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Home My WebLink AboutCT 13-05; State Street Townhomes; Tentative Map (CT) (3)Leighton and Associates, Inc. RECEIVED FE3 C 7 20^'i CiTY OF CARLSBAD PLANNING DIVISION A LEIGHTON GROUP COMPANY To: Attention: February 4, 2014 Project No. 10552.004 Intracorp Companies 4041 MacArthur Blvd. Suite 250 Newport Beach, CA 92660 Mr. Ken Benson Subject: Geotechnical Update, Proposed Redevelopment of 2531 - 2589 State Street, Carlsbad, California References: California Building Standards Commission (CBSC), 2013, California Building Code (CBC) Leighton and Associates, Inc., 2005, Preliminary Geotechnical Investigation, Proposed Redevelopment of 2531, 2541, and 2551 State Street, Carlsbad, California, Project No. 041742-001, dated December 7, 2005 Leighton and Associates, Inc., 2014, Phase I and Limited Phase II Environmental Site Assessment for 2531 through 2601 State Street, Carlsbad, California, Project No. 10552.001, dated January 9, 2014 RBF Consulting, 2014, Preliminary Grading Plans for State Street Townhomes, Carlsbad, California, Sheet C5, dated January 2014 In accordance with your authorization, this letter presents Leighton's geotechnical update for the proposed State Street residential redevelopment, located at 2531 -2589 State Street, in Carlsbad, California. The purpose of this update study was to review our previous geotechnical report and site environmental documentation (Leighton, 2006 and 2014), and to provide seismic design parameters in accordance with the 2013 CBC. We also performed a site visit on January 31, 2014 to observe current conditions. Currently, the site, approximately 1.89 acres, is utilized as commercial and light industrial property. 3934 Murphy Canyon Road, Suite B205 • San Diego, OA 92123-4425 858.292.8030 • Fax 858.292.0771 •www.leightongeo.com 10552.004 As background, Leighton performed the Initial geotechnical investigation of the subject site in 2005 (Leighton, 2005). Recently, Leighton performed a Phase I and Limited Phase II Environmental Site Assessment (ESA) Report for the site. The purpose of the Limited Phase II ESA was to assess the soil and soil vapor throughout the subject site for the presence of contaminants originating from current and historical site uses, such that the potential health risks can be evaluated prior to residential development. The environmental study included eight (8) subsurface borings across the subject site. For the findings and results of the environmental study, please refer to referenced document (Leighton, 2014). Based on our understanding of the site conditions and review of the referenced geotechnical and environmental documents, the geotechnical conditions of the subject site have not changed. Therefore, it is our professional opinion that the geotechnical recommendations presented in the referenced geotechnical report are still applicable for its intended use, provided the following recommendations are incorporated into the design and construction ofthe proposed buildings and improvements. In addition, the following items should be noted: • The undocumented fill associated with the former development is potentially compressible and should be removed down to the dense competent formation material. The depth ofthe removal is generally anticipated to be on the order of 3 to 5 feet below the existing ground surface. However, deeper undocumented fill associated with buried utilities, old septic systems, basements and underground storage tanks should be anticipated. • Impacted soil may be encountered during grading. Please refer to the referenced environmental study (Leighton, 2014). Note that special handling and waste profiling may be required for the excavation and disposal ofthis impacted soil. Seismicitv The following seismic design parameters for the site (latitude 33.164299, longitude-117.35299) are the risk-targeted spectral acceleration parameters for the project determined in accordance with the 2013 California Building Code (CBSC, 2013) and the USGS Worldwide Seismic Design Values tool (Version 3.1.0). 4 -2- Leighton 10552.004 Table 2 2013 CBC Risk-Targeted Mapped Spectral Acceleration Parameters Site Class D Site Coefficients Fa = 1.035 Fv = 1.554 Mapped MCER Spectral Accelerations Ss = 1.163g Si = 0.446g Site Modified MCER Spectral Accelerations SMS = 1.203g SMI = 0.693g Design Spectral Accelerations SDS = 0.802g SDI = 0.462g Utilizing ASCE Standard 7-10, in accordance with Section 11.8.3, the following additional parameters for the peak horizontal ground acceleration are associated with the Geometric Mean Maximum Considered Earthquake (MCEQ). The mapped MCEG peak ground acceleration (PGA) is 0.463g for the site. For a Site Class D, the FPGA is 1.037 and the mapped peak ground acceleration adjusted for Site Class effects (PGAM) is 0.48g for the site. Site Grading All undocumented fill associated with the former development should be removed down to the dense competent formation material. To reduce the potential for differential settlement beneath the proposed buildings (i.e., cut/fill transitions), we recommend the entire buiiding pad be overexcavated at least 3 feet below the lowest foundation and replaced with properly compacted fill. In addition, we recommend that the overexcavation extend laterally at least 10 feet beyond the building perimeter or footprint. Imported soil, if needed to raise the site grades, should be granular and have an expansion index less than 50 (per ASTM Test Method D4829). Import soils and/or the borrow site should be evaluated by the geotechnical consultant prior to importation. Leighton 10552.004 Foundation and Slab Desian for Residential Buildings The proposed residential buildings may be supported by conventional, continuous or isolated spread footings. Footings should extend a minimum of 24 inches beneath the lowest adjacent soil grade. At these depths, footings may be designed for a maximum allowable bearing pressure of 2,000 pounds per square foot (psf) if founded in properly compacted fill soils. The allowable pressures may be increased by one-third when considering loads of short duration such as wind or seismic forces. The minimum recommended width of footings is 18 inches for continuous footings and 24 inches for square or round footings. The interior and garage slab-on-grade floors for the residential buildings should be at least 4 inches and 5 inches thick, respectively, and be reinforced with No. 3 rebars 18 inches on center, each way (minimum), placed at mid-height in the slab. Slabs should be underlain by a 2-inch layer of clean sand (sand equivalent greater than 30), which is in-turn underlain by a minimum 10-mil plastic sheeting (moisture barrier) and an additional 2 inches of clean sand. We recommend that control joints be provided across the slab at appropriate intervals as designed by the project architect. Some moisture sensitive flooring may require additional measures to mitigate moisture migration through the slab as designed by the project architect. Limitations Recommendations contained in this limited report are based on our site reconnaissance, background review, and present knowledge of the proposed construction. It is possible that soil conditions vary between or beyond the points explored. If soil conditions are encountered during grading or foundation excavation, which differ from those described in the original geotechnical reports, our firm should be notified immediately in order that a review may be made and any supplemental recommendations provided. Our firm has performed our services in substantial accordance with the generally accepted geotechnical engineering practice as it exists in the site area at the time of our study. No warranty is made or intended. 4 -4- Leighton 10552.004 We appreciate the opportunity to be of service on this project. Should you have any questions related to this report, please contact our office at your convenience. Respectfully submitted, LEIGHTON and ASSOCIATES, INC William D. Olson, RCE 45283 Associate Engineer Michael R. Stewart, CEG 1349 Vice President / Principal Geologist Distribution: (4) Addressee -5 Leighton