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Home My WebLink AboutCT 06-15; TABATA RANCH; UPDATED GEOTECHNICAL RECOMMENDATIONS; 2016-08-26CTE INC August 26, 2016 Construction Testing & Engineering, Inc. Inspection I Testing I Geotechnical I Environmental & Construction Engineering I Civil Engineering I Surveying CTE Project No. 10-13292G Tabata Family Trust CIO: WMC General Contractors, Inc. Attention: Mr. Bruce Wiegand 760 Garden View Ct., Suite 200 Encinitas, California 92024 Telephone: (760)803-2427 Via Email: bruce(wncommunities.com Subject: Update Geotechnical Recommendations for Proposed Tabata Ranch Subdivision Lemon Leaf Drive Carlsbad California References: At end of document Mr. Wiegand: In accordance with your request, Construction Testing & Engineering, Inc. (CTE) has completed a review of the previously prepared geotechnical reports and documentation for Lots 1-5 at the subject site, as referenced herein. We also recently visited the site on August 23, 2016 to confirm current conditions. Recommendations are based on prior site documentation referenced herein, new and/or updated regulatory requirements, review of current site conditions, and the scope of work we agreed to perform at this time. Based on the project improvement plans, the proposed construction is to consist of two-story, light- framed, residential structures with conventional continuous and/or spread footings. Associated utilities, flatwork, paving, landscaping, and other minor improvements may also be constructed. CTE has found the recommendations in the referenced geotechnical documents to be in compliance with common geotechnical engineering practices and should be considered valid unless revised herein. However, CTE reserves the right to further modify recommendations and/or provide additional recommendations based on the actual conditions encountered at the site during earthwork and/or construction. Updated seismic loading parameters are also provided herein in accordance with the requirements of the current California Building Code. Updated standard grading recommendations (Appendix D) are also attached herewith, though additional grading is anticipated to be relatively minor. 1441 Montiel Road, Suite 115 1 Escondido, cA92026 I Ph(760)746-4955 I Pax(760)746-9806 I www.cte-inc.net Update Geotechnical Recommendations Page 2 Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad California August 26, 2016 CTE Job No. 10-13292G Based on our review, the existing building pads at the subject site were previously graded in accordance with the referenced reports. It appears that site conditions have remained generally consistent to those described in the original and as-graded reports. Significant changes, distress, and/or erosion in the building pad areas was not noted. Therefore, based on this information and on reference review, we provide the following update recommendations. 1.0 SITE PREPARATION Based on the limited preliminary plans and information provided, as well as our understanding of the proposed development, the following remedial recommendations are believed to be appropriate at this time. All proposed building footprints and other distress sensitive improvement areas should be cleared of existing vegetation, construction debris, stockpiled, deleterious, and other loose materials. Objectionable materials, such as construction debris and vegetation, not suitable for structural backfill should be properly disposed of offsite. Following removal of loose and unsuitable soils and approval from the geotechnical representative, exposed areas should be scarified a minimum of eight inches, moisture conditioned, and compacted to a minimum relative compaction of 90 percent, as evaluated by ASTM D 1557, at a minimum two percent above optimum. Fill and backfill should be compacted to a minimum relative compaction of 90 percent as evaluated by ASTM D 1557, at a minimum two percent above optimum. Minimum relative compaction of 95 percent (per ASTM D-1557) should be conducted for the minimum top 12 inches of subgrade beneath proposed pavement and drive areas, and for all/any aggregate base. The optimum lift thickness for backfill soil will depend on the type of compaction equipment used. Generally, fill soil should be placed in uniform lifts not exceeding eight inches in loose thickness. Fill placement and compaction should be performed in overall conformance with the geotechnical recommendations and project specifications in conformance with local ordinances. A CTE geotechnical representative should observe and test the bottom of overexcavation and compaction of soils within the proposed building footprints and other distress sensitive improvement areas. 