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HomeMy WebLinkAboutDEV 2017-0089; ST. JOHN GRADING PLAN; LIMITED SOIL INVESTIGATION AND BEARING VALUE EVALUATION; 2016-12-21Gplkll Geotechnical Exploration, Inc. SOIL AND FOUNDATION ENGINEERING I GROUNDWATER• ENGINEERING GEOLOGY 21 December 2016 Susie St. John Job No. 16-11316 3657 Monroe Street Carlsbad, CA 92008 Subject: Limited Soil Investigation and Bearing Value Evaluation St. John Residence 3657 Monroe Street Carlsbad, California Dear Ms. St. John: In accordance with your request, a representative of Geotechnical Exploration, Inc. has visited the subject site and performed an evaluation of the soil conditions in the area of the proposed new residence. It is our understanding that the proposed single-story, single-family residence, will utilize continuous footings and slab On grade. As part of our investigation, we observed and evaluated the shallow soil conditions at two locations within the proposed new building area. In addition, we reviewed the building plans by Deskin Design and structural plans by Steven D. Reed, in accordance with the requirements of the City of Carlsbad Development Services. It is our opinion that the plans were prepared in accordance with the recommendations included in this report. RECEIVED MAY 04 2017 LAND DEVELOPMENT ENGINEERING 7420 TRADE SIREET• SAN DIEGO, CA. 921210 (858) 549-72221 FAX: (858) 549-16041 EMAIL: geotedi@gei-sd.com St. John Residence Job No. 16-11316 Carlsbad, California Page 2 The field work, conducted on December 1, 2016, consisted of logging two hand- excavated test pits in the location of the proposed new residence. The excavations revealed that the building site is underlain by approximately 1 foot of loose to medium dense, silty sand topsoil over medium dense to dense, silty sand formational materials. The on-site soils are considered to have a very low expansion potential with an Expansion Index of less than 20. Based upon our observation, probing of the on-site soils, it is our opinion that the new foundations for the residence can be founded directly into the existing formational materials. The loose surface soils in the proposed building pad area should be properly compacted as part of site preparation under any new slab areas. Any new fill, such as the existing crawl space area should be compacted to at least 90 percent of Maximum Dry Density. The Maximum Dry Density of the soil has been determined per ASTM D1557-12. We understand that the proposed new slab elevation will remain the same as the current wood floor elevation. No other grading is proposed for the new development. 1. It is our opinion that the existing medium dense to dense formational materials will provide adequate bearing strength for the proposed new addition foundations. New footings placed in the existing medium dense to dense formational soils can be designed for an allowable soil bearing capacity of 2,500 pounds per square foot (psf). We do recommend that the proposed footings and slabs contain at least a nominal amount of reinforcing steel to reduce the separation of cracks should they occur. The allowable soil bearing capacity may be increased one-third for structural design including seismic or wind loads. St. John Residence Job No. 16-11316 Carlsbad, California Page 3 The proposed footings should have a minimum depth of 18 inches and a width of at least 12 inches, founded in the medium dense to dense formational material. A minimum of steel for continuous footings should include at least two No. 4 bars continuous, with two bars 3 inches from the bottom of the footing. Site-specific seismic design criteria to calculate the base shear needed for the design of the residential addition are presented in the following table. The design criteria was obtained -from the California Building Code (CBC) 2013 edition, and is based on the distance to the closest active fault and soil profile classification. The proposed structure should be designed in accordance with Section 1613 of the 2013 CBC, which incorporates by reference the ASCE 7-10 for seismic design and the following parameters should be utilized. We have determined the mapped spectral acceleration values for the site based on a latitude of 33.1607 degrees and longitude of 117.3262 degrees, utilizing a program titled "Design Maps and Tools," provided by the USGS, which provides a solution for ASCE 7-10 (Section 1613 of the 2013 CBC) utilizing