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HomeMy WebLinkAboutCT 75-09B; La Costa Lots 1-43, 59-74, 135-138; Soils Report; 1986-09-10.- - - - - .- EBERHART 8 STONE, INC. DAN 17 EEERHART. CEG GEOTECHNICALCONSULTANTS GERALD L. STONE, RCE 2211 E. WlNSTONROAD.SUlTEF*ANAHEIM.CA92606.l714)991-0163 5205AVENIDAENCINASSUlTE C*CARLSBAD.CA92006*~619)436-9416 W.O. 1343 September 10, 1986 The Anden Group 6544 Corte Montecito Carlsbad, California 92008 Attention: Mr. Bob Fish Subject: Completion of Rough Grading, lots l-43, 59-74, and 135-138, C.T. 75-9(B), Carlsbad, California. Gentlemen: Rough grading has recently been completed on the subject lots by Barry Blanchard, Inc. Upon completion of grading for the remainder of the subject tract, a mass qrading geotechnical reoort will be prepared which will include a review of the testing and observations orovided by this firm during mass grading. For the interim, opinions and recommendations relative to development of the subject lots are presented herein. OPINIONS AND RECOMMENDATIONS Recommendations in this report are opinions based upon this firm's testing and observations performed during rough grading, previous geotechnical studies, and professional judgment. Opinions and recommendations are applicable to the grading performed, and development proposed, under the purview of this report, and should be incorporated into project design and construction practice. GEO-OGY Ground Water No adverse effects from ground water are anticipated. Candslidinq Shallow landslidinq was observed within Lots 4-6 and 29-30. All landslide debris was overexcavated to firm underlyinq bedrock within these lots. A deep- seated landslide was exoosed on lots 72-74 and 135-138. This landslide was sta- bilized by construction of a shear key along Mision Estancia. The upper, hiqhly disturbed, portion of the landslide above the shear key was overexcavated to firm slide debris and replaced with compacted fill. - - Anden -2- W.O. 1343 - - - - - - - .- Seismic Statement No active faults are oresent at the subject site; not anticipated. Because of the proximity of the Southern California area, moderate ground shaking earthquake. liquefaction is not anticipated. therefore, qround rupture is site to active faults in the could occur durinq an Proposed structures should be designed to resist seismic forces in accordance with criteria contained in the 1985 Uniform Buildinq Code for seismic zone 4. GRADING The geotechnical aspects of rough grading under the purview of this report are in compliance with this firm's recommendations and the various codes and/or regulations established by the governing authorities, as well as the Uniform Building Code, Chapter 70. The subject lots, as graded, are considered suitable for the proposed development. Natural Ground Topsoil, colluvium, alluvium, shallow slides, and the upper portion of the deep- seated landslide encountered within the subject lots were removed to expose com- petent bedrock. Both cut and prepared natural ground are considered suitable for support of the compacted fill and/or the proposed structures. Slope Stability Slopes, as-graded, are considered grossly and surficially stable. FOUNDATIONS The following recommendations have been developed for the proposed one- and/or two-story single-family residences. Wood-framed, slab-on-grade construction is oroposed, and is anticipated to yield light structural loading. Based upon the oroposed construction and anticipated liqht structural loading, post-construction settlement should be within generally accepted tolerable limits. Foundations for an individual structure should be embedded within the same bearing material, such as entirely within comoacted fill or entirely within bedrock. Based on soluble sulfate content of the onsite materials, Type V cement should be utilized in concrete for foundations. Concrete utilized for foundations should contain at least 660 pounds of cement per cubic yard, with the water/cement ratio no greater than 0.45. - ERERHART P STONE, INC. Anden -3- W.O. 1343 .- - - - - The bottom outer edge of foundations adjacent a top-of-slope should be set back from the slope surface a horizontal distance of one-half the slope height under consideration. The horizontal distance should not be less than five feet and may be limited to ten feet. Conventional Foundations Val;e: Bearinq For design purposes, an allowable bearing value of 1500 pounds per square oot, based on an embedment of 12 inches into bedrock or compacted fill, may be used for continuous footinqs or square pad foundations. This value is for dead load plus live load conditions and may be increased by one-third in consideration of wind or seismic loadings of short duration. In designing to resist horizontal soil loadinqs, a lateral bearing resistance of 150 pounds per square