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HomeMy WebLinkAboutCDP 96-14; Marja Acres PCS Facility; Coastal Development Permit (CDP) (12)HETHERINGTON ENGINEERING, INC. SOIL & FOUNDATION ENGINEERING • ENGINEERING GEOLOGY • HYDROGEOLOGY May 14, 1996 Project No. 1519.1 Log No. 2265 J. M. Consulting Group 3530 Camino Del Rio North, Suite 101 San Diego, CA 92108 / Attention: Ms. Cheryl M. Hacke Subject: SUPPLEMENTAL GEOTECHNICAL INVESTIGATION Proposed Cox California PCS Station SD154-Marja Acres Carlsbad, California Reference: "Geotechnical Investigation, Proposed Cox California PCS Station, SD154-Marja Acres, Carlsbad, California," by Hetherington Engineering, Inc., dated February 21, 1996. Dear Ms. Hacke: In accordance with the request of Ms. Blanca Bracamontes of your office, we have performed a supplemental geotechnical investigation at the subject site. The purpose of our additional investigation was to evaluate soil and geologic conditions, and provide grading and foundation recommendations for the proposed revised location of the equipment storage area. To assist with our investigation, we were provided with a Site Plan, prepared by Gold Coast Surveying. With the above in mind, our supplemental work included the following: o Subsurface exploration consisting of one backhoe trench to obtain soil and bedrock samples and observe geologic conditions. o Laboratory testing. o Engineering and geologic analyses. o Preparation of this report providing the results of our field and laboratory work, analyses, and our conclusions and recommendations. SITE DESCRIPTION AND PROPOSED DEVELOPMENT It is our understanding that the revised location for the equipment storage area is within the toe area of the existing slope immediately south of the asphalt concrete driveway/parking area and Country Store building. The equipment storage area will reportedly 5245 Avenida Encinas, Suite G • Carlsbad, CA 92008-4369 • (619) 931-1917 • Fax (619) 931-0545 32242 Paseo Adelanto, Suite C • San Juan Capistrano, CA 92675-3610 • (714) 487-9060 • Fax (714) 487-9116 SUPPLEMENTAL GEOTECHNICAL INVESTIGATION Project No. 1519.1 Log No. 2265 May 14, 1996 Page 2 consist of an approximate 10 feet by 20 feet pad excavated into the north facing hillside and bounded by retaining walls up to 10 feet high (maximum) . We understand that the two antenna sites have been relocated approximately 10 feet downslope from their original proposed locations. SUBSURFACE EXPLORATION Subsurface conditions were explored by excavating one backhoe trench into the hillside, to a depth of approximately 10 feet measured at the south wall of the trench. The approximate location of the trench is shown on the attached Plot Plan, Figure 1. The subsurface exploration was supervised by a geologist from this office, who visually classified the soil and bedrock materials, and obtained bulk and undisturbed samples for laboratory testing. The soils were visually classified according to the Unified Soil Classification System. Classifications are shown on the Trench Log, Figure 2. LABORATORY TESTING Laboratory testing was performed on samples obtained during the subsurface exploration. Tests performed consisted of the following: o Dry Density and Moisture Content (ASTM: D 2216) o Maximum Dry Density/Optimum Moisture Content (ASTM: D 1557-91A) o Direct Shear (ASTM: D 3080) Results of the dry density and moisture content determinations are presented on the Trench Log, Figure 2. The remaining laboratory tests are presented on Figure 3. SOIL AND GEOLOGIC CONDITIONS Soil and geologic conditions encountered in this phase of subsurface work are similar to those described in our previous report (Reference 1) . The proposed construction site for the equipment storage area is underlain by surficial soils consisting of colluvium, which is in turn underlain by Santiago Formation bedrock. The colluvium ranges from approximately 5 feet thick on HETHERINGTON ENGINEERING, INC. SUPPLEMENTAL GEOTECHNICAL INVESTIGATION Project No. 1519.1 Log No. 2265 May 14, 1996 Page 3 the south wall of the trench to approximately 3 feet thick near the toe of the slope and consists generally of very moist, soft to stiff, dark brown to dark olive brown fine sandy clay. Bedrock materials encountered in the trench consist of moist, dense to very dense, moderately cemented, light green to light gray silty fine sandstone. No groundwater was observed in the exploratory trench. CONCLUSIONS AND RECOMMENDATIONS 1. General The proposed relocation of the equipment storage area is feasible from a geotechnical standpoint. Grading and foundation plans should take into account the appropriate geotechnical features of the site. 2. Site Grading Following excavation into the north facing hillside to create the relatively level pad for the equipment storage area, any exposed colluvium should be removed down to bedrock and replaced as compacted fill. Fill should consist of sandy soils derived from the underlying bedrock. Fill should be compacted by mechanical means in uniform horizontal lifts of 6 to 8-inches in thickness. All fill should be compacted to a minimum relative compaction of 90 percent based upon ASTM: D 1557-91A. Soils used as compacted fill should be placed at about optimum moisture content. Rock fragments over 6-inches in dimension and other perishable or unsuitable materials should be excluded from the fill. All grading and compaction should be observed and tested as necessary by the Geotechnical Engineer. 