2.0 SEISMIC LOADING PARAMETERS The seismic ground motion values listed in the table below were derived in accordance with the ASCE 7-10 Standard that is incorporated into the California Building Code, 2013. This was accomplished by establishing the Site Class based on the soil properties at the site, and then calculating the site coefficients and parameters using the United States Geological Survey Seismic Design Maps application. These values are intended for the design of structures to resist the effects of earthquake ground motions. \\ESC_SERVER\Projects\10-13292G\Ltr Update Recommendations - 10-1 3292G.doc Update Geotechnical Recommendations Page 3 Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad California August 26, 2016 CTE Job No. 10-13292G TABLE 2.0 SEISMIC GROUND MOTION VALUES 2013CBC AND ASCE 7-10 PARAMETER VALUE CBC REFERENCE (2013) Site Class D ASCE 7, Table 20.3-1 Mapped Spectral Response Acceleration Parameter, 5s 1.108g Figure 1613.3.1 (1) Mapped Spectral Response 0.426g Figure 1613.3.1 (2) Acceleration Parameter, S Seismic Coefficient, Fa 1.057 Table 1613.3.3 (1) Seismic Coefficient, F 1.574 Table 1613.3.3 (2) MCE Spectral Response 1.171g Section 1613.3.3 Acceleration Parameter, SMS MCE Spectral Response Acceleration Parameter, 5M1 0.671g Section 1613.3.3 Design Spectral Response Acceleration, Parameter 5DS 0.780g Section 16 13.3.4 Design Spectral Response Acceleration, Parameter 5D1 0.447g Section 16 13.3.4 Peak Ground Acceleration PGAM 0.466g ASCE 7, Section 11.8.3 3.0 FOUNDATION WALL AND SLOPE SETBACK Footings for structures should be designed such that the horizontal distance from the face of adjacent descending slopes to the outer edge of the footing is a minimum of 10 feet. In addition, foundations should bear beneath an imaginary 1:1 plane extended up from the nearest bottom edge of adjacent parallel trenches or excavations located within 10 feet. Deepening of affected footings should be a suitable means of attaining the prescribed setbacks. In addition, footings located adjacent to retaining walls should bear beneath an imaginary 1:1 plane extending upward from the base of the wall retaining soil in order to minimize additional surcharge load to the wall itself. 4.0 WALLS BELOW GRADE If retaining walls are to be constructed in association with the proposed development, the following parameters are anticipated to be applicable. For the design of subterranean walls where the surface of the backfill is level, it may be assumed that the soils will exert a lateral pressure equal to that developed by a fluid with a density of 35 pcf. The active pressure should be used for walls free to yield at the top at least 0.2 percent of the wall height. For walls restrained so that such movement is not permitted, an equivalent fluid pressure of 60 pcf should be used, based on at-rest soil conditions. The recommended \\ESC_SERVER\Projects\10-13292G\Ltr Update Recommendations - 10-13292G.doc Update Geotechnical Recommendations Page 4 Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad California August 26, 2016 CTE Job No. 10-13292G equivalent fluid pressures should be increased according to Table 4.0 below for walls retaining soils inclined at 2:1 (horizontal: vertical). Walls below the water level are not anticipated for the subject site. In addition to the recommended earth pressure, subterranean structure walls adjacent to traffic loads should be designed to resist a uniform lateral pressure of 100 psf. This is the result of an assumed 300- psf surcharge behind the walls due to normal street traffic. If the traffic is kept back at least 10 feet or a distance equal to the retained soil height from the subject walls, whichever is less, the traffic surcharge may be neglected. The project architect or structural engineer should determine the necessity of waterproofing retaining walls to reduce moisture infiltration. Retaining wall backfill located within a 45-degree wedge extending up from the heel of the wall should consist of soil having an Expansion Index of 30 or less (ASTM D 4829) with less than 30 percent passing the No. 200 sieve. The upper 12 to 18 inches of wall backfill should consist of lower permeability soils, in order to reduce surface water infiltration behind walls. The project structural engineer and/or architect should detail