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HomeMy WebLinkAboutCDP 01-47; Nayudu Residence; Soil Investigation; 2001-11-21( > V SGC South/and Geotechnical Consultants SOILS INVESTIGATION PROPOSED TWO-STORY, SINGLE-FAMILY RESIDENCE 3286 LINCOLN STREET CARLSBAD, CALIFORNIA Project No. 106E61 Novennber 21, 2001 Prepared for: ^ fj KRIS AND NANCY NAYUDU r7^t,„«l^ - /^l^ 320 Walnut Avenue V-t^oV^ Carlsbad, California 92008 1238 GREENFIELD DRIVE, SUITE A EL CAJON, CALIFORNIA 92021 • (619)442-8022 • FAX (619)442-7859 SGC South/and Geotechnical Consultants November 21, 2001 Project No. 106E61 To: Kris and Nancy Nayudu 320 Walnut Avenue Carlsbad, California 92008 Subject: Soils Investigation, Proposed Two-Story, Single-Family Residence, 3286 Lincoln Street, Carlsbad, California Introduction Southland Geotechnical Consultants has performed a soils investigation for the proposed two-story, single-family residence at 3286 Lincoln Street in Carlsbad. This report presents the results of our soils investigation and provides our conclusions and recommendations, from a geotechnical standpoint, relative to the proposed development. Purpose and Scope The purpose of our soils investigation was to evaluate the soil conditions at the property and provide recommendations, from a geotechnical standpoint, relative to the proposed development. The scope of our investigation included the following: Review of geologic maps and literature pertaining to the site and general vicinity. A list of the documents reviewed is presented in Appendix A. Review of preliminary project plans indicating proposed site improvements (Appendix A). Field reconnaissance to observe the existing surficial soil conditions at the subject property and nearby vicinity. Investigation of the subsurface soil conditions by manually excavating, logging and sampling three exploratory borings at the site. Laboratory sulfate content testing of a representative sample of the onsite soils. Geotechnical analysis of the data obtained. • 1238 GREENFIELD DRIVE, SUITE A EL CAJON, CALIFORNIA 92021 • (619)442-8022 • FAX (619)442-7859 Project No. 106E61 Preparation of this report summarizing the results of our soils investigation and presenting recommendations, from a geotechnical standpoint, forthe proposed development. Proiect Description The subject property is located on the northeast corner of Lincoln Street and Walnut Avenue in the City of Carlsbad, California (see Figure 1). The site is bounded by an existing residential development to the north and a bed and breakfast commercial development to the east. The relatively flat lot is currently developed with a single- story residence and detached garage. The site is vegetated with a variety of bushes, trees, and grass. Based on our review of preliminary project plans (Appendix A), we understand that the existing residence will be razed and a two-story residence will be constructed on the central portion of the lot. Retaining walls, sidewalks, curb and gutter and street improvements are also proposed. Site grading is anticipated to prepare the building area and attain design finished grades. Building loads are assumed to be typical for residential construction. Subsurface Investigation On October 29, 2001, three exploratory borings were manually excavated at the site. The exploratory borings were excavated to a maximum depth of approximately 4 feet below the existing ground surface. The borings were logged by a geologist from our firm and samples of the soils encountered were obtained for visual soils classification and laboratory testing. Subsequent to logging and sampling, the borings were backfilled. The approximate locations of the exploratory borings are shown on Figure 2 (Exploratory Boring Location Map). Logs of the exploratory borings are presented in Appendix B. Soil/Geologic Units As encountered in our soils investigation, the subject property appears to be underlain by topsoil and Quaternary-aged terrace deposits. Brief descriptions of these units follow: Topsoil - A natural topsoil, developed on and gradational with the underlying terrace deposits, was encountered in all of our exploratory borings. As SGC Project No. 106E61 encountered, the topsoil generally consisted of dark brown, silty fine sand. The topsoil was encountered to a maximum depth of approximately 1 foot. Localized thicker accumulations of these soils may exist at the site. The topsoil is considered potentially compressible and should not be relied upon for support of fill and/or structural loads. These soils are similar to soils in