foot, per foot of embedment, and a friction factor of 0.2 may be utilized for foundations embedded in bedrock or compacted fill. Expansive Soil Guidelines: The potential expansion of the near-surface materials at rough grade on the subject lots is Hiqh. Guidelines for residen- tial foundations, based on soil expansion, are presented in Table I. Post-Tensioned Structural Slabs AS an alternative to conventional concrete foundations, post-tensioned struc- tural slab systems may be utilized. Post-tensioned structural slab foundation systems should be designed by a struc- tural engineer to resist expansive soil and provide relatively uniform bearing over the slab area. Allowable bearing should be limited to a value of about 1000 pounds per square foot. A cut-off footing should be provided along the perimeter of post-tensioned slabs, with an embedment of about 12 inches below lowest adjacent grade. Though presaturation of slab subgrade is not a requirement, the subgrade area should be kept moist prior to the placement of slab concrete. A visqueen-type moisture barrier should be placed at grade below concrete slabs, in areas to be tiled or carpeted, and be overlain by one inch of protective sand cover. This moisture barrier should be heavily overlapped or sealed at splices. A concrete footing should separate areas requiring a moisture barrier from those that do not. - - -. EBERHART P STONE. INC. - ..~ - - - - - - - Anden -4- W.O. 1343 SITE DEVELOPMENT Should future property owners desire to construct additional structures, such as pools, walls, patios, etc., plans for such improvements should be provided to the geotechnical consultant for review and comment. Prior to the coimnencement of additional gradinq and/or site development, including subgrade preparation of oavement areas, backfilling of trenches and walls, etc., the geotechnical consultant should be notified two working days in advance in order to schedule testinq and observations as needed. Foundation Excavations Foundation excavations should be observed by the geotechnical consultant so that foundation bearing materials may be compared with those upon which the guidelines in Table I are based. This should be accomplished prior to the placement of forms, reinforcement, and concrete. Excavations should be level, square, and free of loose material and debris prior to the placement of concrete. Slab Subgrade For conventional foundations, presaturation is recommended beneath interior slabs. The recommended moisture (see Table I) should penetrate to the depth of the perimeter footings. The moisture content should be tested by the geotech- nical consultant within 24 hours prior to the placement of concrete. Trench Backfill Trench backfill consisting of onsite materials should be olaced in horizontal lifts and mechanically compacted to 90% of the laboratory maximum density. For shallow trenches below slab areas, an alternative to backfilling with onsite materials would be to jet, in place, imported granular material having a sand equivalent. of 30 or more. Wall Backfill Materials to be placed as wall backfill should be tested by the qeotechnical consultant so that retaining wall design oarameters may be formulated. Wall backfill should be placed in horizontal lifts and mechanically compacted to 90% of the laboratory maximum density. The use of heavy compaction equipment adjacent a retaining wall may produce strains greater than those normally associated with the development of active earth pressures, and/or lateral soil loading exceeding design parameters, resulting in excessive wall movement. Therefore, caution should be used during backfill placement and compaction. Pavement Areas Prior to the placement of aggregate base or asphaltic concrete pavement, the exposed subgrade surface should be scarified, moisture-conditioned as needed to obtain near-optimum conditions, and recompacted in place. Isolated overex- cavations may also be required, as field conditions dictate, to eliminate any dry, wet, loose, or soft areas that may exist at the time of subgrade preparation. ERERHART P STONE, INC. - - - - - - - - - _- - Anden -5- W.O. 1343 Slope Maintenance Vegetation planted on slopes should be native to the area, deep rooting, and drought resistant. A landscape architect should be consulted for recomnen- dations on types of plants and planting configuration. Surficial slope instability can be mitigated by minimizing moisture variations near the slope surface. Overwatering of slope surfaces, alteration of drainage patterns and slope configurations, obstruction of drainage devices, and burrowing rodents can be detrimental to slope stability. Provisions should be made to interrupt automatic timing devices of irrigation systems during rainy seasons. Pad Orainaqe Maintenance Roof and pad drainage should be collected and directed