3. Foundation and Slab Recommendations a. The proposed equipment storage area may be supported on conventional continuous footings bearing entirely in bedrock. Footings should extend to a minimum depth of 12-inches into bedrock. Footings located adjacent to utility trenches should extend below a 1:1 plane projected upward from the inside bottom corner of the trench. Continuous footings should be reinforced with a minimum of two #4 bars, one top and one bottom. Footings bearing as recommended may be designed for a dead plus live load bearing value of 2000 pounds per HETHERIIMGTON ENGINEERING, INC. SUPPLEMENTAL GEOTECHNICAL INVESTIGATION Project No. 1519.1 Log No. 2265 May 14, 1996 Page 4 square foot. This value may be increased by one-third for loads including wind or seismic forces. A lateral bearing value of 200 pounds per square foot per foot of depth and a coefficient of friction between foundation soil and concrete of 0.4 may be assumed. These values assume that footings will be poured neat against the foundation soils. Footing excavations should be observed by the Geotechnical Engineer prior to the placement of steel to ensure that they are founded in suitable bearing materials. Floor slabs should have a minimum thickness of 6-inches and should be reinforced with at least #4 bars spaced at 12-inches, center to center, in two directions, and supported on chairs so that the reinforcement is at mid- height in the slab. Prior to placing concrete, the slab subgrade soils should be thoroughly moistened. b. Antennas The antennas located on the slope should be designed in accordance with our previous recommendations contained in the referenced report. 4. Retaining Walls Retaining walls free to rotate (cantilevered walls) should be designed for an active pressure of 40 pounds per cubic foot, equivalent fluid pressure, assuming level backfill consisting of on-site soils derived from the bedrock material or imported granular material. Walls restrained from movement at the top should be designed for an additional uniform soils pressure of 8xH pounds per square foot where H is the height of the wall in feet. Any additional surcharge pressures behind the wall should be added to these values. Retaining wall footings should be designed in accordance with the previous building foundation recommendations. Retaining walls should be provided with adequate drainage to prevent buildup of hydrostatic pressure and should be adequately waterproofed. Temporary construction slopes necessary to facilitate wall installation should be inclined no steeper than 1:1 (horizontal to vertical) and observed by the geotechnical consultant to assure conditions are as anticipated. HETHERINGTON ENGINEERING, INC. SUPPLEMENTAL GEOTECHNICAL INVESTIGATION Project No. 1519.1 Log No. 2265 May 14, 1996 Page 5 5. Trench and Retaining Wall Backfill All trench and retaining wall backfill should be compacted to at least 90 percent relative compaction and tested by the Geotechnical Engineer. 6. Grading and Foundation Plan Review Final grading and foundation plans should be reviewed by the Geotechnical Consultant to confirm conformance with the recommendations presented herein or to modify the recommendations as necessary. LIMITATIONS The analyses, conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our investigation and further assume the excavations to be representative of the subsurface conditions throughout the site. If different subsurface conditions from those encountered during our exploration are observed or appear to be present in excavations, the Geotechnical Engineer should be promptly notified for review and reconsideration of recommendations. This opportunity to be of service is appreciated. If you have any questions, please call Danny Cohen at the Carlsbad office. Sincerely, HETHERING DANNY COHEN Civil Engin (expires 3Geotechnical (expire 3/31/00 BRANDON A. BOKA Senior Engineering Registered Geologist 5913 Certified Engineering Geolog (expires 3/31/98) MDH;DC;BAB/sl HETHERINGTON ENGINEERING, INC. (E) BUILDING ROOF RIDGE LINE 73.4' ASPHALT HAVEMENT PROPOSED ANTENNA ARRAY PROPOSED EASEXOT U.G. CABLE LEGEND APPROXIMATE SCALE: 1'=40' T-1 APPROXIMATE LOCATION OF BACKHOE TRENCH PLOT PLAN HETHERINGTON ENGINEERING, INC. GEOTECHNICAL CONSULTANTS JMCG/COX-SO 154 Marja Acres. Carlsbad PROJECT NO. 1519.1 FIGURE NO. 1 om EARTH MATERIALS COLLUVIUM: Dark brown to dark olive brown fine sandy clay, very moist, soft, rootlets, some random 1/32" caliche veins. @18": Becomes firm to stiff, some 1/32" caliche veins, some 1/2" diameter inclusions of white sandstone, trace rootlets @ 5': Contact dips about 32 to 22 degrees north BEDROCK; Light green to light gray silty fine sandstone, moist, dense to very (Santiago dense, moderately cemented. Formation) @ 5' to 6': weathered, fractured, dense, abundant caliche along fractures, rootlets. @ 7': Decrease in caliche and fracturing, becomes very dense. ATTITUDES 6': Caliche lined iractures, N70E/86SE, :uts laminae @ 7 1/2' 7 1/2': Approximate L/8 to 1/4' caliche .aminae in approximate i" wide orange stained '.s., undulatory, fairly Continuous, N55E/09NW O 5' DryDensity (pcf) 98 104 138 Chn> Moisture Content 19,2 19.9 7.3 Sample Depth X GRAPHIC REPRESENTATION SCALE- r-s- (HV) SANTIAGO FOFIMATION ASPHALT PAJVEMENT 4r n |— O(Q(D CDQ. CD><BH DOm E P z O T CDD 21o" 3 m P m r~OOo$ z SUMMARY OP MAXIMUM DRY DENSITY/OPTIMUM MOISTURE CONTENT DETERMINATIONS (ASTM: D 1557-91A) Sample Location Description Maximum Dry Density (pcf) Optimum Moisture Content TP-1 @ 5'Light gray silty fine sand 113.0 15.0 DIRECT SHEAR (ASTM: D 3080) Sample Location TP-1 @ 5' Cohesion (psf) 225 Internal Angle of friction ( • ) 30 Remarks Remolded to 90% of maximum, consolidated, saturated, drained Project No. 1519, Figure No.