the general site vicinity found to have a very low to low expansion potential when tested in general accordance with UBC test standard 18-2. Terrace Deposits - Quaternary-aged terrace deposits were encountered underlying the topsoil in all of our exploratory borings. As encountered, the terrace deposits generally consisted of gray-brown and orange-brown, silty fine to medium sand. The terrace deposits generally exhibit favorable bearing characteristics. These soils are similar to soils in the general site vicinity found to have a very low to low expansion potential when tested in general accordance with UBC test standard 18-2. Groundwater and Surface Water Indications of a near-surface groundwater table were not encountered in our exploratory borings. Although groundwater is not expected to be a significant constraint to the proposed development, our experience indicates that near-surface groundwater conditions can develop in areas where no such groundwater conditions previously existed, especially in areas where a substantial increase in surface water infiltration results from landscape irrigation or unusually heavy precipitation. It is anticipated that site development will include appropriate drainage provisions for control and discharge of surface water runoff. SGC Project No. 106E61 Conclusions and Recommendations Based on the results of our soils investigation, it is our opinion that construction of the residential development is feasible from a geotechnical standpoint. The following sections provide recommendations, from a geotechnical standpoint, which should be considered for design and construction of the proposed project. Earthwork From our understanding of the project, some site grading may be performed to prepare the site and attain finished design grades. Site earthwork should be performed in accordance with the following recommendations and the general recommendations included in Appendix C (Recommended Earthwork Specifications). Site Preparation - Prior to grading and construction activities, the site should be cleared of vegetation, debris and loose soils. Vegetation and debris should be properly disposed of off site. Holes resulting from removal of buried obstructions which extend below finished site grades should be filled with properly compacted fill soils. Removal/Recompaction of Potentiallv Compressible Soils - The topsoil is considered potentially compressible and unsuitable for the support of fill and/or structural loads in its present condition. We recommend that these soils be removed in areas planned for structures, surface improvements or fill placement. As encountered in our exploratory borings, the topsoil apparently underlies the majority of the site and was encountered to a maximum depth of approximately 1 foot below the existing ground surface. The thickness and extent of these soils may vary and should be evaluated by the geotechnical consultant during removal of these unsuitable soils. These soils are considered suitable for re-use as compacted, structural fill provided they are free of organic material, deleterious debris and oversized materials (rocks with a maximum dimension greater than 6 inches). Excavations - It is anticipated that excavation of the onsite soils can be accomplished by conventional grading equipment in good operating condition. Structural Fill Placement - Areas to receive fill and/or other surface improvements should be scarified to a minimum depth of 6 inches, brought to near-optimum moisture conditions, and recompacted to at least 90 percent relative compaction, based on laboratory standard ASTM D1557. Fill soils should be brought to near-optimum moisture conditions and compacted in uniform lifts to at least 90 percent relative compaction (ASTM D1557). The SGC Project No. 106E61 optimum lift thickness to produce a uniformly compacted fill will depend on the size and type of construction equipment used. In general, fill should be placed in loose lift thicknesses not exceeding 8 inches. Placement and compaction of fill should be observed and tested by the geotechnical consultant. In general, placement and compaction of fill should be performed in accordance with local grading ordinances, sound construction practices, and the Recommended Earthwork Specifications included in Appendix C. Transition (Cut/Fill) Condition - The potential for a transition (cut-fill) condition underlying the area of proposed structures should be checked when project plans are finalized and in the field during grading so that appropriate recommendations can be provided