proper wall backdrains, including gravel drain zones, fills, filter fabric and perforated drain pipes. TABLE 4.0 EQUIVALENT ELLID L]NIT WEIGHTS (pounds per cubic foot) SLOPE BACKFILL WALL TYPE LEVEL BACKFILL 2:1 (HORIZONTAL: VERTICAL) CANTILEVER WALL (YIELDING) 35 45 RESTRAINED WALL 60 80 Lateral pressures on cantilever retaining walls (yielding walls) due to earthquake motions may be calculated based on work by Seed and Whitman (1970). The total lateral thrust against a properly drained and backfilled cantilever retaining wall above the groundwater level can be expressed as: PAE = PA + APAP For non-yielding (or "restrained") walls, the total lateral thrust may be similarly calculated based on work by Wood (1973): PKE = PK + AP1 Where PA = Static Active Thrust (given previously Table 4.0) PK = Static Restrained Wall Thrust (given previously Table 4.0) APAE = Dynamic Active Thrust Increment = (3/8) kh yH2 APKE = Dynamic Restrained Thrust Increment = kh yH2 \\E5C_5ERVER\Projects\10- 13292G\Ltr_Update Recommendations - I0-132926.doc Update Geotechnical Recommendations Page 5 Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad California August 26, 2016 CTE Job No. 10-13292G kh = 2/3 Peak Ground Acceleration = 2/3 (PGAM) H = Total Height of the Wall Total Unit Weight of Soil z 135 pounds per cubic foot The increment of dynamic thrust in both cases should be distributed triangularly with a line of action located at H13 above the bottom of the wall (SEAOC, 2013). These values assume non-expansive backfill and free-draining conditions. Measures should be taken to prevent moisture buildup behind all retaining walls. Drainage measures should include free-draining backfill materials and sloped, perforated drains, as designed and detailed by the wall engineer or architect of record. These drains should discharge to an appropriate off-site location. Any necessary waterproofing should be as specified by the project architect. 5.0 LIMITATIONS As indicated, the updated recommendations herein are based on our evaluation performed to date and could require modification as project improvement plans further progress and/or based on conditions encountered during construction. The field evaluation and geotechnical analysis referenced in our geotechnical documents was conducted according to current engineering practice and the standard of care exercised by reputable Geotechnical Consultants performing similar tasks in this area. No other warranty, expressed or implied, is made regarding the conclusions, recommendations and opinions expressed. Variations may exist and conditions not observed or described may be encountered during construction. Our conclusions and recommendations are based on an analysis of the observed conditions. If conditions different from those described are encountered, our office should be notified and additional recommendations, if required, will be provided upon request. We appreciate the opportunity to be of service on this project. Should you have questions, please contact the undersigned at your convenience. Respectfully submitted, CONSTRUCTION TESTING & ENGINEERING, INC. No.2665 (j (o No.84232 Dan T. Math, GE #2665 EXP.12/31/16 Jj Rodney J. Jones, RCE #84232 1 . 9/30/17 10 Principal Engineer , JJ Project Engineer RJJ/JFL/DTM:nri \\ESC_SERVER\Projects\IO- I 3292G'Ltr_Update Recommendations - 10- 13292G.doc Update Geotechnical Recommendations Page 6 Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad California August 26, 2016 CTE Job No. 10-13292G References: Interim As-Greaded Geotechnical Report Proposed Tabata Ranch Subdivision Lemon Leaf Drive Carlsbad, California CTE Project No. 10-12101G, dated August 2, 2015 Lot 3 Subdrain Recommendations and Elimination of Retaining Wall Proposed Tabata Ranch Subdivision Lemon Leaf Drive - Carlsbad California CTE Project No. 10-12101G, dated July 14, 2014 Transfer of Geotechnical Responsibility Letter Proposed Tabata Ranch Subdivision Lemon Leaf Drive Carlsbad, California CTE Project No. 10-12101G, dated July 1, 2014 Geotechnical and Update Report and Grading Plan Review Proposed Tabata Ranch Subdivision Lemon Leaf Drive, Carlsbad, California Vinje & Middleton Engineering Job # 01-364-P, dated August 2, 2006 Preliminary Geotechnical Investigation Proposed 4-Lot Subdivision, Tabata Ranch Off Camino De Las Ondas and Lonicera Street, Carlsbad, California Vinje & Middleton Engineering Job # 01-364-P, dated October 24, 2001 \\ESC_SERVER\Projects\10-13292G\Ltr Update Recommendations - 10-1 3292G.doc