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HomeMy WebLinkAboutCT 74-15; Las Costa Estates North; Preliminary Soils Report; 1986-11-24City of Carlsbad .' I Engineering Dept. Carlsbad, CA. 92008 2075 Las Palmas Drive c/o Zoltan Kasa Carlsbad,.CA. 92008 2926 Via Pepita 1535 S. Elcondido Blvd. Elcondido. CA 92025 (619) 7414533 LAND DEVELOPMENT Surveying ~ Perc Tats - Lot Splits Grading Plans & Comp.ction TCSII November 24, 1986 , Re: 7336 El Fuerte Street NQ~ 2 6 19" Lot 60 of, Las Costa Estates North, Map 8302 Subject: PRELIMINARY SOILS INVESTIGATION FOR OF. CWS BRD PROPOSED SINGLE FAMILY RESIDENCE ,C\O DaEL~~. PRO'' si~\J. b>\v TO WHOM IT MAY CONCRRN: Per the request of Zoltan Kasa, our firm has conducted a Prelim inary Soils Investigation of the Soils found at the above refer enced site. This report represents the findings and recomrnenda tions for the proposed development based upon the results of this investigation. This investigation consisted of a site geologic reconnaissance, and the drilling of several 12 inch diameter auger borings. Laboratory testing was performed on representative soils samples obtained from these borings in order to evaluate pertinent physical properties. The conclusions and recommendations that follow are based upon review of the proposed development, inspe ction of the site, an analysis of the data obtained and our experience with similar soil conditions. ENGINEERING DEPT. LIBRAW City of Carlsbad 2075 Las Palmas Drive Carlsbad CA 92009-4859 CIVIL ENGINEERING LAND SURVEYING PROJECT SCOPE Explore the subsurface conditions to the depths influenced by the proposed construction. Evaluate, by laboratory tests, the pertinent engineering pro- perties of the various strata which will influence the develop- ment, including their bearing capacities, expansive character- istics and settlement potential. Define the general geology at the site including possible geolo gic hazards which could have an effect on the site develop- ment. Determine potential construction difficulties and provide re- commendations concerning these problems. Recommend an appropriate foundation system for the type of structures anticipated and . develop soil engineering design criteria for the Tecommended design. THE SITE The' site consists of residential lots of approximately 0.5 acre located on the southeast side of El Fuerte Street in the City. of Carlsbad. This lot was created as part of the La Costa Estates North Tract 154. Street grading and improvements along with partial lot grading was completed in November 1976 and certified by Benton Engineering of San Diego. The parcel slopes from its northern most corner at El Fuerte Street to the southerly corner with a difference in elevation of approx- imately 22'. The site is covered with low grass and brush common to the area. Numerous large single family residences have been constructed on other similiar lots surrounding the site. The immediate adjacent lots to the North and East are still vacant. A residential Grading Plan has been prepared by this Engineer for development of this lot. Cuts and fills proposed will approach 8' in height. FIELD INVESTIGATION In November 1986, we made a field review of the site along with the borings of numerous 12" auger borings within the proposed grading area. The soil profile was identical over the site as determined from those borings. The upper 5' of soil found over the site consists of clayey-silts with fractured rock fragments. These soils are considered to be expansive in character. This soil layer is under- lain by Santiago Peaks Volcanic formation. This formation consists of a very hard brittle rock formation that fractures into 2" to 6" pieces when ripped with conventional heavy grading equipment. This fractured rock when mixed with surface soils will form a suitable embankment for the proposed development. Also noted during our investigation was the presence of loose layers of foreign debris that had been scattered over the lot. This material will require removal during clearing operations prior to grading. No ground water was encountered during our investigation and ground water will not be a problem during or after construction. LABORATORY TESTS Laboratory tests were performed in accordance with generally accepted test methods of the American Society for Testing and Materials (ASTM) and other suggested procedures. Selected relatively undisturbed drive samples were tested for their inplace dry density, moisture content and expansive potential. The maximum dry density and optimum moisture content of the near surface soils were determined in accordance with ASTM Test D155770, Method A. In addition, a directshear test and an expansion test were performed on samples remolded to 90% of