to reduce the potential damage due to differential settlement of the structure across the transition. Typically, we recommend that the cut (or natural) portion of the building area be overexcavated to a minimum depth of 3 feet and replaced with moisture- conditioned fill soils compacted to at least 90 percent relative compaction (ASTM Dl 557). The overexcavation and recompaction typically extends for a distance of at least 5 feet beyond the perimeters of the proposed structures. Trench Backfill - The onsite soils are generally suitable as trench backfill provided they are screened of organic matter and clasts over 6 inches in diameter. Trench backfill should be compacted by mechanical means to at least 90 percent relative compaction (ASTM D1557). Graded Slopes - We recommend that graded slopes be constructed at a slope gradient of 2 to 1 (horizontal to vertical) or flatter. The outer portions of fill slopes should be compacted in increments by a sheepsfoot roller as the fill is raised or overbuilt and trimmed back to finished grade. Imported Fill Soils - Imported fill soils should have a very low expansion index (expansion index less than 20) and should be tested by the geotechnical consultant for suitability prior to hauling on site. The imported fill soils should be brought to near-optimum moisture conditions and uniformly compacted to at least 90 percent relative compaction (ASTM D1557). Foundations Foundations should be designed in accordance with structural considerations and the following recommendations. These recommendations assume that the footings will be excavated entirely into properiy compacted fill soils or entirely into dense natural soils with very low to low expansion potential. If the expansion potential of the SGC Project No. 106E61 finished building pad soils differ from that assumed herein, appropriate corresponding modifications to the foundation recommendations may be necessary. The proposed two-story structure may be supported by continuous or spread footings bearing at a minimum depth of 18 inches below the lowest adjacent. Continuous footings should have a minimum width of 15 inches and be reinforced, at a minimum, with two No. 4 rebars (one near the top and one near the bottom). Spread footings should be designed in accordance with structural considerations and have a minimum width of 24 inches. For footings designed in accordance with the above recommendations, an allowable soil-bearing capacity of 2,000 pounds per square foot may be assumed. This value may be increased by one-third for loads of short duration such as wind and seismic loads. Slabs Concrete slab-on-grade floors should be designed in accordance with structural considerations and the following recommendations. Slab-on-grade floors underlain entirely by properly compacted fill soils or entirely by dense natural soils with a very low to low expansion potential should have a minimum thickness of 4 inches and be reinforced at midheight with No. 3 rebars at 18 inches on center each way (or No. 4 at 24 inches on center each way). Care should be taken by the contractor to insure that the reinforcement is placed at slab midheight. Slabs should be designed with crack control joints at appropriate spacings. Nuisance cracking may be reduced by careful control of water/cement ratios. The slabs should be underlain by a 4-inch layer of clean sand (sand equivalent greater than 30). In moisture-sensitive areas or if floor coverings are planned, a 10-mil moisture barrier is recommended midheight within the sand blanket. The soils beneath the floor slabs should be moistened prior to placement of the sand blanket, moisture barrier and concrete. We recommend that a slip-sheet (or equivalent) be utilized if grouted tile or other crack-sensitive flooring is planned directly on the concrete slabs. Please note that the recommendations provided for slabs are minimums. They do not represent an adequate lesser substitute for those that may be recommended by the structural consultant. In addition, our recommendations are not intended to eliminate the possibility of cracks due to concrete shrinkage. Shrinkage cracks develop in nearly all slabs which are not specifically designed to prevent them. We recommend that a structural consultant or qualified concrete contractor be consulted to provide appropriate design and workmanship requirements for mitigation of shrinkage cracks. SGC Project No. 106E61 Lateral Resistance and Retaining Wall Design Parameters Footings and slabs founded in properly compacted fill soils may be designed for a passive