe of the fill be made at the start of site grading. This would allow for allow for placement onsite material in the fill as acceptable materials are encountered. It may be necessary to export more material than indicated on the plans and then import select material. Seismic Design Parameters Current ground accelerations determined from the USGS Website, Earthquake Hazards Program vary from the values presented in our prior report. Current values are provided below. The site is located at latitude 33.1532 and longitude 117.2102. Site spectral accelerations (Ss and SI), for 0.2 and 1.0 second periods and 2 percent probability of exceedance in 50 years (MCE) was determined from the USGS Website, Earthquake Hazards Program, Interpolated Probabilistic Ground Motion for the Conterminous 48 States. The site is considered a Site Class "B". The results are presented in the following table: SITE SEISMIC PARAMETERS Mapped 0.2 sec Period Spectral Acceleration, Ss (g) 1.029 Mapped 1.0 sec Period Spectral Acceleration, S 1(g) 0.399 Site Coefficient, Fa 1.00 Site Coefficient, Fv 1.00 Maximum Considered Earthquake Spectral Response Acceleration Parameter at 0.2 Second, SMS (g) 1.029 Maximum Considered Earthquake Spectral Response Acceleration Parameter at I second, SM I (g) 0.399 Design Spectral Response Acceleration Parameter for 0.2 Second, SDS (g) 0.686 Design Spectral Response Acceleration Parameter for 1.0 Second, SD I (g) 0.286 Seismically resistant structural design in accordance with local building ordinances should be followed during the design of the structures. The California Building Code (CBC) has been 'jG' G EOTEK L I [1 I I I I I I I [1 F] I Frank and Joann Nolan Family Trust January 21, 2014 Carlsbad, California Project No.: 3281 -SD3 Geotechnical Update and Plan Review Page 4 of 4 developed to reduce the potential for structural damage. However, as the result of ground shaking generated by nearby earthquakes some level of damage and associated economic consequences are considered likely in this general area. The opportunity to be of service is greatly appreciated. If you have any questions concerning this report, please do not hesitate to call the undersigned. Respectfully Submitted, GeoTek, Inc. Principal EE c, No. 1142 -f 1142 OF cP.*f Ch kLiac Professional Engineer Distribution: Addressee (I) via Email Dale Clark, bHA, Inc. (4) I-via Email, 3 Delivered 7 pjD LI lIi:'f2 1( oo7 , q u'20 \& C EOTEK