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HomeMy WebLinkAboutHDP 90-03; Leigh Residence; Hillside Development Permit (HDP) (2)REPORT OF SITE PREPARATICN AND POUNDflTICN RECCMMENDflTICNS PROPOSED SINC3i: FAMILY RESIDENCE LA COSTA AVENUE CARLSBAD, CALIFORNIA PREPARED FOR: Mr. Bill Leigh 523 Shadow Hills San Marcos, Califomia 92069 PREPARED BY: Southem Califomia Soil and Testing, Incorporated Post Office Box 20627 6280 Riverdale Street San Diego, Califomia 92120 SOUTHERN CALIFORNIA SOIL AND TESTING, INC. 6Zaa RIVERDALE ST. SAN DIEBD, CALIF. 92120 • TELE 2aD-4321 • P.O. BOX 20627 SAN OIEGO, CALIF. 92120 67a ENTERPRtSe 3T. ESCONOIOa. CALIF. • TELE 746-45^.4 Noventjer 29, 1989 Mr. Bill Leigh 523 Shadow Hills SCS&T 8921165 San Marcos, Califomia 92069 Report No. 1 SUBJECT: Site Preparation and Foundation Recommendations, Proposed Single Family Residence, La Costa Avenue, Carlsbad, Califomia. Dear Mr. Leigh: In accordance with your request, this report has been prepared to present site preparation and foundation reconrtendations for a proposed single family residence, to be located between 2424 and 2440 La Costa Avenue in Carlsbad, Califomia. STFE EESCRIPTICN The subject is legally described as Lot Number 9, La Costa South, Unit Nunfcer 1; County Assessor's Parcel Number 216-160-09. The lot is a nearly rectangular parcel with an average width of 76 feet and an average depth of 135 feet. A vacant lot exists to the east, a flag lot exists to the north with its access on the vvest side of the si±iject lot, and a condominium conplex exists to the west of the flag lots access way. A house slab that appears to have been constructed some time ago exists on the flag lot. The si±)ject lot is vacant of inprovements. It has a relatively level pad that extends back about 77 feet from the front property line. Numerous piles SOUTHERN CALIFORNIA SOIL AND TESTING, INC. SCS&T 8921165 Noventser 29, 1989 Page 2 of end-dumped fill exist on the level pad area. North of the level pad, the lot descends steeply to a lower level that is roughly the same level as the flag lot. The inclination of this slope varies from about 1.5:1 (horizontal to vertical) in the upper portion to 1:1 in the lower portion. The height of this slope is about 28 feet from toe to top. The slope down to the flag lot's access leg is at a ratio of about 1.5:1. SOIL DESCRIPTICNS The subject site has been graded into its present configuration. The upper portion of the lot contains fill material that appears to have been imported. This material is located as approximately delineated on the plot plan included heirewith and consists of a whitish tan, very silty sand/sandy silt. The maximum depth of the fill appears to be about 10 feet near the northwest comer of the upper pad. No documentation was provided on the fill to verify its proper placement, and, our field observations indicated that it is questionable in regards to being suitable to support structures or to exist near the face of slopes. Below the fill, as exposed in the natural portions of the slope below the fill, and on the lot to the east, is a critically expansive, light chocolate brown, clay topsoil. This material has desiccation cracks extending at least two feet deep, has a ^popcorn' texture, and has experienced sloughing and downhill creep v^ere it exists on sloping ground. This material is very veak vAien wet and does not stand veil in cut or fill slopes. Below the topsoil are hard mudstones of the Delmar Formation. This material is exposed in a 1:1 cut made in the lower portion of the slope. This cut has a maximum height of about 12 to 15 feet. The mudstones are comprised of light green, sandy to silty clays. These clays typically do not stand veil in steep slopes and usually require buttressing. This material is also generally found to be highly expansive. Where these mudstones exist near the front of the lot, it appears that they may consist of v^tish tan sandstones. PROPOSED OCNSTRUCTION The proposed structure will be a single-story structure with four different floor levels. The upper two levels will be at elevation 98.94 feet and 95.74 SCS&T 8921165 November 29, 1989 Page 3 feet and will have on-grade concrete floor slabs. The third level down will be at 91.74 feet and will be a raised wood floor. The fourth and lovrest level will be at elevation 81.74 feet and will also have a raised wood floor. An observation deck will be constructed over the lowest level that will basically be at the same elevation as the third lowest level. Construction will consist of wood-framing with stucco siding. Shallow foundations are anticipated and some masonry retaining walls will be necessary, both for the house