lateral bearing pressure of 300 pounds per square foot per foot of depth. A coefficient of friction against sliding between concrete and soil of 0.3 may be assumed. These values may be increased by one-third when considering loads of short duration, such as wind or seismic forces. Cantilever (yielding) retaining walls may be designed for "active" equivalent fluid pressure of 3.5 pounds per cubic foot. Retaining walls which are rigid or restrained at their upper ends (non-yielding) may be designed for an "at-rest" equivalent fluid pressure of 50 pounds per cubic foot. Walls subject to surcharge loading of vehicular traffic within a distance behind the wall equal to the wall height should be designed for an additional uniform pressure of 75 psf. If walls are surcharged by adjacent structures, the wall design should take into account the surcharge load. These values assume horizontal, nonexpansive granular backfill and free-draining conditions. Retaining wall footings should be designed in accordance with the previous foundation recommendations. We recommend that retaining walls be provided with appropriate drainage provisions. Appendix C contains a typical detail for drainage of retaining walls. The walls should also be appropriately waterproofed. Waterproofing treatments and alternative, suitable wall drainage products are available commercially. Design of waterproofing and its protection during construction should be provided by the project architect. Wall backfill should be compacted by mechanical means to at least 90 percent relative compaction (ASTM D1557). Care should be taken when using compaction equipment in close proximity to retaining walls so that the walls are not damaged by excessive loading. Seismic Considerations The principal seismic considerations for most structures in southern California are damage caused by surface rupturing of fault traces, ground shaking, seismically- induced ground settlement or liquefaction. The seismic hazard most likely to impact the site is ground shaking resulting from an earthquake on one of the major active regional faults. The possibility of damage due to ground rupture is considered minimal since no active faults are known to cross the site. The potential for liquefaction or seismically-induced ground settlement due to an earthquake is considered low because of the dense nature of the underlying terrace deposits and anticipated lack of a static, near-surface groundwater table. SGC Project No. 106E61 The effects of seismic shaking can be reduced by adhering to the most recent edition of the Uniform Building Code and current design parameters of the Structural Engineers Association of California. Based on our understanding of the onsite geotechnical conditions, the seismic design parameters from the 1 997 Uniform Building Code, Section 1636, Tables 16-J, 16-S, 16-T and 16-U are provided below. UBC Table 16-J - Based on our understanding of the onsite geotechnical conditions and our review of UBC Table 16-J, the soil profile type for the subject property is Sp ("Stiff Soil Profile"). UBC Table 16-U - Based on our review of the Active Fault Near-Source Zones maps (0-36) prepared by the California Division of Mines and Geology, the nearest known active fault is the Rose Canyon fault zone located offshore to the west of the site. The site is located within approximately 7 kilometers of the Rose Canyon fault. The fault is considered a seismic source type B based on UBC Table 16-U. UBC Table 16-S - Based on our understanding of the onsite geotechnical conditions and minimum distance to the nearest known active fault (Rose Canyon fault zone), the Near-Source Factor (Ng) is 1.0. UBC Table 16-T - Based on our understanding of the onsite geotechnical conditions and minimum distance to the nearest known active fault (Rose Canyon fault zone), the Near-Source Factor (N^) is 1.1. Sulfate Content A sample (Boring 1, sample at 1 to 3 feet) of the onsite soils was tested to assist in an evaluation of the degree of sulfate attack on ordinary (Type 11) concrete. The test was performed in general accordance with California Test Method No. 41 7 and yielded a soluble sulfate content of 80 ppm. The test result indicates a "negligible" degree of sulfate attack based on UBC Table 19-A-4 criteria. The type of concrete specified and used should be determined by the structural engineer. 