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Home My WebLink AboutCT 75-04; El Fuerte Street; Preliminary Soils Report; 1986-05-22PRELIMINARY GEOTECHNICAL STUDY, LOT NO. 100 CARLSBAD TRACT NO. 75-4. CITY OF CARLSBAD, COUNTY OF SAN DIEGO, CALIFORNIA FOR R. G. CRAhTORD COMPANY, INC. BOX 6054 ALHAMBRA, CALIFORNIA 91802 W.O. 624-SD MAY 22, 1986 ENGINEERING DEPT. LIBRARY City of Carlsbad 2075 Las Palmas Orive CarlsbaQ CA 92009-4859 GeoSofls, Inc. IN’I’ROWIYION ....................................................... DESCRIFTION OF SITE ................................................ PIlDpOSED SITE DEVELLlpMENT .......................................... FIELD EXPLORATION .................................................. EaFcH mmms .................................................... RXK HARDNESS AND FILL QUALITY ..................................... CPOUNDVRER ......................................................... SEISMICITY .......................... CONCLUSIONS AND RE(3o”ENDATIONS ..... General Cut and Fill Slopes EOUNDATION DESIGA ................... Bearing Value Lateral Pressure FOUNDATION CONSTRUCTION ............. ............................... ............................. ............................. ............................. GRADING GUIDELINES ................................................. Grading Control WILITY TRENCH BACKFILL ............................................ LIMITATIONS ........................................................ 1 1 2 2 2 3 4 4 4 5 7 8 11 11 CeoSoils, Inc. - - Soil Mechanics- Geology Foundation Engineering - 5751 PalmerWay - SuiteD * Carlsbad, California 92008 * (619)438-3155 Hay 22, 1986 W.O. 624-SD R. G. CRAWFORD COMPANY, INC. Box 6054 Alhambra, California 91802 Attention: Mr. R. G. Crawford Subject : Lot NO. 100, Carlsbad Tract No. 75-4, Preliminary Geotechnical Study, City of Carlsbad, County of San Diego, California Gentlemen: This report presents the results of our preliminary geotechnical study of the subject property. The purpose of this study was to evaluate the nature of the materials underlying the site relative to the feasibility of constructing a single family residence on the site. Our field investigation was performed on May 15, 1986. DESCRIPTION OF SITE The subject property is located on the north side of El Fuerte Street between Acuna Court and Bolero Street, in the La Costa area of the County of San Diego, California. The precise loca- tion is shown on the attached Site Location Map, Figure I. Topographically, the site slopes at gradients between 3:l to 4:l toward the south. A south facing cut slope up to eight (2) feet Los Angeles County Office: (818) 785-2158 * Orange County Gffice:[714) 859-4599 __ R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.0. 624-SD PAGE 2 in height, at gradients of about 2:l to 1JI:l is present along El Fuerte Street. A small fill slope is present near the rear portion of the site. There is a small drainage channel along the west side of the site. Vegetation onsite consists of cereal grasses and a moderate growth of native shrubs. Access to the site is via a ramp in the cut slope along El Fuerte. PROPOSED SITE DEVELOPMENT It is our understanding that a single family residence will be developed on the site. Specific development plans are unknown. FIELD EXPLORATION Subsurface conditions were explored by excavating five backhoe trenches at depths varying from two to four feet. Excavation of the test pits was supervised by one of our geolo- gists. The soils brought out by excavation of the Test Pits were examined, logged and classified by visual examination in accor- dance with the Unified Soil Classification System. Logs of the test pits are enclosed as Table I at the end of this report: locations of the pits are indicated on the Site Sketch, Plate 1. EARTH MATERIALS Earth materials onsite consist of fill, topsoil and bedrock. The fill is comprised of topsoil and bedrock materials. It is found near the north property line and appears to be about 2-4 CeoSofls, Znc. - R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 3 feet thick. It is associated with a small access road above the site. It is rather rocky and poorly compacted. As such, it should not be used for structural support. Most of the site is blanketed by a red-brown sandy, clayey silt which is an incipient topsoil. This material is loose and was dry at the time of our field study. It should not be used for structural support and should be removed in areas to receive fill. Bedrock onsite consists of basaltic rock of the Santiago Peak Volcanics. The upper 1-1% feet is generally well fractured and weathered. It becomes hard and dense and excavation with the backhoe was difficult below depths of three to four feet. Typi- cally, the unweathered rock is a grey to dark grey aphanitic basalt. Fracture spacing varies from several inches to one or two feet. Brown or rust staining is prevalent along joints and fractures. The bedrock should provide good structural support. ROCK HARDNESS AND FILL QUALITY Based on observation of existing cut slopes and a swimming pool excavation on the adjoining property to the east, we anticipate that the bedrock should excavate using conventional equipment to depths on the order of eight to ten feet. The difficulty of excavation will depend on the size of equipment. Local areas or deep excavation could require blasting or similar methods to excavate, however, no specific areas are known. GeoSoils, Znc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.0. 