and for the east property line. The structure will cover roughly 50 percent of the lot area. FPCSPOBED OWDXNS The proposed grading will basically create three level pads and a 2.5:1 slope extending down from the lower pad to the north property line. All other slopes will be constructed at a ratio of 2:1 or flatter. The maximum slope height will be about 20 feet. CCNCLUSIONS AMD RBCCMMENDATICNS (324ERAL; Based on our past experience in the vicinity of the subject site and on the exposed soil conditions as discussed herein, it is our opinion that the subject site can be developed as proposed, provided the recommendations presented herewith eure conplied with. Soil conditions that will create additional construction costs for the proposed residence include moderately to very highly expansive soils, undocumented fill that will be required to be recompacted, and relatively weak soils in regards to foundation and retaining wall design. SITE PREPARMTCN: Site preparation should begin with the removal of all vegetation and deleterious materials from the site in the areas to be graded. The existing fill materials should then be removed and be replaced as uniformly conpacted fill. Any topsoils, subsoils or disturbed formational mterial found beneath the fill, or in any other areas on the site, should SCS&T 8921165 November 29, 1989 Page 4 also be removed and level benches should be cut into the formationcil soils to support the fills. Further, prior to placing any fill in the lower portion of the lot, any loose or disturbed soil should be removed and a level bench mde in the formational soils. The existing cut slope on the lover portion of the lot should be keyed into in a stair-step manner as the fill is brought up to finish grade. All areas to support fill should be scarified to a depth of 12 inches, moisture conditioned and be compacted to at least 90 percent relative cortpaction. SDC3PE OCNSTRDCTTON: All slopes should be constructed at an inclination of two horizontal to one vertical or flatter. All fill slopes should be backrolled with a sheepsfoot conpacter at vertical intervals of four feet or less and should be trackwalked v^en conpleted. The very highly ej^)ansive topsoils should be mixed with the less, expansive soils when placed in fill slopes such that the mixture will have an expansion index of less than 120. EARTHWORK: All earthvrork and grading contenplated for site preparation should be accomplished in accordance with the attached Recormended Grading Specifications and Special Provisions. All special site preparation recaimendations presented in the sections above will supersede those in the Standard Recomnrended Grading Specifications. All errbankments, structiiral fill and fill should be compacted to at least 90% relative compaction at or slightly over optimum moisture content. Utility trench backfill within five feet of the proposed structures and beneath pavements should be conpacted to minimum of 90% of its maximum dry density. The upper six inches of subgrade beneath paved areas should be conpacted to 95% of its maximum dry density. This conpaction should be obtained by the paving contractor just prior to placing the aggregate base material and should not be part of the ness grading requirements. The maximum dry density of each soil type should be determined in accordance with A.S.T.M. Test Method D-1557-78, Method A or C. DRAINAffi: No surface water should be allowed to pond adjacent to foundations and surface runoff should not be allowed to flow over the top of the slopes. SCS&T 8921165 Novenber 29, 1989 Page 5 FOONDflTICNS: Considering the highly expansive soil conditions and a cut/fill transition within the building pad, all footings that will support the proposed structure should be founded at least 24 inches below lowest adjacent grade. When founded on or near slopes, the footings should have a minimum depth that wDuld result in a horizontal distance of at least seven feet between the bottom of the footing and the face of the slope. All continuous footings should have a minimum width of 12 inches and isolated footings should have a minimum width of 24 inches. Footings may be designed for an allowable soil bearing pressure of 1500 psf. FOUMDKnON REINPQRCING: Foundation reinforcing should be determined by the project structural engineer. Howaver, as a minimum, we reconmend that the continuous footings be reinforced with at least four No. 5 bars: two located near the top and two located near the bottom of the footing. di-GRAIK CCNCREIE SLABS: Slabs-on-grade should be at least five inches thick and be reinforced with at least No. 3 bars at 12 inches on center each way. The interior slab reinforcing