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HomeMy WebLinkAboutObservation of Excavation 11.16.2015HEPWORTH-PAWLAK GEOTECHNICAL November 16, 2015 David Iuele P.O. Box 67 Glenwood Springs, Colorado 81602 (aucdave r[msn.com) Hepworth-Paw1 ik Geotechnical, Inc. 5020 County Road 154 Glcn aot1 Springs, Colorado 8160! Phone. 970-945-7988 F.uv 970-947-845.1 email hpgco@hpgcotcch.com Job No. 115 534A Subject: Observation of Excavation, Proposed Residence, 1007 County Road 110, Garfield County, Colorado Dear Mr. Iuele: As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on November 7, 2015 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 November 6, 2015. The proposed residence will be a one to two story wood frame structure supported on spread footings with a structural ground floor above crawlspace and possibly some slab on grade floor areas. We understand the building area was sub -excavated down to remove soft upper soils and it is proposed to re-establish the foundation bearing level on the order of a few feet using compacted aggregate base course or similar granular backfill. At the time of our visit to the site, the building excavation bottom was irregular and from about 7 to 10 feet below the adjacent grades. The materials exposed in the bottom of the excavation consisted of weathered to hard siltstone bedrock and gypsum of the Eagle Valley Evaporite. Results of swell -consolidation testing performed on a sample of the weathered siltstone taken from the site, shown on Figure 1, indicate the materials are slightly compressible under conditions of loading and wetting. No free water was encountered in the excavation and the materials were slightly moist to moist. The bottom of the excavation was blanketed and the subgrade was not frozen. Considering the conditions exposed in the excavations and the nature of the proposed construction, spread footings placed on the undisturbed weathered to hard bedrock or compacted granular backfill should be designed for an allowable maximum soil bearing Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 David Iuele November 16, 2015 Page 2 pressure of 2,000 psf for support of the proposed residence. The weathered bedrock and granular backfill may compress if wetted under load and result in relatively small post - construction settlement, mainly if the bearing materials become wet. Backfill placed below footing areas should be reasonably well -graded granular material such as aggregate base course, free of frozen materials, placed at near optimum moisture in 8 inch maximum lifts and compacted to at least 98 percent of standard Proctor density. Backfill placed below footings should extend laterally beyond the edge of footings a distance at least half the depth of the backfill. Strip footings should be a minimum width of 16 inches and isolated footing pads should be a minimum of 24 by 24 inches. Soils in footing areas that become loose or disturbed should be removed or compacted prior to concrete placement. The subgrade, backfill and bearing soils should be protected against freezing and concrete should not be placed on frozen soils. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Foundation walls acting as retaining structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for on-site soil or granular soil as backfill. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls acting as retaining structures and to prevent wetting of lower areas, such as the crawlspace. Structural fill placed within floor slab areas should consist of granular 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. Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the site. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the property. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. Sinkholes were not observed in the immediate area of the subject building site. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence at the site throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. The recommendations submitted in this letter are based on our observation of the materials exposed within the foundation excavations 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 materials 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 Job No. 115 534A David Iuele November 16, 2015 Page 3 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. Thomas J. Westhoff, CET Reviewed by: Steven L. Pawlak, P.E. TJW/ksw Attachment: Figure 1 -- Swell -Consolidation Test Results Job No, 115534A Compression - Expansion % 1 0 1 2 3 4 5 Moisture Content 12.8 percent Dry Density = 101 pcf Sample of: Weathered Siltstone From: Bottom of excavation • IN\ Compression -upon wetting 0,1 10 APPLIED PRESSURE = ks( 10 100