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Home My WebLink AboutCT 02-19; BRESSI RANCH PA 11; GEOTECHNICAL UPDATE LETTER AND ADDENDUM RECOMMENDATIONS; 2013-03-084 ,,,.,.' c-r Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY March 8, 2013 Project No. 042458-001 To: Manolia Partners, LLC 1094 Cudahy, Suite 212 San Diego, California 92110 Attention: Mr. Paul Colucci Subject: Geotechnical Update Letter and Addendum Recommendations, Lots 6 and 11, Magnolia Estates, Bressi Ranch, PA-1 1, California References:. California Building Standards Commission (CBSC), 2010, California Building Code (CBC) Leighton, and Associates, 2007, Geotechnical Preliminary Investigation for Mass Grading, Bressi Ranch Carlsbad, California, Project No. 971009- 005, dated March 14, 2001 Leighton and Associates, 2004, As-Graded Report of Rough and Fine Grading, Lots 1 through 25, Planning Area PA-1 1, Bressi Ranch, Carlsbad Tract No. 00-06, Carlsbad, California,, Project No. 971009-014, dated October 8, 2004 In accordance with your request, we have prepared an update letter for the geotechnical conditions for Lots 6 and ii, Magnolia Estates located in Bressi Ranch, Carlsbad, California. The purpose of our geotechnical update evaluation was to observe the existing geotechnical conditions of the subject site, review the project geotechnical reports, and update geotechnical recommendations for the seismic design parameters per 2010 California Building Code, and provide appropriate conclusions and additional recommendations, if necessary, for the proposed site improvements. Based on our recent site visits and review of the referenced project geotechnical reports, the geotechnical conditions of the subject site have not significantly changed since the completion of grading in 2004 (Leighton, 2004), excluding a few end dumped 3934 Murphy Canyon Road, Suite B205 • San Diego, CA 92123-4425 858.292.8030 • Fax 858.292.0771 042458-001 piles of soil on Lot 6, some minor fencing and d ri'eway improvements.' Therefore, it is our professional opinion that the geotechnical recommendation presented in the As- Graded Report of Rough and Fine Grading re still applicable for :its intended use, provided the following recorrimendations are in corporated into the design of structures and construction of the buildings/structures. Updated Seismic Recommendations We have provided updated seismic design parameters per California Building Code (CBC 2010) for proposed design of buildings. TheTfoilôwing seismic design parameters have been determined in accordance with the 2010 CBC and the USGS Ground Motion Parameter Calculator (Version 5.10). CBC (2010) Seismic Code - Parameters for the Site Description Values CBC Reference Site Class D Table 1613.5.2 Short Period Spectral Acceleration ' S 1.123 Figure 16135(3) 1-Second Period Spectral Acceleration S1.0.426 Figure 1613 5(4) Short Period Site Coefficient ' F 1.051 Table 1613.5.3(1) 1-Second Period Site Coefficient F 1.575 Table 1613.5.3(2) Adjusted Short Period Spectral Acceleration SMS 1.181 Equation 16-36 Adjusted 1-Second Period Acceleration SM1 0 669 Equation 16-37 Design Short Period Spectral Response Parameter SDS 0.787 Equation 16-38 Design 1-Second Period Spectral Response Parameter SD1 0.446 Equation 16-39 Alternative Foundation Design Recommendations As an alternative to a post-tensioned slab and foundation systems for the mitigation of expansive soils and/or significant fill thickness, 'we recommend conventionally reinforced continuous and spread Jootings with interior grade beams for support of the floor slabs (e.g., a reinforced ribbed mat slab, RRMS). Continuous and spread" footings should extend a ,minimum of 30 and 24 inches, respectively, beneath the lowest adjacent finish grade. At these depths, footings may be designed for a maximum allowable bearing pressure 2,000 pounds per square foot -2- Leighton ' •''•'' .