maximum dry density at optimum moisture content. Field moisture content and dry density were determined for representative undisturbed samples. This information was an aid to classification and permitted recognition of variations in material consistency with depth. The dry unit weight is determined in pounds per cubic foot, and the field moisture content is determined as a percentage of the soils dry weight. Field classifications were verified in the laboratory by visual examination. The final soils classifications are in accordance with the Unified Soil CLassification System. Direct shear tests were performed to evaluate bearing potential along with maximum density testing and expansive analysis were conducted. Maximum dry weight was determined to be 124 #/c.f. @ 12.0% optimum moisture content. Testing was done for internal shear based upon utilizing a 90% relative compaction. The apparent angle of shear for the soils encountered = 18". An expansion analysis of these soils was performed in accordance with the UBC 292 test procedures. The result OF this test indicated an index value of 72, which is considered to be moderately expansive. LABORATORY TESTS Laboratory tests were performed in accordance with generally accepted test methods of the American Society for Testing and Materials (ASTM) and other suggested procedures. Selected relatively undisturbed drive samples were tested for their inplace dry density, moisture content and expansive potential. The maximum dry density and optimum moisture content of the near surface soils were determined in accordance with ASTM Test D155770, Method A. In addition, a directshear test and an expansion test were performed on samples remolded to 90% of maximum dry density at optimum moisture content. Field moisture content and dry density were determined for representative undisturbed samples. This information was an aid to classification and permitted recognition of 'variations in material consistency with depth. The dry unit weight is determined in pounds per cubic foot, and the field moisture content is determined as a percentage of the soils dry weight. Field classifications were verified in the laboratory by visual examination. The final soils classifications are in accordance with the Unified Soil CLassification System. Direct shear tests were performed to evaluate bearing potential along with maximum density testing and expansive analysis were conducted. Maximum dry weight was determined to be 124 #/c.f. @ 12.0% optimum moisture content. Testing was done for internal shear based upon utilizing a 90% relative compaction. The apparent angle of shear for the soils encountered = 18". An expansion analysis of these soils was performed in accordance with the UBC 292 test procedures. The result OF this test indicated an index value of 72, which is considered to be moderately expansive. TECTONIC SETTING La Costa, (Carlsbad) along with most of the southwestern portion of California, San Diego County, and parts of Mexico are located on a relatively stable tectonic plate. This plate is bounded on the east by the San Andreas Fault, system and on the west by the San Clemente Fault System. These faults contain many other parallel faults which are essentially high angle strikeslip faults with a NW/SE trend. Although the bounding fault systems are active, the interior of the tectonic plate is not generally disturbed by the fault movement. Geological references indicate that not much damage will occur from earthquakes of magnitude of less than 6 for sites over eight miles from the fault. Since maximum probable earthquake for the general San Diego area is in the order of 6 and since the site is further than 8 miles from the major active faults we can infer that the shaking problem will not be to great. Current seismic design criteria as imposed by the U.B.C. should be satisfactory to compensate for seismic action. There are no existing landslides, faults or other natural disturbances noted on the site or in the immediate area as determined from air photos, and field inspections. This does not preclude the possibility of sliding by poor grading or other man made practic es. This site can be considered relatively free of geologic hazards. CONCLUSIONS & RECOMENDATIONS The site may he utilized for the proposed development provided the recommendations of this report are carefully followed. The undisturbed native soils, properly compacted native soils or combinations thereof, should adequately support the proposed structures without detrimental settlement. The site shall be cleared of all vegatation prior to any grading or placement of embankment. Any uncompacted native soils shall be compacted to a minimum of 90% relative compaction. The Soil Engineer shall