digitized files for the Spectral Acceleration maps. In addition, we have assigned a Site Classification of D. The response parameters for design are presented in the following table. The design spectrum acceleration vs. Period T is attached. St. John Residence Job No. 16-11316 Carlsbad, California Page 4 TABLE I MaDped Snectral Acceleration Values and Design Parameters SS S1 I Fa - IFy ISMS I Smi I Sds Sd1 1.118 1 0.430 11.053 11.57 11.177 10.675 10.785 0.450 The liquefaction of saturated sands during earthquakes can be a major cause of damage to buildings. Liquefaction is the process by which soils are transformed into a viscous fluid that will flow as a liquid when unconfined. It occurs primarily in loose, saturated sands and silts when they are sufficiently shaken by an earthquake. On this site, the risk of liquefaction of foundation materials due to seismic shaking is considered to be remote due to the relatively shallow, medium dense to dense nature of the natural-ground material and the lack of a shallow static groundwater surface under the site. No soil liquefaction or soil strength loss is anticipated to occur due to a seismic event. Any new concrete slabs on-grade (on properly compacted fill or dense formational soils) should be a minimum of 4 inches actual thickness and be reinforced with at least No. 3 steel bars on 18-inch centers, in both directions, placed at mid-height in the slab. The interior slab should be underlain by a 15-mil vapor barrier (15-mil StegoWrap) placed directly on properly compacted subgrade. The sand base may be waived. We recommend that isolation joints and sawcuts be incorporated to at least one-fourth the thickness of the slab in any slab designs. The joints and cuts, if properly placed, should reduce the potential for and help control floor slab cracking. In no case, however, should control joints be spaced farther than 20 feet apart, or the width of the slab. Control joints should be placed within S. 5-1 St. John Residence Job No. 16-11316 Carlsbad, California Page 5 12 hours after concrete placement as soon as concrete sets and no raveling of aggregate occurs. Slabs spanning any existing loose soils and supported by perimeter deepened foundations should be designed as structural slabs. Although no retaining walls are planned, the active earth pressure (to be utilized in the design of cantilever, non-restrained walls) should be based on an Equivalent Fluid Weight of 38 pounds per cubic foot (for level backfill only) if on-site soils are used. Additional loads applied within the potential failure block should be added to the active soil earth pressure by multiplying the vertical surcharge load by a 0.31 lateral earth pressure coefficient. For restrained wall conditions, we recommend an equivalent fluid weight of 56 pcf. Surcharge loads may be converted to lateral pressures by multiplying by a factor of 0.47. Should seismic soil increment be required, the unrestrained walls with level backfill should be designed for a triangular pressure of 14 pcf, in addition to the regular static loading, with zero pressure at the top and the maximum pressure at the bottom of the wall. The passive earth pressure of the encountered formational soils to be used for design of shallow foundations and footings to resist the lateral forces, should be based on an Equivalent Fluid Weight of 275 pcf. This passive earth pressure is valid for design only if the ground adjacent to the foundation structure is essentially level for a distance of at least three times the total depth of the foundation and is properly compacted or dense natural soil. An allowable Coefficient of Friction of 0.40 times the dead load may be used between the bearing soils and concrete foundations, walls or floor slabs. St. John Residence Job No. 16-11316 Carlsbad, California Page 6 Adequate measures should be taken to properly finish-grade the site after the new structure and other improvements are in place. Drainage waters from this site and adjacent properties should be directed away from perimeter foundations, floor slabs, footings and slope tops, and onto the natural drainage direction for this area or into properly designed and approved drainage facilities. Proper subsurface and surface drainage will help minimize the potential for waters to seek the level of the bearing soils under the foundations, footings, and floor slabs. Failure to observe this recommendation could result in