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Home My WebLink AboutObservation of Excavation 10.15.15Ge<.5'tech HEPWORTH-PAWLAK GEOTECHNICAL October 15, 2015 Aspen Custom Builders Attn: John Davis P.O. Box 966 Basall, Colorado 81621 (john@as pe ncustombuilder.com) t-lqm1 1th 1',111I1k G~lllLch111 ~ 11 lnL 5020 Count; R,' 1.I I 54 Gknn,,.-J Sprini.:' Cul r1J,1 81601 Phunc 9i0 9-15 i9S.., fo'( 970 94; Sfrt Lm,111 hpi:co~hpg<1ULd1 .nm Job No. 115 067B Subject: Observation of Excavation, Proposed Residence, Lot IS-11, Aspen Glen, Wildflower, Garfield County, Colorado Dear John: As requested, a representative of Hepworth-Pawlak GeotechnicaJ, Inc. observed the excavation at the subject site on October 2 and 8, 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 Aspen Custom Builders, dated October 2, 2015. Huddleston-Berry Engineering and Testing, LLC performed a geotechnical investigation for this site, report dated October 16, 2013, their Project# 01273-0007. The proposed residence will be a one and two story wood frame structure over a crawlspace with an attached garage . The garage floor will be slab-on-grade. Foundations were designed for an allowable soil bearing pressure of 2,000 psf based on the Huddleston·Berry Engineering and Testing, LLC geotechnical report. At the time of our initial visit to the site, the foundation excavation had been cut in one level from 2'h lo 6 feet below the adjacent ground surface. The garage area had not been excavated. The soils exposed in the bottom of the excavation consisted of medium dense, silty sandy gravel in the southwest comer and stiff, sandy silty clay in the rest of the excavation. Results of swell-consolidation testing performed on samples of the clay taken from the site, shown on Figures l and 2, indicate the clay soils have a moderate collapse potential (settlement under constant load) when wetted. The samples were highly compressible under increased loading after wetting. 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 laboratory test results, we recommended to excavate the clay soils below footing grade and lower the footing grade down so that all footings are bearing on the relatively dense gravel soils. Pcirker 303·841-7119 • Colur;.idoSprings 719-633-5562 • Stlverrhomc 970-468-1989 Aspen Custom Builders October JS, 2015 Page2 We visited the site on October glh and observed that the footing grade in the clay subgrade areas had been deepened I~ to 4VJ feet, so that all footings will now bear on the gravel subsoils. Cut depths ranged from 5 to 7 feet in the crawlspace excavation footing areas. The garage area had been excavated 4 to 4~ feet below surrounding grade. The south and west sides of the garage footing grade appeared to be down to the gravel soils. The north side appeared to be on the clay soils with a 4VJ foot step down into the crawlspace gravel soils. We recommended that a step be cut on the eastern side of the north footing line in the garage to eliminate the majority of the clay below footing grade in the garage area. The excavator, Jim Marshall, stated that he would step the footing down in that area. Spread footings placed on the undisturbed natural gravel soil designed for an allowable soil bearing pressure of 2,000 psf should be adequate for support of the proposed residence. The upper clay soils at this site tend to compress when wetted and should be removed from below footing areas. Footings should be a minimum width of J 6 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing clay soils in footing areas should be removed and the bearing level extended down to the undisturbed natural gravel soils. The bearing soils should be protected against frost 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. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least IO 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 crawlspace walls and prevent wetting of the lower level. A perimeter drain should not be needed around the garage slab .on-grade area. Structural fill placed within floor slab areas can consist of the on -site soils compilcted 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 lO feet of the building. Landscape that requires regular heavy irrigation, such as sod, and sprinkler heads should not be located within 5 feet of the foundation. 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. Our services do not include determining the presence, prevention or possibility of mold or other biological Job No . 11 S 0678 Aspen Custom Builders October 15, 2015 Page3 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. Rev. by: SLP DEH/ksw attachments Figures 1 and 2 -Swell-Consolidation Test Results cc: Jeff Davis ie ff@_1i avisconst mcti0l).com pan y Job No. 115 0678 Moisture Content "" 6.6 percent Ory Density -91 per Sample of. Sandy Say Clay From : Bottom of Excavation 0 --1'1 } 2 Compression ~ --/ upon / i... i.,.L.. ... wetting i! 4 ~ c: .Q en ~ 6 a. E \ 0 (.) 8 10 \ \ 12 14 \ ' I > 16 \ ' . 18 i\ ~ 20 0 1 10 10 100 APPLIED PRESSURE • ksf 115 0678 ~ HEPWORTH-PAWL.NC Gaml:cHHSCAi. SWELL-CONSOLIDATION TEST RESULTS Figure 1 Moisture Content .,,,. 6.3 percent Dry Density = 98 per Sample ol. Sandy Silty Clay From: Bollom of Excavation 0 -i-. .... 11 1 2 ' ( Compression ~ 1-...._ ~ ... upan 'i/l. 3 wetting c: \ 0 "iii Kl 4 Ci \ ~ (.J 5 6 ' 7 \ B \ ' I 9 \ 10 I\ 11 \ \ 12 1p 0 .1 1.0 10 100 APPLIED PRESSURE • ksl 115 0678 ~ SWELL-CONSOLIDATION TEST RESULTS Figure 2 HEPWORTH-PAWUX Gmtu:HHICA&.