HomeMy WebLinkAboutObservation of Excavation Report 11.12.2015HEPWORTH-PAWLAK GEOTECHNICAL
November 12, 2015
Doolen Construction
Attn: Dave Doolen
3838 County Road 243
New Castle, Colorado 81647
(lave `doo'tenconstruction,corn)
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Job No. 111 395B
Subject: Observation of Excavation, Proposed Adams Residence, 3907 County
Road 243 (East Elk Creek Road), Garfield County, Colorado
Dear Dave:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on November 9, 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 Doolen
Construction dated November 3, 2015.
We previously conducted a subsoil study for design of foundations at the site and
presented our findings in a report dated January 19, 2012, Job No. 111 395A. The
proposed construction has changed since that report. The residence will now include a
basement level and attached garage at the main level. Ground floors will be slab -on -
grade. The building has been designed to be supported on spread footings bearing on the
natural soils using an allowable soil bearing pressure of 1,200 psf based on
recommendations provided in our previous report.
At the time of our site visit, the foundation excavation which was nearly complete had
been cut in three levels from about 4 to 12 feet below the adjacent ground surface. The
southeast portion of the garage area was not complete at this time. The soils exposed in
the bottom of the excavation consisted of medium dense, silty sandy gravel with cobbles
in the deeper cut areas of the basement portion and medium stiff, sandy clayey silt in the
shallower cut areas for the attached garage area at the southeast. The clayey silt soils
were up to about 5 feet deep over the gravel with cobble soils. Results of swell -
consolidation testing performed on samples taken from the site, shown on Figure 1,
indicate the clayey silt soils are moderately to highly compressible under conditions of
loading and wetting with a low hydro -compression potential. No free water was
encountered in the excavation and the soils were moist to very moist, apparently from
recent rain and snow.
The clayey silt soils are compressible and the gravel with cobble soils are relatively
incompressible. Lightly loaded spread footings can be used for support of the residence
provided some risk of differential settlement is acceptable. The risk of settlement is
primarily at the transition from the fine grained (silt) to the coarse granular (gravel)
bearing areas, and especially if the fine grained bearing soils become wetted. Precautions
Parker 303-S41-7119 • Colorado Springs 719-633-5562 • Silverthornc 970-468-1989
Doolen Construction
November 12, 2015
Page 2
should be taken to prevent wetting of the fine grained bearing soils. Extending the
footings down to bearing entirely on the coarse granular soils or providing several feet of
compacted structural fill below the footings in the fine grained soil bearing areas could be
done to reduce the risk of settlement.
The soil conditions exposed in the excavation are somewhat different than those
previously encountered on the site. Spread footings bearing on the natural soils designed
for an allowable bearing pressure of 1,200 psf can be used for support of the residence
with the risk of settlement as discussed above. The settlements could be up to 1 to 11/2
inches and differential. Prior to the footing construction, loose disturbed soils should be
removed to expose the undisturbed natural soils, and the subgrade compacted. The
bearing soils should be protected from frost and concrete should not be placed on frozen
soils. 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 55 pcf for on-site soil as
backfill. A perimeter foundation drain should be provided to prevent temporary buildup
of hydrostatic pressure behind the basement walls and prevent wetting of the basement
level. A drain should not be provided around the garage area. A minimum 4 inches of
free draining gravel should be provided below the basement floor slab. Providing
positive surface drainage away from foundation walls should be done as well as limiting
irrigation near the building. Other recommendations presented in our previous report
which are applicable should also be observed.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and the previous limited subsurface exploration
at the site. Variations in the subsurface conditions below the excavation could increase
the risk of foundation movement. We should be advised of any variations encountered in
the excavation conditions for possible changes to 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 -- PAW AK C L tI*j�CAL, INC.
David A. Young, P.E. i113 -
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attachment Figure 1, Swet'tr'dhtion Test Results
cc: Denise Doolen (denise doolenconstruction.com)
Job No 111395E