determine when the native soils are ready for placement of embankment. Adverse soil conditions also exist in the form of minor quanti- ties of uncompacted fill and loose topsoils. These materials will require complete removal and recompaction. Retaining walls, which are not fixed at the top and have a level backfill should be designed for an active soil pressure equivalent to a fluid pressure of 60 pcf. Where the backfill will be sloping 2:1, an active soil pressure of 65 pcf is reco- mmended. These v,alues assume a drained backfill condition. Wall drainage details should be provided by the project archi- tect. All cut slopes shall be 2:l or flatter and all fill slopes shall not exceed 2:l slope ratio. When retaining walls are restrained at the top an active soil pressure of 85 pcf shal.1 he used for design of the wall. A passive soil pressure value of 160 psf shall be used. A coefficient of friction of 0.45 may be assumed for resistance to sliding between concrete and soil. The anticipated total and/or differential settlements for the proposed structures may be considered to he within tolerable limits provided the recommendations presented in this report are followed. The recommendations presented in this report are contingent on review of final plans and specifications. It is recommended that H.L. Land Development, Engineering and Surveying, be re- tained to provide continuous soil engineering services during the earthwork operations. This is to observe compliance with the design concepts, specifications, and recommendations and to allow design changes in the event subsurface conditions differ from those anticipated prior to start of construction. All existing topsoils shall be removed to a depth of 2.0 feet below existing ground level. This material shall then be recom- pacted into a dense embankment of at least 90% relative compac- tion, with a moisture content within 2.0% of optimum. Minimum foundation shall be 15 inches wide and founded 18 inches below grade and shall have two #4 bars of reinforcing steel. One bar shall be placed 4 inches below the top of foundation and the other bar shall be placed 4 inches from the bottom of foundation. These recommendations are based upon soil charac- teristics only and do not reflect any possible design considera- tions imposed by building design which might increase these recommendations. The proposed foundations may be designed util- izing an allowable bearing pressure of 1300 lb/sf. This value may be increased by 1/3 for the design of loads that include wind and seismic analysis. Slab on grade shall be reinforced with 6x6xlOga wire mesh over visqueen vapor barrier and 4 inches Of clean sand. These recornfiendations are made based upon the assumption that the expansive soils found on the site will be used for foundation embankment. It should be noted that additional or final foundation recommen dations will be made and contained in the Final As Graded Report when a more complete evaluation of actual soil that the foundat ions will be founded in can be made. The above minimum foundations requirements are bssed upon a single story structure and the foundation depth would need to be increased to 24 inches for a two story structure. An alternate solution to elj.minate .the need for expansive design foundations kould be to utilize a good quality non-expansive imported material over the top 3.0 feet of the building pad. All embankment shal.1 be co~~~pac~etl to a nIiniluum of 90% relative compacti.on and all existing ground wi.thin 5.0 feet laterally and to a depth of 1.0 foot below the bottom of foundations shall be compacted to at ].east 908 relative compaction. The cut portion of the pad site shall be sub-excavated to a depth of 2.0 feet below finish pad grade and the material watered to optimum and recompacted to provide a uniform finished pad. The recommendations and opinions expressed in this report re- flect our best estimate .OF the project requirements based on an evaluation of the subsurface soil conditions encountered at the subsurface exploration locations and the assumption that the soil conditi~ons do not deviate appreciably from those encountered . It should be recognized that the performance of the foundations may be influenced hy undisclosed or unforseen variations in the sojl conditions .tl~at may occur in the interme diate and unexplored areas. Any un~~sual conditions no't covered in this report that may be encountered during site development should be brought 'to the attentio~~ of the Soils Engineer SO that he may make modifications jf necessary. ,. It is the responsib.ility of the Ilcveloper or their representa- tive to ensure that the information and recommendations con- tained herein are brought to the attention of the Engineer and Architect for the project and incorporated into the project plans and specifications. It is further their responsibility to take the necessary measllres to insure that the contractor and his subcontractors carry out sucl~ recommendations during construction. . All grading shall be monitored by a qualified Soils Engineer to insure compliance with these recommendations and all applicable City of Carlsbad ordinances. Recommended grading specifications are made part of the report and attached as Exhibit "A". Respectfully submitted, RCE 33220 . 