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HomeMy WebLinkAboutCT 73-24; Carlsbad Lake Community; Soils Report; 1981-01-21Wobdward-Clyde Consultants . UPDATE GEOTECHNICAL INVESTIGATION CARLSBAD LAKE COMMUNITY (CARLSBAD TRACT NO. 73-24) CARLSBAD, CALIFORNIA prepared for: Plaza Builders, Inc. 16800 Devonshire Street Granada Hilla, California 91344 -i 3467 Kurlz Street San Diego Callforrta 921 10 714-224-2911 Telex 697-841 . January 21, 1981 Project No. 50316W-UD01 . Woodward=Clyde Consultants Plaza Builders, Inc. Granada Hills, California 91344 16800 Devonshire Street Attention: Mr. Ed Hamner UPDATE GEOTECHNICAL INVESTIGATION CARLSBAD LAKE COMMUNITY (CAFLSBAD TRACT NO. 73-24) CARLSBAD, CALIFORNIA Gentlemen: In accordance with your request and our proposal dated December 2, 1980, we have prepared an update report of the soil and geologic conditions on the subject site. This conforms to Phase I of the studies presented in our pro- posal. SCOPE AND PURPOSE OF STUDY Our study consisted of reviewing our files concerning the site, making a field reconnaissance and reviewing up to date applicable geologic literature. The purpose of this work was to provide a basis for our report and to provide criteria for development of the site including conclusions and recommendations regarding: 0 The possible presence of geologic hazards on the 0 The stability of proposed cut and fill slopes, 0 Foundation design, including allowable soil site, bearing pressures in natural ground and compacted fill, and, 0 Earthwork and grading specifications. Plaza Suilders, Inc. January 21, 1981 Project No. 50316W-UD01 Page 2 Woodward-Clyde Consultants . PREVIOUS SOIL STUDIES A preliminary soil investigation of the site was prepared by Woodward-Gizienski and Associates. The results of that study were issued in a report entitled "Soil Investi- gation for the Proposed Carlsbad Lake Community, Carlsbad, California," dated July 30, 1973. A copy of that report is attached (Appendix A). DESCRIPTION OF PROPOSED PROJECT The subject site is east of Interstate 5 and south of Poinsettia Lane in Carlsbad, California. For our study, we were provided a copy of "Plot and Grading Plan, Carlsbad Tract No. 73-24," prepared by Lawrence R. Williams Consulting Civil Engineer dated August 5, 1974 with latest revision dated June 14, 1976. Except for minor changes this plan is essentially the same as that studied for the 1973 Woodward- Gizienski & Associates investigation. The plan indicates that the site will be graded to form building pads for construction of 67 multi-story condominium type structures. The plans also indicate that cut slopes will have maximum heights on the order of 15 feet and slope inclinations of 2 to 1 and that fill slopes will have maxi- mum heights on the order of 25 feet and 15 feet with slope inclinations of 2 to 1 and 1-1/2 to 1, respectively. The 1-1/2 to 1 inclined fill slope is proposed along the western property line approximately 30 feet east of the top of an existing 2 to 1 inclined cut slope extending down to Inter- state 5. This existing cut slope has a maximum height on the order of 30 to 40 feet. The plans also indicate that two shallow, on the order Of 4 feet deep decorative ponds will be constructed in the central portion of the site. -~ ! -I Plaza Builders, Inc. January 21, 1981 Project No. 50316W-UD01 Page 3 Woodward-Clyde Consultants . CURRENT SITE CONDITIONS The site is located on a relatively flat north-south trending ridge. The ridge top slopes gently to the west. The major portion of the site has been cultivated to depths of 1 to 2 feet sometime in the past. These cultivated topsoils consist of silty to clayey sands. Underlying the topsoils are the Quaternary age Lindavista Formation and the Eocene age Santiago Formation. A review of the grading plans and our field reconnaissance indicates that all pro- posed cuts will be in the Lindavista Formation. These materials are dense to very dense silty to clayey sands. As noted in the 1973 report a small area of uncompacted fill was found in the west-central portion of the site. This fill extended to a depth of 13 feet and consists of silty to clayey sand. NO new fill was noted on the site. Man-made structures on the site consist of a waterline (shown on the plans) present along the eastern site boundary and a sewerline (not shown on the plans) present along the western site boundary. Although not observed during our site reconnaissance, abandoned irrigation lines may be present. Vegetation consists of a dense growth of native weeds and grasses. GEOLOGIC HAZARDS Faulting For this study we have reviewed our files and published literature including the "Geology of the Encinitas Quadrangle," prepared by Kenneth iiilson in 1972. The review did not indicate the presence of any known faults on the site, nor was any surface evidence of faulting noted on the site or the ridge side slopes during our site visit. -I I Plaza Builders, Inc. January 21, 1981 Project No. 50316W-UD01 Page 4 Woodward-Clyde Consultants. . The nearest known active fault along which seismic events of Richter Magnitude 4.0 or greater have been re- corded is the Elsinore Fault zone, mapped approximately 25 miles northeast of the