HomeMy WebLinkAboutSoils Report 07.29.1999.pdfGtech
July 29, 1999
Terrell and Patty Tankersley
c/o American Overseas Pet. Ltd. (RUMBAI)
P.O. Box 770249
Houston, Texas 77215-0249
Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
hpgeo@hpgeotech.com
Job No. 199 512
Subject: Subsoil Study for Foundation Design and Percolation Testing, Proposed
Residence, Parcel 1, Simmons Subdivision, Canyon Creek Area,
Garfield County, Colorado
Dear Mr. & Mrs. Tankersley:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for
foundation design at the subject site. Percolation testing was also conducted to evaluate
the infiltration characteristics of the subsoils for design of the on-site septic disposal
system. The study was conducted in general accordance with our proposal for
geotechnical engineering services to you dated June 17, 1999. The data obtained and
our recommendations based on the proposed construction and subsurface conditions
encountered are presented in this report.
Proposed Construction: The proposed construction includes a single family residence
over a walkout basement and a guest cottage, located on the site as shown on Fig. 1.
Cut depths are expected to be up to about 8 to 10 feet. Foundation loadings for this
type of construction are assumed to be relatively light and typical of the proposed type
of construction. An septic disposal system is proposed to be located northeast of the
proposed main residence as shown on Fig. 1.
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 site is located on a southwesterly facing hillside near the
confluence of Canyon Creek and East Canyon Creek. The ground surface in the
building areas is relatively flat with a strong slope down generally the south. Beyond
the west, south and east sides of the building sites the slope becomes steep down to East
Canyon Creek. There are several shallow abandoned irrigation ditches on the site.
Vegetation is mostly grass and weeds with scrub oak and brush on the steep slopes.
Terrell and Patty Tankersley
July 29, 1999
Page 2
Cobbles and boulders up to several feet in diameter were observed on the ground
surface.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits in the proposedbuilding areas and one profile pit in the
proposed septic disposal site at the locations shown on Fig. 1. The logs of the pits are
presented on Fig. 2. The subsoils encountered, below about 11 to 2 feet of topsoil,
consist of slightly silty sandy gravel and cobbles with boulders. Some of the boulders
observed in the pits were up to about 5 feet in diameter. Results of a gradation
analysis performed on a sample of the gravel (minus 5 inch fraction) obtained from the
site are presented on Fig. 3. No free water was observed in the pits at the time of
excavation and the subsoils were slightly moist.
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 gravel and cobble soil designed for an
allowable bearing pressure of 3,000 psf for support of the proposed buildings. Footings
should be a minimum width of 16 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 granular soils. Voids created by boulder removal should be
backfilled with compacted gravel or concrete. 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 10 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 45 pef for the on-site soil,
excluding vegetation, topsoil and oversized rock, as backfill.
Floor Slabs: The natural granular soils, exclusive of topsoil, are suitable to support
lightly to moderately loaded slab -on -grade construction. 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
H -P GEOTECH
Terrell and Patty Tankersley
July 29, 1999
Page 3
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 beneath basement level slabs to facilitate drainage.
This material should consist of minus 2 inch aggregate with less than 50% passing the
No. 4 sieve and less than 2% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95 % of
maximum standard Proctor density at a moisture content near optimum. Required fill
can consist of the on-site granular soils devoid of vegetation, topsoil and oversized
rocks.
Underdrain System: Although free water was not encountered during our exploration,
it has been our experience in the area that local perched groundwater can develop
during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can also create a perched condition. We recommend below -grade construction,
such as retaining walls, crawlspace and basement areas, be protected from wetting and
hydrostatic pressure buildup by an underdrain system.
The drains should consist of drainpipe placed in the bottom of the wall backfill
surrounded above the invert level with free -draining granular material. The drain
should be placed at each level of excavation and at least 1 foot below lowest adjacent
finish grade and sloped at a minimum 1% to a suitable gravity outlet. Free -draining
granular material used in the underdrain system should contain Less than 2% passing the
No. 200 sieve, less than 50% passing the No: 4 sieve and have a maximum size of
2 inches. The drain gravel backfill should be at least 11/2 feet deep.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the buildings have been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at least 95 % of the maximum standard Proctor density in
pavement and slab areas and to at least 90 % of the maximum standard
H -P GEOTECH
Terrell and Patty Tankersley
July 29, 1999
Page 4
Proctor density in landscape areas. Free draining wall backfill should be
capped with about 2 feet of the on-site, finer graded soils to reduce
surface water infiltration.
3) The ground surface surrounding the exterior of the buildings should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of 6 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.
4) Roof downspouts and drains should discharge well beyond the limits of
all backfill.
Percolation Testing: Percolation tests were conducted on July 8, 1999 to evaluate the
feasibility of an infiltration septic disposal system at the site. One profile pit and three
percolation holes were excavated 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 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 sandy gravel and cobbles with boulders. The percolation test results,
presented in Table I, indicate average percolation rates at the test sites of from 8 to 20
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.
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 and to the depths shown on Fig. 2, 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.
HP GEOTECH
Terrell and Patty Tankersley
July 29, 1999
Page 5
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
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
If you have any questions or if we may be of further assistance, please let us lcnow.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
David A. Young, P.E.
Reviewed By:
Daniel E. Hardin, P.E.
DAY/ksm
attachments
H -P GEO TECH
LEGEND
• EXPLORATORY PIT
A PERCOLATION TEST HOLE
PROPOSED
RESIDENCE
APPROXIMATE SCALE
1"=120'
. EAST C44, `EEK
PROPOSED
GUEST
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DISPOSAL
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PROPERTY
BOUNDARY
199 512
HEPWORTH - PAWLAK
GEOTECHNICAL, INC.
LOCATION OF EXPLORATORY PITS
Fig. 1