1535 S. Escondido Hlvd. Escondido. CA 92025 (619) 741-0533 LAND DEVELOPMENT Surveying. Pcrc Tests . Lo1 Splits Grading Plans dr Compaction Tesls RECOMMENDED GRADING SPECIFICATIONS Prepared for: Zoltan Kasa 2926 Via Pepita Carlsbad, CA. 92008 for the Of: A single family residende 7336 El Fuerte Street Carlsbad, CA. 92008 Herald R. Lantis hereinafter described as the Soil Engineer, should be consulted prior to any site work connected with site development to ensure compliance with these specifications. These specifications they are attached and part thereto. shall only be used in conjunction with the soil report of which 1.1 This item shall consist of all clearing and grubbing, prepar- ation of land to be filled, filling of the land, spreading, compact- ing, and control of the fill and all subsidiary work necessary to complete the grading of the filled areas to conform with the lines, grades and slopes as shown on the accepted plans. tions. In the event that any unusual conditions not covered by the special provisions are.encountered during the grading operations, the Soil Engineer shall be contacted for further information. 2.0 Tests work shall be the 'ASTM Test Procedure D1557-70. All densities shall The standard test used to define maximum density of all compaction be expressed as a relative compaction in terms of the maximum density obtained in the laboratory by the foregoing procedure. 3.0 Any trees not utilized in landscaping, structures, weeds and other Clearing, Grubbing and Preparing Areas to be Filled. rubbish shall be removed, piled or otherwise disposed of so as to leave the areas that have been disturbed with a neat and finished appearance, free from unsightly debris. 1.2 The Soil Engineer shall test and observe all grading opera- . 3.2 Any septic tanks, if encountered, and debris must be removed from the site prior to any building, grading or fill operations. Septic tanks, including all con- nectimg drain fields and other lines, must be totally removed and the resulting depressions properly reconstruct- ed and filled to the complete satisfaction of the super- vising soil engineer. 3.3 All water wells on the site shall be capped according to the requirements of the San Diego County Health Depart- ment. The strength of the cap shall 'be at least equal to the adjacent soil. The final elevation of the top of the adjacent grade prior to any grading of fill oper- ations. 3.4 All buried tanks, if encountered, must be totally removed and the resulting depressions properly reconstructed and filled to the complete satisfaction of the supervising soil engineer. 3.5 All vegetable matter and soil designated as unsuitable by the soil engineer shall be removed under the direction of the direction of the soil engineer. The then exposed surface shall then be plowed or sacrified to a depth of at least ,eight inches and until the surface is free from ruts, hummocks, or other uneven features which would tend to prevent uniform compaction by the equipment to be used. 3.6 The original ground upon which the fill is to be placed shall be plowed or sacrified deeply, and where the slope ratio of the original ground is steeper than 6.0 horizon- tal to 1.0 vertical, the band shall be stepped or benched. At the toes of the major fills and on the sideslope fills, the base key shall be, as described elsewhere in this report, at least 10 feet in width, cut at least three feet into the undisturbed or native soil, and sloped back into the hillside at a gradient of not less than . . .. two percent. Subsequent keys should be cut into the hillside as the fill is brought up the slope. The con- struction of subsequent keying operations shall be deter- mined- by the soil engineer during grading operations. Ground slopes which are flatter than 6 to 1 shall be bench- ed when considered necessary by the soil engineer. 3.7 After the foundation for the fill has been cleared, plowed, or scarified it shall be disced or bladed until it is uniform and. .free from large clods; brought to .the proper moisture content by adding water or aerating; and recom- pacted to a relative compaction of not less than 90 percent. 4. Materials 4.2 5. 