site. The hypothesised northern extension of the Rose Canyon Fault zone is located approxi- mately 5 miles west of the site. No earthquakes greater than Richter Magnitude 4 have been recorded for this fault. Landslides In addition to other work previously mentioned, our evaluation of the potential for landslides included re- viewing aerial photographs and general topography of the site. No landslides or landforms that could be construed as landslides were noted. Ground Water No water seeps or springs were noted during our recent site visit. No ground water was encountered in the test borings made for the 1973 investigation. We do not expect that a ground water table is present within proposed grading depths. DISCUSSIONS, CONCLUSIONS, AND RECOMMENDATIONS The discussions, conclusions, and recommendations presented in this report are based on results of our field studies, analyses and professional judgment. The results of our field reconnaissance, made by a senior staff geologist of our firm on January 7, 1981, indicates that the site is in essentially the same condition as it was for the 1973 field investigation, although no actual surveying was done. Plaza Builders, Inc. January 21, 1981 Project No. 50316X-UD01 Page 5 Woodward-Clyde Consultants Potential Geologic Hazards Faulting - Our investigation identified no faults crossing the site. Seismicity - All faults have a potential for earth- quake activity. The fault zone closest to the site along which earthquakes of Richter Magnitude 4 or greater have been recorded is approximately 25 miles from the site, whereas the closest significant faulting to the site is approximately 5 miles from the site but this fault has not exhibited earthquakes greater than Richter Magnitude 4. Liquefaction Potential - We estimate that the depth to the regional ground water table is in excess of 50 feet. In our opinion, this depth, together with the dense nature of the soils on the site generally precludes the possibility of liquefaction. Landslides - Our field investigation and review of maps and vertical aerial photographs disclosed no evidence of landslides on the site. Ground Water In our opinion, no shallow permanent ground water table exists on the site and the conditions of the proposed development indicate a low probability of ground water seepage problems resulting from existing subsurface water. We recommend that positive measures be taken to prop- erly finish grade the building areas after structures and other improvements are in place so that drainage water from lots is directed off the lots to the street and away from foundations, floor slabs, and slope tops. Even with these provisions, experience has shown that a shallow ground water or surface water condition can and may exist in areas where no such ground water condition existed prior to site develop- -I -1 Plaza Builders, Inc. January 21, 1981 Project No. 50316W-UD01 Page 6 Woodward-Clyde Consultants ment. This is particularly true where a substantial in- crease in surface water infiltration results from landscape irrigation. Slope Stability For the proposed slopes we have performed stability analyses using the Janbu method of analyses and the follow- ing soil strength parameters: Area - ra' C' (psf) Y (pcf) Cut Slopes 30' 300 125 Fill Slopes 25 O 250 120 These parameters are based on laboratory tests per- formed for the 1973 field investigation report and our experience with similar soils in nearby areas. Our analyses indicates the proposed slopes will have calculated factors of safety in excess of 1.5 under static conditions for both deep-seated and shallow sloughing. Stability analyses require the use of soil parameters selected from a range of possible values, thus, there is a finite possibility that slopes having calculated factors of safety as indicated above could become unstable. In our opinion, the probability of the slopes becoming unstable is low and it is our professional judgment that the slopes can be constructed as planned. IJe recommend that all slopes be planted, drained and properly maintained to help control erosion. Our experience indicates that slopes constructed at steeper than 2 to 1 are particularly susceptible to shallow slope sloughing during periods of heavy rainfall, heavy irrigation or upslope surface runoff. Periodic maintenance, including the re- building of the outer 18 to 36 inches of the slope, may be Woodward-Clyde Consultants January 21, 1981 Plaza Builders, Inc. Projec-t No. 50316W-UD01 Page 7 . required. Sloughing of fill slopes can be reduced by overbuilding the slope by at least 3 feet and cutting back to the desired slope inclination. Sloughing can also be reduced by backrolling slopes at frequent intervals. We recommend that all fill slopes be backrolled at least every 4 feet of fill height and that 2 to 1 inclined fill slopes be trackwalked upon completion. Earthwork We recommend that the site be graded in accordance with the attached Specifications for Controlled Fill (Appendix B) . Me also recommend that the grading be observed by and compacted fills be tested by Woodward-Clyde Consultants. We recommend that a pre-construction conference be held at the site with the developer, civil engineer, contractor