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HomeMy WebLinkAboutCDP 04-11; CASA DI MARE; INTERIM REPORT OF SITE CONDITIONS AND PRELIMINARY RECOMMENDATIONS; 2003-03-17I I I I I I I I I I I I I I I I I I I GEOTECHNICAL EXPLORATION, INC. SOIL & FOUNDATION ENGINEERING • GROUNDWATER HAZARDOUS MATERIALS MANAGEMENT • ENGINEERING GEOLOGY 17 March 2003 Mr. Ted Viola 6817 Adolphia Drive Carlsbad, CA 92009 Job No. 04-8622 Subject: Interim Report of Site Conditions and Preliminary Recommendations Casa Di Mare Remodel and Additions 5019 Tierra Del Oro Street Carlsbad, California Dear Mr. Viola: In accordance with your request, Geotechnical Exploration, Inc. is in the process of investigating soil conditions at the residential lot on Tierra Del Oro Street in Carlsbad, California. Our work to date has included review of preliminary building plans, observation of the site and surrounding vicinity, subsurface soil testing (5 hand-auger borings) in the proposed new addition areas and improvement locations, and laboratory soil testing. We found the proposed new addition building site to be underlain at depth by medium dense to dense formational materials and slopewashjcolluvium with approximately 2 to 3 feet of loose to medium dense fill soils that will not provide adequate bearing strength for the proposed new additions and improvements. In addition, up to 6 feet of backfill soil exists adjacent to the lower-level basement walls. As suchr we recommend that the fill soils either be removed and recompacted as part of site preparation for the main level improvements or that the additions and any new structural loads be supported on a system of caissons founded in the underlying dense formational (terrace deposits) soils. It is our understanding that a new two-story addition will be added on the east side of the existing residence, including a new garage and upper level bedrooms. In addition, 7420 TRADE STREET • SAN DIEGO, CA 92121 • (858) 549-7222 • FAX: (858) 549-1604 • E-MAIL: geotech@ixpres.com I I I I I I I I I I I I I :1 I I I I I Casa Di Mare Remodel and Additions Carlsbad, California Job No. 04-8622 Page 2 a second-story addition will be built over the existing building footprint that will be supported by new caissons CONCLUSIONS AND PRELIMINARY RECOMMENDATIONS The following preliminary conclusions and preliminary recommendations are based upon the practical field investigation conducted by our firm, and resulting laboratory tests, in conjunction with our knowledge and experience with the soils in this area of the City of Carlsbad. Additional or modified recommendations will be included in our final geotechnical report. A. Preparation of Soils for Site Development 1. The existing concrete patio, foundations, debris and vegetation on the building site must be removed prior to the preparation of the areas to receive new structural improvements and be properly disposed of. 2. If the site is to be re-graded to provide a more uniform, firm soil base for the proposed new additions and improvements on the main level, the existing fill soils on the building pad shall be completely removed to a depth of at least 2 to 3 feet, and as per the indications of our field technician based on field observations. The excavated soils shall be cleaned of any debris and deleterious materials and watered to approximately optimum moisture content. Deeper removal and recompaction (up to 6 feet) may be required adjacent to the basement wall in the front courtyard area. 3. All fill shall be compacted to at least 90 percent of the maximum dry density determined per ASTM D1557-98, and the soil moisture content shall be at least 3 percent over the optimum. Should any expansive soils be I I I I I I I I I I I I I I I I I I I Casa Di Mare Remodel and Additions Carlsbad, California Job No. 04-8622 Page 3 encountered they shall be compacted to a moisture content equal to at least 5 percent over the optimum. 4. The allowable soil bearing capacity of the proposed recompacted fill in the north addition is 2,500 psf for foundations embedded at least 18 inches into the ground, measured from the lowest adjacent grade. The passive resistance is 300 pcf. The friction coefficient is 004. The anticipated maximum settlement of properly compacted fill up to 10 feet in thickness is on the order of 1 inch. Differential angular rotation should be less than 1/240. 5. All foundation excavations and grading excavations shall be observed and evaluated by a representative of our firm before they are backfilled with soil or concrete. B. Design Parameters for Caissons All new loads for the proposed second-story addition are to be supported by a caisson foundation system and the following recommendations should be followed. We encountered between 2 to 6 feet of loose to medium dense fill soil over medium dense to dense terrace deposits in the area of the proposed second-story addition. 6. Where caissons are utilized, they shall be designed by the project Civil/Structural Engineer to support all vertical and lateral loads of the proposed addition. 