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HomeMy WebLinkAbout18-7-434 (06-28-17) Deck Addn Exc Obs signed.pdf jmAR5020 County Road 154 Glenwood Springs, CO 81601 Geotechnical Engineering I Engineering Geology Phone: (970)945-7988 Materials Testing I Environmental Fax: (970)945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver(HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs,Summit County, Colorado June 28, 2018 Howard Rapson Construction Attn: Howard Rapson P. O. Box 5664 Vail, Colorado 81658 Hrapski7@gmail.com Project No. 18-7-434 Subject: Observation of Excavation, Proposed Deck Addition to Hanson Residence, Unit 15, 770 Potato Patch Drive, Vail, Colorado Dear Howard: As requested, a representative of H-P/Kumar observed the excavation at the subject site on June 25, 2018 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations are presented in this report. The services were performed in accordance with our agreement for professional engineering services to Howard Rapson Construction dated June 22, 2018. The deck will be attached southeast portion of the residence. The deck has been designed to be supported on a stem wall and continuous footing extending from the existing building foundation wall and three isolated footing pads located away from the building. The footings have been sized assuming an allowable soil bearing pressure of 1,500 psf. The existing building is apparently founded on shallow spread footings. At the time of our site visit, the foundation excavation for the addition area had been cut from 3 to 6 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted primarily of relatively dense, silty sand and gravel with cobbles. Adjacent the existing foundation was a shallow depth of fill. The results of a gradation analysis performed on a sample of the natural granular soils (minus 3 inch fraction) obtained from the site are presented on Figure 1. 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 granular soil designed for an allowable soil bearing pressure of 1,500 psf can be used for support of the proposed deck addition. It should be feasible to leave the limited area of existing fill in place and span the new foundation wall over the fill. There could be some differential settlement with respect to the Howard Rapson Construction June 28, 2018 Page 2 existing structure which should be considered in the design. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils in footing areas should be removed and the bearing level extended down to the undisturbed natural granular soils, and the subgrade compacted. The 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 10 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 granular soil (minus 6-inch fraction) as backfill. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind any foundation or site walls acting as retaining structures. Backfill of the foundations can consist of the on-site soils compacted to at least 90% of standard Proctor density (95% in pavement areas) at a moisture content near optimum. Backfill placed around the addition should be compacted and the surface graded to prevent ponding within at least 10 feet of the building and foundations. 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 excavation, drilling would be required. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. If you have any questions or need further assistance, please call our office. Sincerely, H-P�KUM 0, -� wR i 7,4e.. r•'ae . y,,N /C r. c� I .1= GSA i E % 32.216,,,,,z _ David A. Young, P. 0yn DAY/kac dB,�-'°Oa�l ,s e� �o G G ' '1;000,..(1;v00 attachment Figure 1, Gradation Test Results cc: Town of Vail—CJ Jarecki (cjarecki@vailgov.com) H-P*KUMAR Project No. 18-7-434