and geotechnical engineer in attendance. Special soil handling and the grading plans can be discussed at that time . We recommend, in areas to receive new fill or structures, that the existing fill and underlying porous topsoil in the west-central part of the site be excavated, watered as required and compacted. The 1973 report indicates the maxi- mum combined depth of existing fill and porous topsoil is on the order of 13 feet, however, we recommend that the actual depth and extent be evaluated in the field at the time of grading. We recommend that all other loose and/or cultivated topsoils not removed by planned grading operations be ex- cavated, watered as required, and compacted prior to placing additional fill or structures. Although no expansive soil were noted during either the 1973 or current investigations, our experience in nearby Woodward-Clyde Consultants Plaza Builders, Inc. January 21, 1981 Project No. 50316W-LID01 Page 8 areas indicates that residual clayey soils may be en- countered locally beneath the topsoils. If expansive clay soils are found during grading, we recommend that they be excavated where encountered, a minimum depth of 2 feet below rough grade in building areas and 1 foot in pavement areas. The building area is defined as the area of the building plus 5 feet beyond the building limits. The clay soils can be replaced with properly compacted nonexpansive granular soils available on the site. We recommend that the upper 2 feet of fill in building areas and 1 foot in pavement areas be composed of properly compacted nonexpansive granular soil available on the site. We recommend that all cut areas be examined by our firm during grading to evaluate actual geologic conditions. Foundations We recommend that foundations for the proposed build- ings founded in either undisturbed or properly compacted nonexpansive materials be designed for an allowable soil bearing presure not exceeding 2,000 psf (dead plus live loads) at 12 inches below compacted fill or undisturbed cut lot grade. In our opinion, this soil bearing pressure can be increased by no more than one-third for loads that in- clude wind or seismic forces. We recommend that all con- tinuous footings have minimum widths of 12 inches. We recommend that structures that cannot tolerate differential settlements (foundations, floor slabs, decks, etc.) not be located within 8 feet of the face of a slope. For structures located in this zone we recommend the foot- ings be extended in depth until the outer bottom edge of the footing is at least 8 feet from the face of the slope. I -I Plaza Builders, Inc. January 21, 1981 Pro jecc No. 50316W-UD01 Page 9 Woodward-Clyde Consultants . Recommendations for pond construction are not within the scope of this report. Recommendationa in this regard can be presented in an addendum report upon request. RISK AND OTHER CONSIDERATIONS We have only observed a small portion of the pertinent soil, and ground water conditions. The recommendations made herein are based on the assumption that soil conditions do not deviate appreciably from those found during our field investigation. If the plans for site development are changed, or if variations or undesirable geotechnical condi- tions are encountered during construction, the geotechnical consultant should be consulted for further recommendations. We recommend that the geotechnical consultant review the foundation and grading plans to verify that the intent of the recommendations presented herein has been properly interpreted and incorporated into the contract documents. We further recommend that the geotechnical consultant observe the site grading, subgrade preparation under con- crete slabs and paved areas, and foundation excavations. It should also be understood that California, including San Diego, is an area of high seismic risk. It is generally considered economically unfeasible to build totally earth- quake-resistant structures; therefore, it is possible that a large or nearby earthquake could cause damage at the site. Professional judgments presented herein are based partly on our evaluations of the technical information gathered, partly on our understanding of the proposed con- struction, and partly on our general experience in the geotechnical field. Our engineering work and judgments rendered meet current professional standards. \Ye do not guarantee the performance of the project in any respect. Plaza Builders, Inc. January 21, 1981 Project. No. 50316W-UD01 Page 10 Woodward-Clyde Consultants . This firm does not practice or consult in the field of safety engineering. We do not direct the contractor's operations, and we cannot be responsible for the safety of other than our own personnel on the site; therefore, the safety of others is the responsibility of the contractor. The contractor should notify the owner if he considers any of the recommended actions presented herein to be unsafe. If you have any questions, or if we can be of further service, please give us a call. Very truly yours, WOOD:qARD-CLYDE CONSULTANTS Richard P. While R.E. 21992 RPW/DS/MRR/rs Attachments (4) Plaza Builders, Inc. (4) Lawrence R. Williams Consulting Civil Engineer Daryi Streiif C.E.G. 1033