should be turned down into the perimeter footings. The reinforcing for exterior slabs that abut footings should be doveled into the footings. Interior slabs should be underlain by a moistxire barrier consisting of one inch of clean sand over 6 mil visqueen over four inches of clean, coarse, poorly graded sand or crushed rock (maximum size 1/2 inch). MDISTURE CCNDITICNING: The clayey soils should not be allowed to dry out before placing concrete. They should be kept in a moist condition by sprinkling when necessary until the concrete is placed. If allowed to dry out, pre-soaking may be required. RETAINING WAUJS: Earth retaining walls should be designed for an equivalent fluid pressure of 50 and 60 pounds per cijbic foot for unrestrained and restrained walls respectively. These pressures reflect a drained and level backfill condition. All retaining walls should have a si±drain and have a 12-inch-wide rock or a Miradrain drain that extends up to one foot below the SCS&T 8921165 November 29, 1989 Page 6 top of the retained soil. A passive pressure of 300 psf per foot of depth may be used to resist lateral movement. In addition the coefficient of friction or concrete to soil may be assumed to be 0.30. When carbining friction and passive pressure, the friction value should be reduced by one-third. If you should have any questions after reviewing this report, please do not hesitate to contact this office. This opportunity to be of professional service is sincerely appreciated. Respectfully submitted, SOUTHERN CALIFORNIA SOIL & TESTING, INC. '7 . ^^,7 Charles H. Christian, R.G.E. #00215 CHC:mw cc: (4) Submitted (2) E. Brian Smith (1) SCS&T, Escondido i SCALE 1"=20' O 10 20 30 40 LEGEND <C-£2.r5 EXISTING TOPOGRAPHY APPROXIMATE LOCATION OF DAY LIGHT LME LA COSTA AVENUE SOUTHIRM CALIFORNIA ^^1^ SOiL A TKSTINQ,INC. SOUTHIRM CALIFORNIA ^^1^ SOiL A TKSTINQ,INC. ,Y: CHC/WDW DATE: 11-30^89 SOUTHIRM CALIFORNIA ^^1^ SOiL A TKSTINQ,INC. JOI NUMtER: M21165 PLATE #1 SINC3UE FAMILY RESIDENCE, LA COSTA AVENUE, CARLSBftD RECCWMENEED GRADING SPBCIFICATICNS - (3MERAL PROVISICNS GQIERAL nnSSfT The intent of these specifications is to establish procedures for clearing, conpacting natural ground, preparing eureas to be filled, and placing and conpacting fill soils to the lines and grades shown on the accepted plans. The reconmendations contained in the preliminary geotechnical investigation report and/or the attached Special Provisions are a part of the Reconmended Grading Specifications and shall supersede the provisions contained hereinafter in the case of conflict. These specifications shall only be used in conjunction with the geotechnical report for which they are a part. No deviation from these specificatipns will be allowed, except v^ere specified in the geotechnical report or in other written comnunication signed by the Geotechnical Engineer. OBSERVZXnCN AM) TESmC Southem California Soil and Testing, Inc., shall be retained as the Geotechnical Engineer to observe and test the earthwork in accordance vdth these specifications. It will be necessary that the Geotechnical Engineer or his representative provide adequate observation so that he may provide his opinion as to whether or not the work was acconplished as specified. It shall be the responsibility of the contractor to assist the Geotechnical Engineer and to keep him appraised of work schedules, changes and new information and data so that he may provide these opinions. In the event that any unusual conditions not covered by the special provisions or preliminary geotechnical report are encountered during the grading operations, the Geotechnical Engineer shall be contacted for further reconmendations. (R-9/89) SCS&T 8921165 November 29, 1989 Appendix, Page 2 If, in the opinion of the Geotechnical Engineer, substandard conditions are encountered, such as questionable or unsuitable soil, unacceptable moisture content, inadequate conpaction, adverse weather, etc.; construction should be stopped until the conditions are remedied or corrected or he shall recommend rejection of this work. Tests used to determine the degree of conpaction should be performed in accordance with the following American Society for Testing and Materials test methods: Maximum Density & Optimum Moisture Content - ASTM D-1557-78. Density of Soil In-Place - ASTM D-1556-64 or ASTM D-2922. All densities shall be expressed in terms of Relative Compaction as determined by the foregoing ASTM testing procedures. PREPARATICN OF AREAS TO RECEIVE FILL All vegetation, brush and debris derived from clearing operations shall be removed, and legally disposed of. All areas disturbed by site grading should be left in a neat