7. For vertical loading, all end bearing caissons should be embedded at least 5 feet into dense natural materials, to be approved upon observation by a representative of this firm. Dense natural soils are typically encountered between 2 and 6 feet from the surface. For lateral loading deSign, the I I I I I I I I I I I I I I I I I I I Casa Oi Mare Remodel and Additions Carlsbad, California Job No. 04-8622 Page 4 caisson length may require additional embedment. It is important, when drilling excavations for caissons that utilize end-bearing strength, to limit the amount of loose material on the bottom of the excavation. Therefore, we recommend that caissons be designed with a minimum diameter of 24 inches in order to facilitate observation of the excavations and allow for easy hand- tool removal of material on the bottom. For end-bearing capacity caissons, no slough over 1 inch in thickness shall remain at the bottom of the excavation before concrete placement. The drilling contractor shall provide an appropriate cleaning tool to satiSfy this requirement. Otherwise, shoring installation and hand-tool cleaning (or another acceptable option) will be required. Frictional caissons may start getting positive vertical frictional resistance starting at an approximate depth equal to -5 feet from the surface. The average allowable frictional resistance per shaft square foot is 300 psf from - 5 ft. to -15 feet in depth; and 500 psf from -15 to -25 feet in depth. Wet caisson weight to be deducted is 1/3 of their weight in the soil embedded part. The part of caisson above ground needs to be considered in its full weight. 8. The minimum center-to-center spacing of caissons, in a direction perpendicular to the temporary seismic or with a lateral load, shall be 3 caisson diameters. For caissons located in the same line of the applied lateral load, the shadow effect produces a reducing effect in their combined individual lateral load capacity. For 24-inch diameter caissons spaced 10 feet center-to-center, the reduction factor shall be equal to 2.2, for the sum of the individual capacities. If the spacing is 12 feet center-to-center, the reduction factor is 1.8. The reduction factor for caissons spaced at least 16 feet apart is 1. I I I I I I I I I I I I I I I I I I I Cas a Di Mare Remodel and Additions Carlsbad, California Job No. 04-8622 Page 5 9. The allowable end-bearing capacity is 6,000 psf for caissons penetrating at least 5 feet into dense formational soils (terrace deposits). This end-bearing capacity has already deducted the down drag force produced by existing fills. The caisson weight to be considered is only one-third of the actual weight of buried caisson. The actual needed caisson length and embedment into formational soils shall be established by the structural engineer based on the length needed to adequately support the total vertical and lateral loads included in the design. 10. For lateral earthquake or wind load resistance, the structural engineer may use any method that conSiders the equilibrium of forces and moments. Some structural engineers like to use the fixity concept. Based on a free- head caisson with diameter equal to 24 inches, the concrete modulus of elasticity, and a horizontal sub-grade reaction of the loose fills, we recommend that fixity depth to be considered in the calculation be not less than 8 feet from the soil surface. The maximum moment produced by the lateral load may be calculated by multiplying the lateral load times one-half the total distance between the point of application of lateral load at top of caisson load application above the ground plus the buried depth to fixity depth. If a balance of forces is calculated based on the applied lateral forces and reaction soil forces, the following allowable passive (equivalent fluid) forces are recommended: for existing loose fill soils 130 pcf and for formational soils 300 pcf. The passive resistance shall be measured from a depth of caissons at least 3 feet from the surface. The passive resistance of the caissons may be considered applicable on a projected surface equal to 2.5 times the diameter of the caisson multiplied by the vertical length being considered. I I I I I I I I I I I I I I I I I I I Cas a Di Mare Remodel and Additions Carlsbad, California Job No. 04-8622 Page 6 11. Another option chosen by some engineers is the use of the pole equation presented in USC Section 1806.8.2. to calculate minimum depth of embedment of the caissons due to lateral loads. The maximum lateral bearing of fill soil is 1,300 psf; 3,600 psf for formational soils. 12. The recommended allowable end bearing vertical capacity for the caissons already includes the effect of negative friction produced by the existing fills as well as the buried caisson weight. Any caisson weight above the soil surface shall be considered as dead load and shall be deducted from the net end bearing capacity. The depth to medium dense to dense formational soils in the explored area was encountered to range from approximately 2 to 6 feet from the existing split-level pad surface. 13. Caisson drilling operations shall be performed under the continued observations of a representative of our firm to confirm the penetration into formational soils. 14. The design and construction of the caissons shall be in accordance with the recommendations presented above, the current USC reqUirements accepted by the City of Carlsbad, and also in accordance with ACI 336, 3R-93 Design and Construction of Drilled Piers, of the American Concrete Institute. The contractor shall follow all the safety procedures required by Cal OSHA. 15. It is also our recommendation that the caissons excavations be filled with concrete within 2 days after the excavations are completed, to help reduce the risk of soil caving, mud or slough intrusion, etc. We will keep you advised as to the completion of our final report. We appreciate this opportunity to be of service. Should you have any questions, please feel free to contact our office. Reference to our Job No. 04-8622 will help to expedite a response to your inquiries. I I I I I I I I I I I I I I I I I I I Casa Di Mare Remodel and Additions Carlsbad, California Respectfully submitted, GEOTECHNICAL EXPLORATION, INC. J iser, Project Coordinator Senior Project Geologist Jaime A. Cerrosl P.E. Job No. 04-8622 Page 7 R.C.E. 34422/G.E. 2007 Senior Geotechnical Engineer . -