8 SGC Project No. 106E61 Site Drainage Drainage at the site should be directed away from foundations, collected and tightlined to appropriate discharge points. Consideration may be given to collecting roof drainage by eave gutters and directing it away from foundations via non-erosive devices. Water, either natural or from irrigation, should not be permitted to pond, saturate the surface soils or flow over the tops of slopes. Landscape requiring a heavy irrigation schedule should not be planted adjacent to foundations or paved areas. Plan Review/Construction Observation and Testing The recommendations provided in this report are based on our understanding of the project and subsurface conditions disclosed in our widely-spaced exploratory borings. Final project drawings for the proposed development should by reviewed by Southland Geotechnical Consultants prior to construction to check that the recommendations contained in this report are incorporated into the project plans. Subsurface conditions should be checked in the field during construction. Geotechnical observation during site grading and field density testing of compacted fill should be performed by Southland Geotechnical Consultants. Geotechnical observation of footing excavations should also be performed by the geotechnical consultant to check that construction is in accordance with the recommendations of this report. SGC Project No. 106E61 If you have any questions regarding our report, please contact our office, appreciate the opportunity to be of service. We Sincerely, SOUTHLAND GEOTECHNICAL CONSULTANTS Gene Custenborder, CEG 1319 Principal Engineering G< Attachments: Charles R. Corbin, RCE 36302 Project Engineer Figure 1 - Site Location Map Figure 2 - Exploratory Boring Location Map Appendix A - References Appendix B - Logs of Exploratory Borings Appendix C - Recommended Earthwork Specifications Distribution: (1) Addressee (3) Edward M. Eginton, Inc. 10 SGC SITE LOCATION IVIAP N Project No. 106E61 3286 Lincoln Street Carlsbad, California Scale (approxinnate): 1 inch = 2,000 feet Base Map: Recent Slope Failures, Ancient Landslides, and Related Geology of the North-Central Coastal Area, San Diego County, California by F. Harold Weber, Jr., 1982. FIGURE 1 SGC WALNUT AVENUE ^"^un?^^ / N EXPLORATORY BORING LOCATION MAP LEGEND Project No. 106E61 Proposed Nayudu Residence 3286 Lincoln Street Carlsbad, California Scale (approximate): 1 inch = 20 feet Base Map: Site Plan prepared by Edward M. Eginton, Architect, Inc., dated December 1998 « Approximate location of B-3 exploratory boring FIGURE 2 SGC '•7'.77-77777/777}07i7^7^77777^^ Project No. 106E61 APPENDIX A REFERENCES 1. California Division of Mines and Geology, 1 994, Fault activity map of California and adjacent areas: CDMG Geologic Data Map No. 6. 2. Hart, E.W., 1997, Fault-rupture hazard zones in California: California Division of Mines and Geology, Special Publication 42, revised. 3. Tan, S.S., 1995, Landslide hazards in the northern part of the San Diego metropolitan area, San Diego County, California: California Division of Mines and Geology, Open-File Report 95-04. 4. Southland Geotechnical Consultants, in-house geologic/geotechnical information. 5. Weber, F.H., Jr., 1982, Recent slope failures, ancient landslides, and related geology of the north-central coastal area, San Diego County, California: California Division of Mines and Geology, Open-file Report 82-12LA. PLANS Pasco Engineering, undated. Site Development Study. Edward M. Eginton, Architect, Inc., 1998, Single Family Residence for Mr. and Mrs. Kris Nayudu, 3286 Lincoln St., Carlsbad, dated December. SGC '7/-7'777$7^7( 777-77777 Project No, 106E61 APPENDIX B LOGS OF EXPLORATORY BORINGS BORING NO. DEPTH DESCRIPTION B-1 0-1' Topsoil - Dark brown, dry, loose, silty fine sand (SM); with abundant roots in upper 4", gradational with: 1-4' Terrace Deposits - Dark orange-brown, moist, medium dense, siity fine sand (SM) Total depth = 4 feet No refusal No groundwater encountered Bulk sample at 1 to 3 feet Excavated and backfilled 10-29-01 B-2 0-1' Topsoil - Dark brown, dry, loose, silty fine sand (SM); with roots, gradational with: 1-3' Terrace Deposits - Dark orange-brown, moist, medium dense, silty fine sand (SM); with roots Total depth = 3 feet No refusal No groundwater encountered Bulk sample at 0 to 1 foot Excavated and backfilled 10-29-01 SGC Project No. 106E61 LOGS OF EXPLORATORY BORINGS (continued) BORING NO. DEPTH DESCRIPTION B-3 0-1' Topsoil - Dark brown, dry, loose, silty fine sand (SM); with roots, gradational with: 1-3' Terrace Deposits - Dark orange-brown, moist, medium dense, silty fine sand (SM) Total depth = 3 feet No refusal No groundwater encountered Bulk sample at 1 to 3' Excavated and backfilled 10-29-01 SGC