and finished appearance, free from unsightly debris. After clearing or benching the natural ground, the areas to be filled shall be scarified to a depth of 6 inches, brought to the proper moisture content, conpacted and tested for the specified minimum degree of compaction. All loose soils in excess of 6 inches thick should be removed to firm natural ground v^iich is defined as natural soils wtiich possesses an in-situ density of at least 90% of its neximum dry density. (R-9/89) SCS&T 8921165 NovCTiber 29, 1989 Appendix, Page 3 When the slope of the natural ground receiving fill exceeds 20% (5 horizontal units to 1 vertical unit), the original ground shall be stepped or benched. Benches shall be cut to a firm conpetent formational soils. The lower bench shall be at least 10 feet wide or 1-1/2 times the the equipment width v^chever is greater and shall be sloped back into the hillside at a gradient of not less than two (2) percent. All other benches should be at least 6 feet wide. The horizontal portion of each bench shall be conpacted prior to receiving fill as specified herein for conpacted natural ground. Ground slopes flatter than 20% shall be benched when considered necessary by the Geotechnical Engineer. Any abandoned buried structiires encountered during grading operations must be totally removed. All underground utilities to be abandoned beneath any proposed structure should be removed from within 10 feet of the structure and properly capped off. The resulting depressions from the above described procedures should be backfilled with acceptable soil that is conpacted to the requirements of the Geotechnical Engineer. This includes, but is not limited to, septic tanks, fuel tanks, sewer lines or leach lines, stojnn drains and water lines. Any buried structures or utilities not to be abandoned should be brought to the attention of the Geotechnical Engineer so that he may determine if any special recommendation will be necessary. All water wells v^iich will be abandoned should be backfilled and capped in accordance to the requirements set forth by the Geotechnical Engineer. The top of the cap should be at least 4 feet below finish grade or 3 feet below the bottom of footing whichever is greater. The type of cap will depend on the diameter of the well and should be determined by the Geotechnical Engineer and/or a qualified Structural Engineer. (R-9/89) SCS&T 8921165 November 29, 1989 i^pendix. Page 4 FHii MATERIAL Materials to be placed in the fill shall be approved by the Geotechnical Engineer and shall be free of vegetable matter and other deleterious substances. Granular soil shall contain sufficient fine material to fill the voids. The definition and disposition of oversized roclcs eind expansive or detrimental soils are covered in the geotechnical report or Special Provisions. Ejqansive soils, soils of poor gradation, or soils with low strength characteristics may be thoroughly mixed with other soils to provide satisfactory fill material, but only with the explicit consent of the Geotechnical Engineer. Any import material shall be approved by the Geotechnical Engineer before being brought to the site. PLACING AND CCMPACTICN OF FILL Approved fill material shall be placed in areas prepared to receive fill in layers not to exceed 6 inches in conpacted thickness. Each layer shall have a uniform moistiare content in the range that will allow the conpaction effort to be efficiently applied to achieve the specified degree of compaction. Each layer shall be uniformly compacted to the specified minimum degree of compaction with equipment of adequate size to economically conpact the layer. Conpaction equipment should either be specifically designed for soil conpaction or of proven reliability. The minimum degree of conpaction to be achieved is specified in either the Special Provisions or the recomnendations contained in the preliminary geotechnical investigation report. When the structural fill material includes rocks, no rocks will be alloved to nest and all voids must be carefully filled with soil such that the minimum degree of compaction recommended in the Special Provisions is achieved. The maximum size and spacing of rock permitted in structxxral fills and in non-structural fills is discussed in the geotechnical report, v^en applicable. (R-9/89) SCS&T 8921165 November 29, 1989 i^ppendix. Page 5 Field observation and conpaction tests to estimate the degree of conpaction of the fill will be taken by the Geotechnical Engineer or his representative. The location and frequency of the tests shall be at the Geotechnical Engineer's discretion. When the conpaction test indicates that a particular layer is at less than