HomeMy WebLinkAbout03566 • Hepworth- Pawlak Geotechnical, Inc.
c led te� 5020 County Road Co8
Glenwood springs, lorado 81601
Phone: 970- 945 -7988
Fax: 970.945.8454
hpgeo @hpgeotech.com
March 6, 2000
Ron and Cathy Chase
P.O. Box 1083
Glenwood Springs, Colorado 81602 Job No. 100 219
Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed
Residence, County Road 335, 11/4 Mile West of Garfield Creek, Garfield
County, Colorado
Dear Mr. and Mrs. Chase:
As requested, Hepworth - Pawlak Geotechnical, Inc. performed a subsoil study and
percolation test for foundation and septic disposal system designs at the subject site.
The study was conducted in accordance with our agreement for geotechnical
engineering services to you dated February 22, 2000. The data obtained and our
recommendations based on the proposed construction and subsurface conditions
encountered are presented in this report. Evaluation of potential geologic hazard
impacts on the site are beyond the scope of this study.
Proposed Construction: The proposed residence will be a single story modular home,
about 40 feet by 60 feet plan size, located on the site as shown on Fig. 1. A parking
level may also be provided below the residence. Ground floors for the basement will be
slab -on- grade. Cut depths are expected to range between about 3 to 8 feet. Foundation
loadings for this type of construction are assumed to be relatively light and typical of
the proposed type of construction. The septic disposal system is proposed to be located
to the northeast of the residence.
If building conditions or foundation loadings are significantly different from those
described above, we should be notified to re- evaluate the recommendations presented in
this report.
Site Conditions: The property is vacant and located on a northerly facing hillside
above County Road 335. The ground surface in the building area is strongly sloping,
becoming steeper to the south. The lot is vegetated with pinon and juniper trees,
sagebrush and grass. There are scattered boulders up to about 4 feet in diameter on the
hillside above the building area.
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Ron and Cathy Chase
March 6, 2000
Page 2
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits in the building area and one profile pit in the septic
disposal area at the approximate locations shown on Fig. 1. The logs of the pits are
presented on Fig. 2. The subsoils encountered, below about 1 to 1 feet of topsoil,
consist of medium stiff sandy clayey silt. The silt soils extended to the depths dug of 6
and 8 feet at Pits 1 and 2, respectively, and to a depth of 6 feet at the Profile Pit
where silty sandy gravel with cobbles and boulders was encountered to the pit depth of
8 feet. Results of swell - consolidation testing performed on relatively undisturbed
samples of the silt soils, presented on Figs. 3 and 4, indicate low compressibility under
existing moisture conditions and light loading and moderate to high compressibility
when wetted and loaded. One sample showed a low collapse potential (settlement under
constant load) when wetted. The laboratory testing is summarized in Table I. No free
water was observed in the pits at the time of excavation and the soils were moist to
slightly moist with depth.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread
footings placed on the undisturbed natural soil designed for an allowable bearing
pressure of 1,000 psf for support of the proposed residence. The soils tend to compress
after wetting and there could be post - construction foundation settlement of 1 to 2 inches
or more. 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.
All topsoil and loose or disturbed soils encountered at the foundation bearing level
within the excavation should be removed and the footing bearing level extended down to
the undisturbed natural soils. The footing subgrade should then be moistened and
compacted. Exterior footings should be provided with adequate cover above their
bearing elevations for frost protection. Placement of footings at least 36 inches below
the exterior grade is typically used in this area. 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 the on -site soil as backfill.
Floor Slabs: The natural on -site soils, exclusive of topsoil, are suitable to support
lightly loaded slab -on -grade construction. There could be some slab settlement if the
subgrade becomes wetted. To reduce the effects of some differential movement, floor
slabs should be separated from all bearing walls and columns with expansion joints
which allow unrestrained vertical movement. Floor slab control joints should be used
to reduce damage due to shrinkage cracking. The requirements for joint spacing and
slab reinforcement should be established by the designer based on experience and the
intended slab use. A minimum 4 inch layer of free - draining gravel should be placed
H -P GEOTECH
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Ron and Cathy Chase
March 6, 2000
Page 4
recommend a minimum slope of 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in pavement
and walkway areas. A swale should be provided uphill to direct surface
runoff around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of
all backfill.
5) Landscaping which requires regular heavy irrigation, such as sod, should
be located at least 5 feet from the building. Consideration should be
given to the use of xeriscape to limit potential wetting due to irrigation.
Percolation Testing: Percolation tests were conducted on March 1, 2000 to evaluate
the feasibility of an infiltration septic disposal system at the site. One profile pit and
three percolation holes were dug at the locations shown on Fig. 1. The test holes
(nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow
backhoe pits and were soaked with water one day prior to testing. The soils exposed in
the percolation holes are similar to those exposed in the Profile Pit shown on Fig. 2 and
consist of very sandy clayey silt. The percolation test results, presented in Table 11,
indicate percolation rates from about 16 to 34 minutes per inch. Based on the
subsurface conditions encountered and the percolation test results, the tested area should '
be suitable for a conventional infiltration septic disposal system. We recommend the
infiltration area be oversized due to the variable percolation rates.
Limitations: This study has been conducted in accordance with generally accepted
geotechnical engineering principles and practices in this area at this time. We make no
warranty either expressed or implied. The conclusions and recommendations submitted
in this report are based upon the data obtained from the exploratory pits excavated at the
locations indicated on Fig. 1, the proposed type of construction and our experience in
the area. Our findings include interpolation and extrapolation of the subsurface
conditions identified at the exploratory pits and variations in the subsurface conditions
may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be
notified at once so re- evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes.
We are not responsible for technical interpretations by others of our information. As
the project evolves, we should provide continued consultation and field services during
construction to review and monitor the implementation of our recommendations, and to
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications to the recommendations
presented herein. We recommend on -site observation of excavations and foundation
H -P GEOTECH