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Home My WebLink AboutCDP 03-07; Thompson Residence; UPDATE TO GEOTECHNICAL INVESTIGATION; 2006-09-08GEOCON I N C O R P 0' R A T .E D Project No. 06716-32-01 September 8, 2006 SeaBourne Development Company Post Office Box 4659 Carlsbad, California 92018-4659 Attention: Mr. Ken Cablay Subject: THOMPSON PROPERTY 7066 CRYSTALLINE DRIVE CARLSBAD, CALIFORNIA UPbATE TO GEOTECHNICAL INVESTIGATION - Nffi. RECEIVED OCT 13 2006 ENGINEERING DEPARTMENT References: 1. Geotechnical Investigation, Thompson Property, Carlsbad, California, prepared by Geocon Incorporated, dated June 28, 2001 (Project No. 06716-32-01). Gentlemen: 2. · Grading Plans For: 70.66 Crystalline Drive, Lot 242, CT 98-14, Carlsbad, California, prepared by Buccola Engineering, Inc., Sheet 2 of 4, sent via e-mail, dated August 30, 2006 (Project No. CDP 03-07). In accordance with your request, we have reviewed the referenced plans for the proposed residential development in Carlsbad, California. The purpose of our review was to determine whether the plans and details have been prepared in substantial conformance with the recommendations presented in the referenced geote:;chnical report and provide additional and/ or revised recommendations, if necessary. The referenced grading plan shows a 6-foot high (maximum), mechanically stabilizea. earth (MSE) retaining wall and an approximately 20 foot high, 2:1 (horizontal:vertical) slope to create a relatively level surface in the rear yard of the existing residence. Import material will be necessary to achieve finish grade elevation. In accordance With the recommendations provide~ the referenced geotechnical report, remedial grading will be performed to remove and compact unsuitable surficial soils prior to placing new fill. The maximum depth of removal is approximately 8 feet. the temporary excavation should begin at the property boundary and be sloped back. into the property in accordance with current OSHA guidelines. Building restrictions or setbacks are not anticipated based on the proposed remedial 6960 flanders.Drive • San Diego, California 92121-2974 • Telephone (858) 558-6900 • Fox {858) 558-6159 grading. The wall foundation should be excavated such that the bottom outside edge of footing is at least 7 feet horizontally from the face of slope, i.e., for a 2:1 slope, the wall footing would need to extend approximately 3.5 feet below existing grade. Based upon our review of the referenced plans and the information contained within the referenced geotechnical report, it is the opinion of Geocon Incorporated that the plans and details have been prepared in substantial conformance with recommendations presented in the referenced documents. In addition, the recommendations provided in the referenced report remain applicable for the project. MSE Retaining Walls Based on information obtained from the referenced geotechnical investigation, we recommend the following geotechnical parameters be used for design of the MSE retaining wall: Parameter Reinforced Zone Retained Zone Foundation Zone Angle of internal Friction 30 degrees 30 degrees 30 degrees Cohesion 200 psf 200 psf 200 psf Wet Unit Weight 125 pcf 125 pcf 125 pcf The above soil parameters are based on direct shear-strength tests performed during the referenced investigation ·our experience at the subject site. Geocon has no way of knowing whether these materials will actually be used as backfill behind the wall during construction. It is up to the wall designer to use his judgment in selection of design parameters. As such, once backfill materials have been selected and/or stockpiled, sufficient shear tests should be conducted on samples of the proposed backfill materials to verify they conform to actual design values. Results should be provided to the designer to re-evaluate stability of the walls. Dependent upon test results, the designer may require modifications to the original wall design (e.g., longer geogrid embedment lengths). Backfill. materials within the reinforced zone Should_ be compacted to a dry density of at least 90 percent of the laboratory maximum dry density at or slightly above optimum moisture content in accordance with ASTM D 1557-02. This is applicable to the entire embedment length of the geogrid reinforcement. Typically, wall designers specify that heavy compaction equipment be excluded from . ' ' within 3 feet of the face of the wall; however, smaller equipment ( e.g., walk-behind, self-driven compactors or hand whackers) should be used to compact the materials without causing deformation of the wall. If the designer specifies no compactive effort for this zone, the ~terials are essentially Project No. 06716-32-01 -2-September 8, 2006 .) -- not properly compacted and the geogrid withiJ:1 the uncompacted zone should not be relied upon for reinforcement and overall embedment lengths should be increased to account for the difference. The wall ~esigner should con~ider providing a drainage system sufficient to dissipate hydrostatic pressure behind the wall and to mitigate seepagetbrqugh and beneath the wall. Geosynthetic-reinforceme_nt must elongate to develop full tensile resistance. This elongation generally results m movement at the top.-of the wall. The amour1t of movement is dependent upon the height of the wall (e.g., hi-gher walls rotate more), construction, c:llld the cype of geosynthetic used. In addition, over time reinforced-earth retaining walls have been known to exhibit creep and can undergo additional movement. Given this condition, the owner should be aware that structures and pave:r;nent placed within the !einforced and retained zones of the wall may undergo movement and should be designed to accommodate this movement. If you have any questions regarding this review, or ifwe may be of further service, please contac;t the- undersigned at your convenience. Very truly -yours, . GEOCON INCORPORATED TEM:DBE:anh (2) Addressee (3) Buccola Engineering _ Attention: Mr: Phil Buccloa Project No. 0(i716-32-0i -3-September 8, 2006