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HomeMy WebLinkAboutPD 2018-0008; CHESTNUT AVENUE RESIDENCE; PRELIMINARY GEOTECHNICAL REPORT; 2018-02-08P Q1D Xo'fD© Inc. SOIL AND FOUNDATION ENGINEERING 0 GROUNDWATER 0 ENGINEERING GEOLOGY 08 February 2018 Mr. Tom Scott Job No 17-11714 3320 Highland Drive Carlsbad, CA 92008 Subject: Preliminary Geotechnical Investigation Scott Residential Lot 1161 Chestnut Avenue Carlsbad, California Dear Mr. Scott: 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 lot is being developed to receive a new two-story, single-family residence and associated improvements which will utilize continuous footings and slab on grade. As part of our investigation, we observed and evaluated the shallow soil conditions at three locations within the proposed new building area. In addition, we reviewed an architectural site plan and the as-built grading plan by RCE, Inc., dated November 15, 2005. We should review the final plans to confirm they have been prepared in accordance with the recommendations included in this report. APR 0 5 2018 LAND DEVELOPMENT ENGINEERING 7420 TRADE STREETO SAN DIEGO, CA. 921210 (858) 549-72220 FAX: (858) 549-1604 0 EMAIL: geotech©gei-sd.com =9; Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 2 The field work, conducted on November 21, 2017, consisted of logging three hand- excavated test pits/hand auger borings in the location of the proposed new residence. The excavations revealed that the building site is underlain by approximately 3 to 6 feet of medium dense, silty sand fill soil 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 in the existing medium dense fill soil. Any loose fill soils in the proposed building pad area should be removed and properly compacted as part of site preparation for the new foundation and slab areas. All fill 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. It is our opinion that the medium dense fill soil will provide adequate bearing strength for the proposed new foundations. New footings placed in the existing medium dense fill or dense formational soils can be designed for an allowable soil bearing capacity of 1,500 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. The proposed footings should have a minimum depth of 18 inches and a width of at least 15 inches, founded in the medium dense fill soil. A minimum of steel for continuous footings should include at least four No. 5 bars continuous, with two bars 3 inches from the bottom of the footing. Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 3 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) 2016 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 2016 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 33.1583 degrees and longitude of 117.3364 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 2016 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. TABLE I Mapped Spectral Acceleration Values and Design Parameters Si I Fa I F, I Smg J S11 I Sds Sd1 1.136g I 0.436g 1 1.045 1 1.564 1.188g 0.682g I 0.792g 0.455g 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. Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 4 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. New concrete slabs on-grade (on existing fill soils) should be a minimum of 5 inches actual thickness and be reinforced with at least No. 4 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 12 hours after concrete placement as soon as concrete sets and no raveling of aggregate occurs. If any 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. Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 5 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 an additional 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 fill soil to be used for design of shallow foundations and footings to resist the lateral forces, should be based on an Equivalent Fluid Weight of 200 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 0.35 times the dead load may be used between the bearing soils and concrete foundations, walls or floor slabs. 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. Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 6 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. 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. 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 Scott Residential Lot Job No. 17-11714 Carlsbad, California Page 7 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. Should you have any questions regarding this matter, please contact our office. Reference to our Job No. 17-11714 will help to expedite a response to your inquiries. Respectfully submitted, ECHNICAL EXPLORATION, INC. -K Heiser nior Project Geologist A;~ - Jaime A. Ce rros, P. E. R.C.E. 34422/G.E. 2007 Senior Geotechnical Engineer 3Ii No. -)02007 iu; Exp573O/!7 I LJSGS Design Maps Summary Report User—Specified Input Report Title 1161 Chestnut Avenue, Carlsbad, CA Thu February 8, 2018 18:09:57 UTC Building Code Reference Document ASCE 7-10 Standard (which utilizes USGS hazard data available in 2008) Site Coordinates 33,15830N, 117.3364°W Site Soil Classification Site Class D "Stiff Soil" Risk Category 1/11/111 1_ .r - C O&eanidt' cat -i Carlsbad* iln I Marcos -I f EconcIuclo' USGS—Provided Output = 1.136 g S = 1.188 g 0.792 g = 0.436 g SM1 = 0.682 g S,, = 0.455 g For information on how the US 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" budding code reference document. 'U_c L-wir Souct- 1Jct Sjtjri For PGA,,, I, C, and C51 values, please view the detailed repo. Altncjah this rtormation is ti product ci the U.S. Gecioutcel Survey, iso prosio'o no war aOtf, expressed or imphed, as to th accuracy of the data contained therein. This tool is oct a substitute for tech -acal subject-matter knowledge. HUM Design Maps Detailed Report ASCE 7-10 Standard (33.15830N, 117.3364°W) Site Class D - "Stiff Soil", Risk Category 1/11/111 Section 11.4.1 - Mapped Acceleration Parameters Note: Ground motion values provided below are for the direction of maximum horizontal spectral response acceleration. They have been converted from corresponding geometric mean ground motions computed by the USGS by applying factors of 1.1 (to obtain S) and 1.3 (to obtain S1). Maps in the 2010 ASCE-7 Standard are provided for Site Class B. Adjustments for other Site Classes are made, as needed, in Section 11.4.3. From figure 22-1113 Ss = 1.136 9 From Figure 22-2 (2] S1 = 0.436 g Section 11.4.2 - Site Class The authority having jurisdiction (not the USGS), site-specific geotechnical data, and/or the default has classified the site as Site Class D, based on the site soil properties in accordance with Chapter 20. Table 20.3-1 Site Classification Site Class V5 N or NCh Hard Rock >5,000 ft/s N/A N/A Rock 2,500 to 5,000 ft/s N/A N/A Very dense soil and soft rock 1,200 to 2,500 ft/s >50 >2,000 psf Stiff Soil 600 to 1,200 ft/s 15 to 50 1,000 to 2,000 psf Soft clay soil <600 ft/s <15 <1,000 psf Any profile with more than 10 ft of soil having the characteristics: Plasticity index P1 > 20, Moisture content w ~! 