HomeMy WebLinkAboutSubsoil StudyG EM411, 1-1
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HEPWORTH - PAWLAK GEOTECHNICAL
March 31, 2004
Rapids Development Corporation
Attn: Gene Hilton
Hepworth-Pawlak Geotechnical, Inc.
5020 Count} Road 154
Glen -wood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email: hpgeo@hpgeotech.com
2102 West Arapahoe Drive
Littleton, Colorado 80120-3008
Job No. 103 198
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 5, The
Rapids on the Colorado, County Road 335, Garfield County, Colorado.
Dear Mr. Hilton:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design
of foundations at the subject site. The study was conducted as a supplement to our
previous agreement for geotechnical engineering services to Rapids Development
Corporation dated March 5, 2003. The data obtained and our recommendations based on
the proposed construction and subsurface conditions encountered are presented in this
report. Hepworth-Pawlak Geotechnical, Inc. previously conducted percolation testing for
the subdivision development and presented our findings in reports dated May 12, 1995
and August 8, 1996, Job No. 195 217.
Proposed Construction: The proposed residence will be a one or two story wood frame
structure located in the area of the pits shown on Figure 1. Ground floor will be either
structural over a crawlspace or slab -on -grade. Cut depths are expected to range between
about 3 to 4 feet. Foundation loadings for this type of construction are assumed to be
relatively light and typical of the proposed type of construction.
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 was vacant at the time of our field exploration. The ground
surface in the building envelope is relatively flat with a gentle slope down to the
northwest. There is about 5 feet of elevation difference across the building envelope.
The Colorado River is adjacent to the northwest of the property and is about 10 feet lower
in elevation. Vegetation consists of grass and scattered brush.
Parker 303-841-7119 . Colorado Springs 719-633-5562 . Silverthorne 970-468-1989
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Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits at the approximate locations shown on Figure 1. The
logs of the pits are presented on Figure. 2. The subsoils encountered, below about 1/2 feet
of topsoil, and up to 3 1/2feet of loose to medium dense, silty sands consist of relatively
dense, slightly silty sandy gravels and cobbles. A clayey sand lens was encountered at 3
feet in Pit 2 and swell -consolidation testing presented on Figure 3, indicates a minor
expansion potential. No free water was observed in the pits at the time of excavation and
the soils were slightly moist to 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 soil designed for an allowable soil bearing
pressure of 1,000 psf for support of the proposed residence. There could be a potential for
some differential settlement due to the variable bearing conditions. Footings bearing
entirely on the underlying dense gravels can be designed for an allowable soil bearing
pressure of 3,000 psf. Footings should be a minimum width of 18 inches for continuous
walls and 2 feet for columns. The topsoil and loose disturbed soils encountered at the
foundation bearing level within the excavation should be removed. The exposed soils
should then be moistened and compacted. Voids created by the removal of large rocks
should be backfilled with compacted sand and gravel or with 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 12 feet.
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 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. The upper fine grained soils could be compressible
when wetted under load. 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 beneath
interior slabs for subgrade support. This material should consist of minus 2 inch
aggregate with less than 50% passing the No. 4 sieve and less than 12% passing the
No. 200 sieve.
Job No. 103 198
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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 soils devoid of vegetation, topsoil and oversized rock.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the residence has 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
Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building 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.
5) Landscaping which requires regular heavy irrigation should be located at
least 5 feet from the building.
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 express 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 Figure 1 and to the depths shown on Figure 2, the proposed type of
construction, and our experience in the area. Our services do not include deterinining 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. 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.
Job No. 103 198
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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 know.
Respectfully Submitted,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Scott W. Richards
Reviewed by:
Steven L. Pawlak, P.E.
S WR/ksw
attachments Figure 1— Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Swell Consolidation Test Results
Job No. 103 198
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SCALE 1"=50'
5490
NOTE: CONTOUR LINES
SHOWN ARE PRIOR TO SITE
GRADING FOR SUBDIVISION
DEVELOPMENT.
5495 r
LOT 4
1
LOT 5
F- -- T -7
BUILDING
ENVELOPE
PIT 2
I
— 5490
' _ -- 5495
LOT 6
RAPIDS VIEW LANE
LINE
1 103 198 1
GEOWEOCHNICALWINC. LOEA RAPIDSF ON EXT HOE COLORA ORATORY S I Figure 1 I
0
5
10
PIT 1 PIT 2
ELEV.=5501' ELEV.=5499'
LEGEND: .
F� TOPSOIL; silt and sand, organics, moist, dark brown.
SAND (SM); silty to very silty, medium dense, slightly moist, light brown, stratified.
e o•
GRAVEL AND COBBLES (GM—GP); sandy, slightly silty, dense, slightly moist, light brown,
•••� subrounded to rounded rock.
�j 2" Diameter hand driven liner sample.
Disturbed bulk sample.
_J
TPractical digging refusal.
NOTES:
•o' .a
Exploratory pits were excavated on
March 25, 2004 with a Cat D3 backhoe.
an§
;•� :DT MC -7.6
Locations of exploratory pits were
�,.:• DD=120
.1
T
LEGEND: .
F� TOPSOIL; silt and sand, organics, moist, dark brown.
SAND (SM); silty to very silty, medium dense, slightly moist, light brown, stratified.
e o•
GRAVEL AND COBBLES (GM—GP); sandy, slightly silty, dense, slightly moist, light brown,
•••� subrounded to rounded rock.
�j 2" Diameter hand driven liner sample.
Disturbed bulk sample.
_J
TPractical digging refusal.
NOTES:
1.
Exploratory pits were excavated on
March 25, 2004 with a Cat D3 backhoe.
2.
Locations of exploratory pits were
measured approximately by pacing from features shown
on the
site plan provided.
3.
Elevations of exploratory pits were
obtained by interpolation between contours on the site
plan
provided.
4.
The exploratory pit locations and elevations should be considered accurate only to the degree
implied by the method used.
5.
The lines between materials shown
on the exploratory pit logs represent the approximate
boundaries between material types
and transitions may be gradual.
6.
No free water was encountered in
the pits at the time of excavating. Fluctuation in water
level
may occur with time.
7.
Laboratory Testing Results:
WC = Water Content ( % )
DD = Dry Density ( pcf )
HEPWORTH—PAWLAK LOGS OF EXPLORATORY PITS
Figure
103 198
GEOTECHNICAL, INC. LOT 5
K
= 7.6
percent
Moisture
Content
Dry
Density
= 120
pcf
Sample
of:
Clayey
Sand
Lens
From:
Pit
2 at
3 Feet,
Lot 5
b\
c
y 1
c
v
CL
X
w
I �
0
NC
N
N 1
N
CQ
E
O
U 2
Expansion
upon
wetting
0.1
1.0
10
100
APPLIED
PRESSURE
—
ksf
103 198
HEPWORTH—PAWLAK I SWELL—CONSOLIDATION TEST RESULTS Figure 3
GEOTECHNICAL, INC.
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