HomeMy WebLinkAboutObservation of Excavation Report 09.02.2009Gtech HEPWORTH - PAWLAK GEOTECHNICAL September 2, 2009 Whit Dean 1136 County Road 129 Glenwood Springs, Colorado 81601 Hepwoith-Patv�,ak(m.otechnical, Inc. 5020 County Rn.aJI 154 Glenwood Springs, Cnlorl1Llo 81601 Phone:970-945-7985 F❑>.: 970-945.8454 email hpgec,Kip,c;eoiech.com Job No. 109 297A Subject: Observation of Excavation, Proposed Garage/Shop, 1136 County Road 129, No Name, Garfield County, Colorado Dear Mr. Dean: As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on September 1, 2009 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 you, dated September 1, 2009. The proposed garage/shop will be a two story wood frame structure with a slab -on -grade floor. The upper floor will be living space. Foundations were designed based on an assumed allowable soil bearing pressure of 1,000 psf Retaining walls on the north and east sides of the garage were designed to resist an earth pressure loading of 60 pcf with a passive earth pressure resistance of 250 pcf. At the time of our visit to the site, the foundation excavation had been cut in two levels from 2'/2 to 8 feet below the adjacent ground surface. There was a 2 foot step down to the west and south between levels. The soils exposed in the bottom of the excavation consisted of 4 to 6 inches of 3/ -inch screened rock overlying relatively dense, slightly silty sandy gravel with cobbles and boulders. No free water was encountered in the excavation and the soils were slightly moist. Considering the conditions exposed in the excavation and the nature of the proposed construction, spread footings placed on a thin layer of compacted screened rock over the undisturbed natural granular soil designed for an allowable soil bearing pressure of 3,000 psf can be used for support of the proposed garage/shop. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils (and existing screened rock fill) in footing areas should be compacted. 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 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 45 pcf with a Prirker 303-841-7119 Colorado $p'ing« 719-033-J562 9 SiLlerrhoine 97C-168-1959 Whit Dean September 2, 2009 Page 2 passive earth pressure resistance of 300 pcf for the on-site granular soil as backfill. The coefficient of friction against sliding on the bottom of the footings can be taken as 0.50. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind the retaining walls and prevent wetting of the garage level on the north and east sides of the building. 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. 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 gaziI NICAL, INC. Daniel E. Hardin, P.R. . 24443 t tti 9,(,t„,:4 Rev. by: SLP DEH/ksw cc: BPSE — Attn: Brian Puppe Job No. 109 297A GecPtech