40%, and Undrained shear strength s < 500 psf F. Soils requiring site response See Section 20.3.1 analysis in accordance with Section 21.1 For SI: lft/s = 0.3048 m/s 1lb/ft2 = 0.0479 kN/m2 Section 11.4.3 - Site Coefficients and Risk-Targeted Maximum Considered Earthquake Spectral Response Acceleration Parameters Table 11.4-1: Site Coefficient Fa Site Class Mapped MCE R Spectral Response Acceleration Parameter at Short Period S < 0.25 S5 = 050 S5 = 0.75 S = 1.00 S5 ~t 1.25 A 0.8 0.8 0.8 0.8 0.8 B 1.0 1.0 1.0 1.0 1.0 C 1.2 1.2 1.1 1.0 1.0 D 1.6 1.4 1.2 1.1 1.0 E 2.5 1.7 1.2 0.9 0.9 F See Section 11.4.7 of ASCE 7 Note: Use straight-line interpolation for intermediate values of S. For Site Class = D and S = 1.136 g, Fa = 1.045 Table 11.4-2: Site Coefficient F Site Class Mapped MCE R Spectral Response Acceleration Parameter at 1-s Period S1 0.10 S1 = 0.20 S1 = 0.30 S1 = 0.40 S1 > 0.50 A 0.8 0.8 0.8 0.8 0.8 B 1.0 1.0 1.0 1.0 1.0 C 1.7 1.6 1.5 1.4 1.3 D 2.4 2.0 1.8 1.6 1.5 E 3.5 3.2 2.8 2.4 2.4 F See Section 11.4.7 of ASCE 7 Note: Use straight-line interpolation for intermediate values of S1 For Site Class = 0 and S1 = 0.436 g, F, = 1.564 Equation (11.4-1): SMS = FaSs = 1.045 x 1.136 = 1.188 g Equation (11.4-2): SmI = FS1 = 1.564 x 0.436 = 0.682 g Section 11.4.4 - Design Spectral Acceleration Parameters Equation (11.4-3): S = % S = %x 1.188 = 0.792 g Equation (11.4-4): SDI = % S 1 = % x 0.682 = 0.455 g Section 11.4.5 - Design Response Spectrum From figure 22-12 [3] TL = 8 seconds Figure 11.4-1; Design Response Spectrum T<T0:SS(O.4 +O.8T/T,) T<TTi:Sa Sni /T > S01T, IT I' Il I' I------------------------- 4 1 Cr I I I I 1XX f. Section 114.6 - Risk-Targeted Maximum Considered Earthquake (MCER) Response Spectrum The MCER Response Spectrum is determined by multiplying the design response spectrum above by I.S. T (c) Section 11.8.3 - Additional Geotechnical Investigation Report Requirements for Seismic Design Categories D through F From figure 22-7 [4) PGA = 0.448 Equation (11.8-1): PGA, = FPGAPGA = 1.052 x 0.448 = 0.471 g Table 11.8-1: Site Coefficient FpGA Site Mapped MCE Geometric Mean Peak Ground Acceleration, PGA Class PGA < PGA = PGA = PGA = PGA > 0.10 0.20 0.30 0.40 0.50 A 0.8 0.8 0.8 0.8 0.8 B 1.0 1.0 1.0 1.0 1.0 C 1.2 1.2 1.1 1.0 1.0 D 1.6 1.4 1.2 1.1 1.0 1 E 2.5 1.7 1.2 0.9 0.9 F See Section 11.4.7 of ASCE 7 Note: Use straight-line interpolation for intermediate values of PGA For Site Class = D and PGA = 0.448 g, FPGA = 1.052 Section 21.2.1.1 - Method 1 (from Chapter 21 - Site-Specific Ground Motion Procedures for Seismic Design) From Figure 22-17 Cps = 0.947 From Figure_22_18 [6J CR1 = 0.999 Section 11.6 - Seismic Design Category Table 116-1 Seismic Desiqn Category Based on Short Period Response Acceleration Parameter VALUE OF S05 RISK CATEGORY lorli III IV SOS < 0.1679 A A A 0.167g S0 < 0.33g B B C 0.33g S05 < 0.50g C C D 0.509<S05 D D D For Risk Category = I and S0s = 0.792 g, Seismic Design Category = D Table 11.6-2 Seismic Desiqn Catepory Based on 1-S Period Response Acceleration Parameter VALUE OF S01 RISK CATEGORY lorll III IV S01 < 0.067g A A A 0.067g 5 S01 < 0.133g B B C 0.133g :5 S01 < 0.209 C C D 0.209S01 D D D For Risk Category = I and S01 = 0.455 g, Seismic Design Category = D Note: When S1 is greater than or equal to 0.75g, the Seismic Design Category is E for buildings in Risk Categories I, II, and HI, and F for those in Risk Category IV, irrespective f the above. Seismic Design Category "the more severe design category in accordance with Table 11.6-1 or 11.6-2" = D Note: See Section 11.6 for alternative approaches to calculating Seismic Design Category. References 1, Figure 22-1: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2010_ASCE-7_Figure22-1.pdf Figure 22-2: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2O10_ASCE-7_Figure22-2.pdf Figure 22-12: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2O1OASCE-7_Figure_22-12.pdf Figure 22-7: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2O1OASCE-7_Figure_22-7.pdf Figure 22-17: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2O1OASCE-7Figure_22-17.pdf Figure 22-18: https ://earthquake.usgs,gov/hazards/designmaps/downloads/pdfs/2010_ASCE-7jigure....22-18.pdf