the required degree of conpaction, the layer shall be reworked to the satisfaction of the Geotechnical Engineer and until the desired relative conpaction has been obtained. Fill slopes shall be conpacted by means of sheepsfoot rollers or other suitable equipment. Cotpaction by sheepsfoot rollers shall be at vertical intervals of not greater than four feet. In addition, fill slopes at a ratio of two horizontal to one vertical or flatter, should be trackrolled. Steeper fill slopes shall be over-built and cut-back to finish contours after the slope has been constructed. Slope conpaction operations shall result in all fill material six or more 'inches inward from the finished face of the slope having a relative conpaction of at least 90% of maximum dry density or the degree of conpaction specified in the Special Provisions section of this specification. The conpaction operation on the slopes shall be continued until the Geotechnical Engineer is of the opinion that the slopes will be stable surficially stable. Density tests in the slopes will be made by the Geotechnical Engineer during construction of the slopes to determine if the required conpaction is being achieved. Where failing tests occur or other field problems arise, the Contractor will be notified that day of such conditions by written conmunication from the Geotechnical Engineer or his representative in the form of a daily field report. If the method of achieving the required slope conpaction selected by the Contractor fails to produce the necessary results, the Contractor shall rework or rebuild such slopes until the required degree of conpaction is obtained, at no cost to the Owner or Geotechnical Engineer. (R-9/89) SCS&T 8921165 November 29, 1989 Appendix, Page 6 CUT SLOPES The Engineering Geologist shall inspect cut slopes excavated in rock or lithified formational material during the grading operations at intervals determined at his discretion. If any conditions not anticipated in the preliminary report such as perched water, seepage, lenticular or confined strata of a potentially adverse nature, unfavorably inclined bedding, joints or fault planes are encountered during grading, these conditions shall be analyzed by the Engineering Geologist and Soil Engineer to determine if mitigating measures are necessary. Unless otherwise specified in the geotechnical report, no cut slopes shall be excavated higher or steeper than that alloved by the ordinances of the controlling govemmental agency. ENGINEERING GBSERVTVTIGN Field observation by the Geotechnical Engineer or his representative shall be nade during the filling and conpacting operations so that he can express his opinion regarding the conformance of the grading with acceptable standards of practice. Neither the presence of the Geotechnical Engineer or his representative or the observation and testing shall not release the Grading Contractor from his duty to compact all fill material to the specified degree of conpaction. SEASCN UMITS Fill shall not be placed during unfavorable weather conditions. When vrork is interrupted by heavy rain, filling operations shall not be resumed until the proper moisture content and density of the fill materials can be achieved. Damaged site conditions resulting from weather or acts of God shall be repaired before acceptance of work. (R-9/89) SCS&T 8921165 November 29, 1989 Appendix, Page 7 RBCOMMENIM) OUTING SPECIFICAnCNS - SPECIAL PROVISICNS RELftnVE CCMPACTICN: The minimum degree of conpaction to be obtained in conpacted natural ground, conpacted fill, and conpacted bacJcfill shall be at least 90 percent. For street and parking lot subgrade, the upper six inches should be compacted to at least 95% relative conpaction. EXPANSIVE SOUS: Detrimentally expansive soil is defined as clayey soil which has an expansion index of 50 or greater v^en tested in accordance with the Uniform Building Code Standard 29-C. OVERSIZED MMERIAL: Oversized fill material is generally defined herein as rocks or lunps of soil over 6 inches in diameter. Oversize materials should not be placed in fill unless reconmendations of placement of such material is provided by the geotechnical engineer. At least 40 percent of the fill soils shall pass through a No. 4 U.S. Standard Sieve. TRAHBinON LOTS: Where transitions between cut and fill OCCTUT within the proposed building pad, the cut portion should be undercut a minimum of one foot below the base of the proposed footings and reconpacted as stmctvural backfill. In certain cases that would be addressed in the geotechnical report, special footing reinforcement or a combination of special footing reinforcement and undercutting may be required. (R-9/89)