away from proposed struc- tures. As-graded drainage patterns and devices created durinq fine gradinq of the lots should be maintained to orovide positive drainaqe. Alteration and/or obstruction of these may cause foundation distress. SUWARY Testing and observations were provided , and opinions and recommendations were developed, in accordance with generally accepted engineering and geologic prin- ciples and practices. No warranty is expressed nor implied. Upon completion of rough qrading for the,remainder of the subject tract, a mass grading geotech- nical report will be prepared. This report will include a review of the testing and observations provided by this firm during mass grading operations, opinions and recommendations relative to foundations and site development, a sumnary of field and laboratory test results, and plans depicting the approximate loca- tions of field tests and geologic data, This report is subject to review by the governing agencies. Respectfully submitted, c-8-d;l R. Eberhart President CEG 965 and ORE:G?S:pm (35) Enclosure: Table I - Guidelines for Residential foundations Based on Soil Expansion Distribution: (6) Addressee - EBERHART P STONE. INC. *“CL” I I I I I / I TALE 1 / I , I I I I W.O. 1343 CiIllf-lFI INFS FOR RFClt-tFNTlAl Enllk,t-t.%~,nktc DACC~ n., en,, FYPdNC,nM --.-- -... -- . -,. ..--.I-.. . .,.L 1 ““I.YrxI 8”I.J OI-XJC” “I-l J”li I... ..,.-,-,. LOT POTENTIAL ~FOOTINCS EXPANSION ’ SLABS (UBC 29-2) EMBEDMENT REINFORCEMENT THICK. REINFORCEMENT PRESAT. BASE ‘r-act 75-9 1 STORY 2 STORY COURSt Very Low (O-20) 12” perimeter 18” perimeter 12” Interior 18” Interior Low (21-50) 12” perimeter 18” perimeter 2 No. 4 bars : 12” Interior 18” interior 1 top, 1 bottom Medium (51-90) 1V’ perimeter 18” perimeter 2 No. 4 bars: 12” interior 4” net 6”xV’; #6x#6 120% of 4” 18” interior 1 top, 1 bottom W.W.F. optimum or moisture - No. 3 bars @ 24” O.C. each way l-43 High 59-74 24” perimeter 24” perimeter 4 No. 4 bars: 4” net 6”x6” (91-130) w.w.i. #6X#6 120% of 4” 12” interior 18” interior 135-138 2 top, 2 bottom optimum or or moisture - 2 No. 6 bars: No. 3 bars B 18” 1 top, 1 bottom O.C. each way Very High 24” perimeter 24” perimeter 4 No. 5 bars: (?1301 12” Interfor 6” net No. 4 bars B 10” 18” interior 120% of 6” 2 top, 2 bottom O.C. each way optimum I or or moisture - - 2 No. 8 bars: No. 3 bars Q 12” I 1 top, 1 bottom O.C. each way . ’ 1 _;- Notes on the following page are considered part of this table. I I I I I I I I I / I I I I I ! ! Anden NOTES TO TABLE I W.O. 1343 11 2) 3) 4) 5) 6) 7) 8) 9) 10) These guidelines are based on site soil expansion and should not preclude more restrictive structural or agency requirements. As an alternative to conventionally reinforced concrete foundations, post- tensioned structural slab systems, designed bye a structural engineer, may be utilized. Footing embedments should be measured below lowest adjacent grade. At the time of concrete placement, footing excavations should be moist and free of desiccation cracks. A reinforced concrete grade beam should be. constructed across garage entrances, with similar depth and reinforcement as adjacent perimeter footings. For soil with a potential expansion greater than “Low”, interior isolated spread footings and/or partial length footings are not recommended. Where presaturation is recommended beneath interior slabs, the recommended moisture should penetrate to the depth of the perimeter footings. The moisture content should be .tested by the geotechnical consultant 24 hours prior to the placement of concrete. Though presaturation of slab subgrade is not a requirement for Very Low and Low expansion potentials, optimum moisture conditions should be maintained or reestablished just prior to the placement of slab concrete. Below proposed slabs-on-grade in areas to be tiled or carpeted, a visqueen-type moisture barrier should be placed at grade and be overlain by one inch of protective sand cover. This moisture barrier should be heavily overlapped or sealed at splices. Where a base course is recommended beneath interior slabs, it should consist of pea gravel, clean sand, or other granular material acceptable to the geotechnical consultant. The above moisture barrier/sand cover requirement may be included as part of the recommended base course thickness. Slab reinforcement should be supported at mid-slab height, Garage slabs should be placed separate from footings. Garage slab reinforcement may be omitted if slab cracking can be tolerated, provided that the slabs are saw cut or jointed for crack control. For soil with a potential expansion greater than “Low”, slabs should be free-floated or structurally tied to perimeter footings, Structural ties could be provided by the placement of No. 3 bars @ 24” O.C., bent from perimeter footings about three feet into the slab.