HomeMy WebLinkAboutObservation of Excavation Report.pdf~tech HEPWORTH -PAWLAK GEOTECHNICAL June 25,2013 Mark Favro 20 I Larkspur Drive Carbondale, Co lorado 81623 mfavro 19@9mail. com i i\ !'\\," lh l',\\ l. tk 1i"",.,hnl\ l it -il.'\~l~l
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'lll SCANNED Job No. 112 255A Subject: Observation of Excavation, Proposed Detached Garage, Lot 7, Cerise Ranch, 201 Larkspur Drive, Garfield County, Colorado Dear Mr. Favro: As requested,
a representative of HepwOl1h-Pawlak Geoteclmical, Inc. observed the excavation at the subject site on May 13 & 15,20 13 to evaluate the soi ls exposed for foundation suppol1. The findings
of our observations and recommendations for the foundation design are presented in this repOl1. The services were performed as additional services in accordance with our agreement for
professional engineering services to you, dated July 24, 2012. Previous Study: We previously performed a subsoil study for a proposed house on this site dated July 27, 2004, Job No.
104430. The study indicated that the natural soils at this site, below about I foot of topsoil, consist of about 17 feet of silty sandy clay overlying dense gravel. Groundwater was encountered
about 4 to 6 feet below the ground surface. Proposed Garage: The proposed detached garage will be a one story wood fi'ame structure located west of the ex isting house and adjacent to
the existing driveway. Originally the foundation was to consist of spread footings bearing on the natural clay soils with up to 8 foot high foundation walls and a slab-on-grade floor.
Due to potential sett lement of the underslab fill and of the underlying clay under the new fill loading, the design was changed to a thickened edge slab foundation placed on compacted
structural fill. The structural fill was placed and monitored for settlement for a month prior to construction of the new foundation. Site Observations: At the time of our initial visit
to the site, the foundation excavation appeared to be in old fill and likely still above the original topso il. We recommended that the old fill be removed and the excavation be extended
down to the natural clay so ils below any topsoil. We understand that the garage was moved northward to provide room Parker 303-841-7 11 9 • Colorado Spr ings 719-633 -5562 • Sil\'e
rthorne 970-468-1989
Mark Favro June 25,2013 Page 2 for fill construction without extending into an existing utility easement. Upon our second site visit the old fill and topsoil had been removed with cut
depths up to 5Yz feet below the adjacent ground surface. The soils exposed in the bottom ofthe excavation consisted of sandy silty clay fill soils. No free water was encountered in the
excavation and the soils were moist to very moist. Recommendations: We understand that geogrid was placed over the exposed subgrade and imported "pit run" type fill was placed and compacted
in lifts up to thickened slab subgrade. We recommended that the fill be left in place for at least one month to allow for settlement of the underlying clay soils. We recommended that
level measurements be done across the fill initially and at least every two weeks until settlement had essentially stopped. The initial level measurement was made on May 17'h. We have
been provided with the level measurements which indicate that the fill settled 118 to Yz an inch over the first two weeks (reading made May 30'h) and showed essentially no movement over
the next two weeks (reading made June 17th). Conclusions: Considering the conditions exposed in the excavation and the nature ofthe proposed construction, a thickened edge slab placed
on the structural fill designed for an allowable soil bearing pressure of I ,000 psfshould be adequate for support of the proposed garage. The natural clay soils tend to compress when
loaded and there could be some post-construction settlement of the foundation over the long term. Loose and disturbed soils should be compacted prior to placing concrete. Exterior slab
edges should be insulated in accordance with the International Residential Code or provided with adequate soil cover above their bearing elevations for frost protection. A perimeter
foundation drain should not be provided for the proposed slab-on-grade foundation. Structural fill placed within 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 the previous subsurface exploration.
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 contaminants (MOBC) Job No. 112 255A
Mark Favro June 25, 2013 Page 3 developing in the future. If the client is concemed about MOBC, then a professio nal in this special field ofpractice should be consulted. If you have
any questions or need ful1her assistance, please call our office. Sincerely, HEPWORTH -PAWLAK GEOTCHNICAL, INC. Rev. by: SLP DEH/ksw Job No. 112 255A ~tech