ç,, 042458-001. (psf) The allowable pressures may be increased by one-third when considering loads of short duration such as wind, but not for seismic forces Theminimm recommended width is 15 inches for cortinuous footings and 24 inches for square round footings Minimum recommended width and depth of the ribs (i.e., grade beams used tc5 support floor slabs) is 12 and 18 inches, respectively: In addition, 'werecorrmthd .a ribs spacing (i.e., an unsupported floor slab span length) ofj 6-feet or less. Footing reinforcement should be' desi9. hed in accordance with. the' stctural engineer's requirements, but not less than four No 5 reinforcing bars two top ànd two bottom) for continuous footings Additional design and construction information for the RRMS foundation ôan be obtained in US Army Corps of Engineer, Technical Instructions (TI) 809-28' (USACOE, 1999). LateralEärth Pressures for Basement Wall The. recommended lateral .'pressures.for the 'onsjtvery low to low expansive soil (expansion index less than 50) or mediu'm expansi\ie soil (expansion index between 51 anJ 70) áid level or sloping backfilbre presented on the table below Higher medium to very high 'expansive soils (having an expansion potential greater than 70) should not be used asbckfilI soils! ...'. . Lateral Earth Pressures Conditions Equivalent Fluid Weight (pcf) Very Low to Low Expansive' Soils Medium Expansive Soils Expansion index less thn 50 Expansion index'between 51 and 70 Level., 2:1' Slope ' Level T 2:1 Slope Active 35 ' 55 60 70 At-Rest 55 ' 65 70 80 Passive 350 150 350 ' 150 Embedded structural walls shOuld. be designed for; lateral earth pressures exerted on them The magnitude of these pressures depends on the amount of deformation that the wall can yield under load If the wall can yield enough to mobilize the full shear strength of the soil, it can be designed for "active" pressure If the wall cannot yield under the applied load, the shear strength of the.soil cannot be mobilized and the earthpressure will be higher. Such walls should be designed for "at rest" conditions (i.e., a basement wall). If a Leighton - t •' i •. - 10. ills 042458-001 structure moves toward the soils, the resulting resistance developed by the soil is the "passive" resistance To account for potential redistribution of forces during a seismic seismic pressure distribution equal to 8. H psf where H equals the overall retained height in feet (ASCE. 2006). For design purposes, the recommended equivalent fluid pressure for each case for walls founded above the static ground water and backfilléd with soils of very low to low expansion potential or medium expansion potential is provided on the table above. The equivalent fluid pressure values assume free-draining conditions. If conditions other than those assumed above are anticipated, the equivalent fluid pressures values should be provided on an individual-case basis by the geoteöhnical engineer. The geotechnical and structural engineer shOuld evaluate surcharge-loading effects from the adjacent structures. All retaining Wall structures should be provided with appropriate drainage and appropriately waterproofed. The outlet pipe should be sloped to drain to a suitable outlet. Typical wall drainage design is illustrated in Appendix D. For sliding resistance, the friction coefficient of 0.35 may be used at the concrete and soil interface In combining the total lateral resistance, the passive pressure or the frictional resistance should be reduced by 50 percent. Wall footings should be designed. in accordance with structural considerations. The passive resistance value may. be increased by one-third when considering-loads of short duration including wind or seismic loads. The horizontal distance between fOundation elements providing passive resistance should be minimum of three times the depth of the elements to allow full development of these passive pressures. The total depth of retained earth for the design of cantilever walls should be the vertical distance below, the ground surface measured at the wall face for stem design or measured at the heel of the footing for overturning and sliding. All wall backcuts should be made in accordance with the current OSHA requirements. The. granular and native backfill soils should be compacted to at least 90 percent relative compaction (based on ASTM Test Method D1557). The granular fill should extend horizontally to a minimum distance equal to one-half the wall height behind the walls. The walls should be constructed and backfi lied ässoon as possible after backcut excavations. Prolonged exposure.of backcut slopes may result in some localized slope instability The recommendations provided in this, letter are based on as-graded subsurface conditions observed during the previous site grading. The interpolated subsurface conditions should be checked in the field during construction. Construction observation of all onsite excavations and field density testing of all compacted fill should be performed by a representative of this office. All footing excavations should be reviewed by this office prior to placing steel or concrete. -4- , 4 Leighton