624-SD PAGE 4 Moderate quality fill material would be generated from bedrock materials. While it should be suitable for placement in struc- tural fill, some difficulties are anticipated due to the blocky nature of the material and oversize rock fragments. GROUNDWATER No groundwater was encountered in the test pits and no evidence of near surface groundwater was observed. Groundwater is not - anticipated to affect site development. It is. our understanding that a wet area was noted in the north- west corner of the site. This is believed to be the result of surface water concentration in the drainage swale not a ground- water condition. SEISMICITY The site, as all of Southern California, is in a seismically active region. There are no known active or potentially active faults on or in immediate proximity of the site. There are faults in close enough proximity to cause earthquake induced - ground shaking on the site. Moderate to intense ground shaking should be anticipated within the next 50 to 100 years. Seis- mically resistant structural design is recommended. - CONCLUSIONS AND RECOMMENDATIONS - General - 1. Based on the geotechnical data presented herein, it is our GeoSoiZs, Inc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 5 - opinion that the subject site is suited for the proposed residential development from a geologic viewpoint. The - recommendations presented below should be incorporated into the design, grading and construction considerations. - 2. Groundwater is not expected to be a factor in development of the site. - Cut and Fill Slopes Cut slopes should be no steeper than (2) horizontal to (1) ver- tical. Fill slopes should be no steeper than (2) horizontal to (1) vertical. - FOUNDATION DESIGN Optional methods for design and construction of foundations are offered for this site. Which method is most reasonable for the site will depend upon the specific site grading and house design. If grading results in a cut/fill transition beneath any proposed structures, then one of the following methods apply: 1. Extend all structural footings beneath the fill and into bedrock. 2. Overexcavate and recompact the cut area to a minimum depth of three feet below finish grade and so that a minimum of 18 inches of compacted fill is present below the footings. GeoSoils, Znc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.0. 624-SD PAGE 6 Bearing Value Allowable bearing capacity will depend on the foundation material. 1. Allowable bearing values of 2,000 pounds per square foot in compacted fill and 3,000 pounds per square foot in bed- rock may be used for design of continuous footings 12 inches wide and 12 inches deep. Lateral Pressure 1. 2. 3. 4. The active earth pressure to be utilized for retaining wall design may be computed as an equivalent fluid having a density of 30 pounds per cubic foot when the slope of the backfill behind the wall is level. Where the slope of the backfill is 2:1, an equivalent fluid pressure of 45 pounds per cubic foot may be used. Passive earth pressure may be computed as a equivalent fluid having a density of 250 pounds per cubic foot, with a maximum earth pressure of 2,000 pounds per square foot. An allowable coefficient of friction between soil and con- crete of 0.4 may be used with the dead load forces. When combining passive pressure and frictional resistance, the passive pressure component should be reduced by one- third. For design of isolated poles, the allowable passive earth pressure may be increased by one hundred percent. GeoSoils, Znc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 7 FOUNDATION CONSTRUCTION The following construction procedures are tentatively recom- mended, modifications may be recommended at the completion of grading: 1. 2. 3. 4. Exterior footings should be founded at a minimum depth of 18 inches below the lowest adjacent ground surface. In- terior footings may be founded at a depth of 12 inches below the lowest adjacent ground surface. All footings should have one No. 4 reinforcing bar placed at the top and bottom of the footing. A grade beam 12 inches by 12 inches should be provided across the garage or other large entrances. The base of the grade beam should be at the same elevation as the adjoining footings. Concrete slabs, in areas where moisture condensation is undesirable, shall be underlain with a vapor barrier con- sisting of a minimum of six mil polyvinyl chloride membrane with all laps sealed. This membrane whall be covered with a minimum of one inch of sand to aid in uniform curing of the concrete. Concrete slabs should be reinforced with six inch by six inch, No. 10 by No. 10 welded wire mesh. All slab rein- forcement should be supported to provide property position- ing near the vertical midpoint during placement of concrete. GeoSoils, Inc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 8 GRADING GUIDELINES 1. Prior to placement of fill, all vegetation, rubbish and other deleterious material should be disposed of off-site. 2. Sidehill fills should have an equipment-width key at their toe excavated into competent material. As the fill is elevated, it,should be benched through surficial soil and slopewashed into competent bedrock or other materials deemed suitable by the Soil Engineer. 