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Home My WebLink About108400A (07-22-08) Western Half Duplex ExcObs.pdf Hepworth-Pawlak Geotechnical, liu_ 5020 County Road 154 Glenwood Springs,Colorado 81601 Phone: 970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 email:hpgeo@hpgeotech.com July 22, 2008 Sunder, Inc. Attn: John Sunderland P.O. Box 1393 Edwards, Colorado 81632 Job No. 108 400A Subject: Observation of Excavation, Proposed Western Half of Existing Duplex, Lot 26, Block 7, Vail Village 1st Filing, 185 Forest Road, Vail, Colorado. Dear Mr. Sunderland: As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on July 17, 2008 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations for the foundation design are presented in this report. The services were performed in accordance with our agreement for professional engineering services to Sunder, Inc., dated July 16, 2008. The western half of the existing duplex was razed as part of the proposed construction. The proposed construction will consist of a two story wood frame structure over a walkout basement level. We understand that due to over-excavation of unsuitable bearing soils in the building area, a structural basement floor is being planned. The attached garage floor will be slab-on-grade. Spread footings sized for an allowable soil bearing pressure of 2,000 psf were assumed in the design. At the time of our visit to the site, the foundation excavation had been cut in multiple levels from 3 to 10 feet below the adjacent ground surface. The uphill, southern portion of the excavation had not been completed. The soils exposed in the bottom of the excavation consisted of slightly clayey silty sand and gravel in the northern portion of the excavation and silty sand in the southwest portion of the excavation. The cut depths extended through the old fill from the previous construction. Results of swell- consolidation testing performed on a sample of silty sand taken from the site, shown on Figure 1, indicate the soils have low to moderate compressibility under conditions of loading and wetting. The results of a gradation analysis performed on a sample of silty Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-198 Sunder, Inc. July 22, 2008 Page 2 sand and gravel(minus 3 inch fraction) obtained from the site are presented on Figure 2. No free water was encountered in the excavation and the soils were slightly moist to moist. Considering the conditions exposed in the excavation and the nature of the proposed construction, spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 2,000 psf should be adequate for support of the proposed western half of the duplex. The bearing soils are variable and the silty sand soils tend to compress when loaded and wetted. There could be some post-construction settlement of the foundation that could be differential between the different bearing materials and the eastern half of the duplex. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose disturbed soils and any remaining fill in footing areas should be removed and the bearing level extended down to the undisturbed natural soils. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 12 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for on-site soil as backfill. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind the basement walls and prevent wetting of the lower level. Structural fill placed within floor slab areas can consist of the on-site soils compacted to at least 95%of standard Proctor density at a moisture content near optimum. Backfill placed around the structure should be compacted and the surface graded to prevent ponding within at least 10 feet of the building. The recommendations submitted in this letter are based on our observation of the soils exposed within the foundation excavation and do not include subsurface exploration to evaluate the subsurface conditions within the loaded depth of foundation influence. This study is based on the assumption that soils beneath the footings have equal or better support than those exposed. The risk of foundation movement may be greater than indicated in this report because of possible variations in the subsurface conditions. In order to reveal the nature and extent of variations in the subsurface conditions below the Job No. 108 400A Geaech • Sunder, Inc. July 22, 2008 Page 3 excavation, drilling would be required. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH—PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E. Rev. by: SLP JZA/vam attachments Figure 1 — Swell-Consolidation Test Results Figure 2 — Gradation Test Results Job No. 108 400A Gla tech Moisture Content = 14.8 percent Dry Density = 110 pcf -200 = 46 percent Sample of: Silty Sand From: Bottom of Excavation at Southwest Corner 0 • 1 0 • 0- 2 E • 0 U 3 No movement upon wetting 0.1 1.0 10 100 APPLIED PRESSURE-ksf 108 400A ~ C7QCI7 SWELL-CONSOLIDATION TEST RESULTS Figure 1 HEPWORTH•PAWLAK GEOTECHNICAL HYDROMETER ANALYSIS SIEVE ANALYSIS 24M.175 R. 7 HR TIME READINGS I U.S.STANDARD SERIES I CLEAR SQUARE OPENINGS 1 0 45 MIN.15 MIN.60MIN19MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8" 3/4" 1 1/2" 3" 5"6" 8" 100 10 90 20 80 30 70 40 60 Z N W i Q c 50 50 1— Z Z W W U U a. W w a 60 40 70 30 80 20 90 10 100 0 .001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 2.36 4.75 9.5 19.0 37.5 76.2 152 203 12.5 127 DIAMETER OF PARTICLES IN MILLIMETERS CLAY TO SILT SAND GRAVEL FINE I MEDIUM I COARSE FINE I COARSE COBBLES GRAVEL 36 % SAND 35 % SILT AND CLAY 29 % LIQUID LIMIT % PLASTICITY INDEX SAMPLE OF: Silty Sand and Gravel FROM: Bottom of Excavation at East Side N 108 400A Lech GRADATION TEST RESULTS Figure 2 HEPWORTH^PAWLAK GEOTECHNICAL