HomeMy WebLinkAboutObservation of Excavation 05.29.14~ech
HEPWORTH-PAWLAK GEOTECHNICAL
May 29, 2014
Tim Wittenberg
7026 County Road 233
Silt, Colorado 81652
(twwoodworks@ msn.com)
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Job No. 114 163A
Subject: Observation of Excavation, Proposed Detached Garage/Shop, 7026
County Road 233 (Silt Mesa Road), Garfield County, Colorado
Dear Tim:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on May 20, 2014 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 May 8,
2014.
The structure will be 1 Yi stories of wood frame construction with a slab-on-grade ground
floor at an elevation near to slightly above the existing ground surface. The building has
been designed to be supported on spread footings assuming an allowable soil bearing
pressure of 1,500 psf.
At the time of our site visit, the foundation excavation had been cut in one level from
about 2 to 4 Yi feet below the adjacent ground surface. The soils exposed in the bottom of
the excavation consisted of medium stiff to stiff, sandy silt and clay. Results of swell-
consolidation testing performed on samples taken from the site, shown on Figure 1,
indicate the soils are moderately compressible under conditions of loading and wetting
with a low hydro-compression potential. The laboratory testing is summarized in Table
1. 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 nature of the proposed
construction, spread footings placed on the undisturbed natural soil designed for an
allowable bearing pressure of 1,500 psf can be used for support of the proposed building.
The exposed soils may tend to compress some when wetted and there could be some post-
construction settlement of the foundation ifthe bearing soils become wet. Precautions
should be taken to prevent wetting of the bearing soils.
Footings should be a minimum width of 18 inches for continuous walls and 2 feet for
columns. Existing fill and loose disturbed soils in footing areas should be removed and
the bearing level extended down to the undisturbed firm natural soils. The footing
Pmkcr 303-841-7119 • Colorado Springs 719-633-5562 • Silwrthumc 970-468-1989
Tim Wittenberg
May 29, 2014
Page 2
subgrade should then 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 12 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 around
the building should not be needed due to the floor "slab-at-grade" construction.
Structural fill placed within floor slab areas can consist of the on-site soils compacted to
at least 95% of standard Proctor density a (SPD} at a moisture content near optimum.
Backfill placed around the structure should be compacted to at least 90% SPD (95 % in
pavement areas) and the surface graded to prevent ponding within at least l 0 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
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,
David A. Young, P.E.
DAY/ljg
attachments: Figure 1, Swell-Consolidation Test Results
Table 1, Summary of Laboratory Test Results
Job No . 114 l63A
Moisture Content -16.7 percent
Dry Density -98 pct
Sample of : Sandy Sil t and Clay
From : Bottom of South-Central Portion of
Excavation
0 I" ,...
'if-1 .... v
c: " ( Compression
0 ~ -upon ·c;; t-t-"'"' ,.... C/) wetting Cl> 2 ...
0. "' E
0
(.) I\ 3
4
0 .1 1.0 10 100
APPLIED PRESSURE -ksf
114 163A ~
Heoworth-Powlak Geotechnlcal
SWELL-CONSOLIDATION TEST RESULTS Figure 1
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 114163A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL GRADATION ATTERBERG LIMITS NATURAL PERCENT ASSHTO MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC CLASSIFICATION SOIL OR
IN BOTTOM OF CONTENT DENSITY (%) (%) NO. 200 LIMIT INDEX BEDROCK TYPE
EXCAVATION SIEVE
(%\ tpcO t'lo\ Wo)
South-Central 16.7 98 Sandy Silt and Clay Portion
North-Central 8.7 116 Sandy Silt and Clay Portion