5.1 5.2 5.3 5.4 5.5 Native soil, free of organic material and undesirable de- leterious material, may be used as fill. Native soil which building pads without the approval of the soil engineer. is expansive, shall not be placed on the top two feet of The materials for fill shall be approved by the soil engin- eer before commencement of grading operations. Any import- ed material must be approved for use before being brought to the site. The materials used shall be free from veget- able matter and other deleterious material, and be nonex- pansive. Expansive soil is defined as soil which expands more than three percent when saturated at 90 percent relat- charge of 150 psf. ive compaction and optimum moisture content under a sur- Placing, Spreading, and Compaction Fill Material The selected fill material shall be placed in layers which, Each layer shall be spread evenly and shall be thoroughly when compacted. shall allow adequate bonding and compaction. material in each layer. blade mixed during the spreading to ensure uniformity of When the moisture content of the fill material is below that specified by the soil engineer, water shall be added until the moisture content is as specified to assure thor- ough bonding during the compaction process. When the moist- by the soil engineer, the fill material shall be aerated ure content of the fill material is above that specified by blading or other satisfactory methods until the moist- ure content is as specified. After each layer has been placed, mixed and spread even- ly, it shall be thoroughly compacted to a relative compac- tion of not less than 90 percent. When fill material includes rock, no rock will be allowed to nest, and all voids must be carefully filled with small stones or earth and properly compacted. No rocks larger than six inches in diameter will be permitted in the fill. Compaction shall be by sheepsfoot rollers, multiple wheel pneumatic-tired rollers, or other types of acceptable com- pacting rollers. Rollers shall be of such design that they will be able to compact the fill to the specified moisture content range. Rolling of each layer shall be continuous over it's entire area and until the required density has been obtained. 5.6 5.1 5.8 5.9 6. 6.1 6.2 6.3 6.4 Field density tests shall be made by the soil engineer where sheepsfoot rollers are used, the soil may be dis- be taken in compacted material below the disturbed sur- turbed to a depth of several inches. Density tests shall face. any-layer of fill or portion thereof is below the required When these tests indicate that the density of 90 percent relative compaction, the particular layer or portion shall be reworked until the required density has been obtained. as specified above, until the fill has been brought to The fill operation shall be continued in compacted layers the finished slopes and graded as shown on the accepted plans. Fill slopes shall be compacted by means of sheepsfoot rollers or other suitable equipment. Compaction oper- ations shall be continued until the slopes are stable. All earthmoving and working operations shall be controlled to prevent water from running into excavated areas. All water shall be promptly removed and the site kept dry. Disposal of Oversize Rock Oversize rock shall be either exported from the site, used for landscaping purposes, oc placed in designated non-structural fill areas. Prior to grading, the soil engineer shall be consulted to approve any proposed nonstructural fill area, In General, nonstructural fill areas will be acceptable structures: e.g. rear yard fills at least 15 feet from if located outside of the"zone of influence" of proposed posed utility lines).. structures and beneath street areas (deeper than pro- Oversize ' rock shall not exceed four feet in greatest dimension, shall be placed in lifts not exceeding four feet in thickness, and shall be placed in a manner that will not produce "nesting" of the rock. The voids between the rocks shall be completely filled with fine granular material. NO oversize rock shall be placed within four feet of finish lot grade nor within the anticipated depth of utility lines, whichever is deeper. - 7. Engineering Observation during the fill and compaction operations so that he Field observations by the soil engineer shall be made can express his opinion regarding the conformance of the ^grading with the accepted specifications. 0. Seasonal Limits No.fil1 material shall be placed, spread, or rolled while unfavorable weather conditions. When the work is inter-' it is at an unsuitable high moisture 'content, nor during rupted by heavy rain, fill operations shall not be resumed' until field tests by the soil engineer indicate that the moisture content and density of fill are as previous- ly specified .