undermining, differential settlement of the building foundation or other improvements on the site, or moisture-related problems. It is not within the scope of our services to provide quality control oversight for surface or subsurface drainage construction or retaining wall sealing and base of wall drain construction. It is the responsibility of the contractor and/or their retained construction inspection service provider to provide proper surface and subsurface drainage. Due to the possible build-up of groundwater (derived primarily from rainfall and irrigation), excess moisture is a common problem in below-grade structures or behind retaining walls that may be planned. These problems are generally in the form of water seepage through walls, mineral staining, mildew growth and high humidity. In order to minimize the potential for moisture-related problems to develop, proper cross ventilation and water- proofing must be provided for below-ground areas, in crawl spaces, and the backfill side of all structure retaining walls must be adequately waterproofed and drained. St. John Residence Job No. 16-11316 Carlsbad, California Page 7 Proper subdrains and free-draining backwall material (such as gravel or geocomposite drains such as Miradrain 6000 or equivalent) should be installed behind all retaining walls on the subject project in addition to wall waterproofing. Geotechnical Exploration, Inc. will assume no liability for damage to structures that is attributable to poor drainage. 11. Planter areas and planter boxes should be, sloped to drain away from the foundations, footings, and floor slabs. Planter boxes should be constructed with a closed bottom and a subsurface drain, installed in gravel, with the direction of subsurface and surface flow, away from the foundations, footings, and floor slabs, to an adequate drainage facility. The finish grade around the addition should drain away from the perimeter walls to help reduce or prevent water accumulation. Exterior slabs or rigid improvements should also be built on properly compacted soils and be. provided with concrete shrinkage reinforcement and adequately spaced joints. Geotechnical Exploration, Inc. recommends that we be asked to verify the actual soil conditions revealed in footing excavations prior to form and steel reinforcement placement. In addition, any new fills or loose soils should be properly compacted under the observations and testing of our firm. St. John Residence Job No. 16-11316 Carlsbad, California Page 8 Should you have any questions regarding this matter, please contact our office. Reference to our Job No. 16-11316 will help to expedite a response to your inquiries. Respectfully submitted, GEOTECHNICAL EXPLORATION, INC. AveiAx---- i4R1eiser Senior Project Geologist A~~ - R.C.E. 34422/G.E. 2007 Senior Geotechnical Engineer cc Exp -7 OF C VICINITY MAP St. John Residence 3657 Monroe Street Carlsbad, CA. Figure No. I Job No. 16-11316 IJSGS Design Maps Summary Report User-Specified Input Report Title 3657 Monroe Street, Carlsbad, CA Mon December 19, 2016 17:36:04 UTC Building Code Reference Document ASCE 7-10 Standard (which utilizes USGS hazard data, available in 2008) Site Coordinates 33.16070 N, 117.3262°W Site Soil Classification Site Class D - "Stiff Soil" Risk Category I/H/Ill -I wt I - .: C. ell '-.. 4.. '0ei&IcIe Muni Alvpnt - -. 'Vista \.' '• Oteansid Il .r• .: Carlsbad ;,.. 5n Marcos M,.Ckin PIornr Anpor(j.. Escondido' USGS-Provided Output S5 = 1.118g SMS = 1.177g SDS = 0.785 g S1 = 0.430 g SM1 = 0.675g S01 = 0.450 g For information on how the SS and Si values above have been calculated from probabilistic (risk-targeted) and deterministic ground motions in the direction of maximum horizontal response, please return to the application and select the "2009 NEHRP" building code reference document. Design Response Spectrum MCE, Response Spectrum 1.20 1.09 0.94 2 0.72-- 0.60. 0. 49 0.36-- 0.24 0.12 0.00 I 0.00 0.20 0.40 0.60 0.20 1.00 1.20 1.40 1.60 1.20 2.00 Period, T (sec) 0.88 0.90-- 0.72 -- 0.64 -- 0.56 -- 0.48-- U 0.40 0.32 - 0.24 F- OAG 0.08-- 0.00 0.00 0.20 0.40 0.GO 0.20 1.00 1.20 1.40 1.60 1.20 2.00 Period, T (sec) For PGA., TL, CRS, and C51 values, please view the detailed report. Although this information is a product of the U.S. Geological Survey, we provide no warranty, expressed or implied, as to the accuracy of the data contained therein. This tool is not a substitute for technical subject-matter knowledge.