3. Rock fragments less than six inches in diameter may be utilized in the fill, provided: a) they are not placed in concentrated pockets; b) there is a sufficient percentage of fine-grained c) the distribution of the rocks is observed by the material to surround the rocks: Soil Engineer. " 5 " 4. Rocks greater than six inches in diameter should be taken off site or placed in accordance with the recommendations of the Soil Engineer in areas designated as suitable for rock disposal. . Representative samples of materials to be utilized as com- pacted fill should be analyzed in the laboratory by the. Soil Engineer to determine their physical properties. If any material other than that previously tested is encountered GeoSofls, Inc. " R. G. CRAWPORD COMPANY, INC. MAY 22, 1986 W.Q. 624-SD PAGE 9 during grading, the appropriate analysis of this material should be conducted by the Soil Engineer as soon as possi- ble. 6. Material used in the compacting process should be evenly spread, moisture conditioned, processed and compacted in thin lifts (generally six to eight inches thick) to obtain a uniformly dense layer. The fill should be placed and compacted on a horizontal plane, unless otherwise agreed to by the Soil Engineer. 7. if the moisture content or relative density varies from that required by the Soil Engineer, the Contractor should rework the fill until it is approved by the Soil Engineer. 8. Each layer should be compacted to 90 percent of the maxi- mum density in compliance with the testing method specified by the controlling governmental agency. In this case, the testing method is ASTM Test Designation D-1557-78. 9. The Contractor should be required to obtain a minimum relative compaction of 90 percent out to the finish slope face of fill slopes. This may be achieved by either overbuilding the slope and cutting back to the compacted core, or by direct compaction of the slope face with suitable equipment. If a method other than overbuilding and cutting back to the compacted core is to be employed, slope tests should GeoSoils, Inc. - R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 10 " be made by the Soil Engineer during construction of the slopes to evaluate if the required compaction is being achieved. Each day the Contractor should receive a copy of the Soil Engineer's "Daily Field Engineering Report" which indicates the results of field density tests for that day. If the method of achieving the required slope compaction selected by the Contractor fails to produce the necessary - results, the Contractor should rework or rebuild such slopes until the required degree of compaction is obtained, at no additional cost to the Owner or Soil Engineer. Grading Control 1. Observatiun of the fill placement should be provided by the Soil Engineer during the progress of grading. 2. In general, density tests should be made at intervals not exceeding two feet of fill height or every 500 cubic yards of fill placed. This criteria varies depending on soil conditions and the size of the fill. In any event, an adequate number of field density tests should be made to verify that the required compaction is being achieved. 3. Density tests may also be made of the surface material to receive fill, as deemed appropriate by the Soil Engineer. 4. All cleanout, processed ground to receive fill, key exca- vations, subdrains and rock disposal should be observed by GeoSoils, Inc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 11 the Soil Engineer prior to placing any fill. It should be the Contractor's responsibility to notify the Soil Engineer when such areas are ready for inspection. UTILITY TRENCH BACKFILL Utility trench backfill shall be placed to the following stan- dards : 1. 2. 3. Ninety percent of the laboratory standard if native material is used as backfill. As an alternative, clean sand may be utilized and flooded inplace with no specific relative compaction required. Inspection, probing and, if deemed necessary, testing may be required however, to verify adequate results. Exterior trenches, paralleling a footing and extending below a 1:l plane projected from the outside bottom edge of the footing shall be compacted to 90 percent of the laboratory standard. Sand backfill, unless it is similar to the inplace fill, should not be allowed in these trench backfill areas. Density testing along with probing shall^ be accomplished to verify the desired results. LIMITATIONS - The materials encountered on the project site and utilized in our laboratory study are believed representative of the total - CeoSoils, Inc. R. G. CRAWFORD COMPANY, INC. MAY 22, 1986 W.O. 624-SD PAGE 12 area; however, soil and bedrock materials vary in characteris- tics between excavations and natural outcrops. Since our study is based upon the site materials observed, selective laboratory testing and engineering analysis, the conclusions and recommendations are professional opinions. These opinions have been derived in accordance with current standards of practice and no warranty is expressed or implied. The opportunity to be of service is sincerely appreciated. If you should have any questions regarding this report, please do - not hesitate to call our office. Very truly yours, - .. Albert R. Kleist, RCE 16351 Principal/Civil Engineer - TEM/ARK/smh Enclosures Distribution: (6) Addressee GeoSoiZs, Inc. "- "_ _" "_ "_ __ x rv 4 I I x rl rl W QIV) con NW N _. El F Y u !+ 0 a Q a, a c m 4 4 0 m a +J 0 ICI 0 .rl x E ar m 0 4 0 il il H PI I. x X 4 N I I x x 0 4 N W x x rl m I I x x 0 rl m rl N I 0 I rl I Not sc I 0 e - R. G. CRAWFORD COMPANY, INC. I