HomeMy WebLinkAboutSoils report.pdfHEPWORTH - PAWLAIK GEOTECHNICAL
August 12, 2013
Crawford Design Build, LLC
Attn: Brad Crawford
P.O. Box 1236
Carbondale, Colorado 81623
(crawford des ign comcast.net)
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Fax: 970-945-ti434
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Job No.113 278A
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 12,
Callicotte Ranch, Garfield County, Colorado
Dear Mr. Crawford:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design
of foundations at the subject site. The study was conducted in accordance with our
agreement for geotechnical engineering services to Crawford Design Build, LLC dated
July 31, 2013. 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 performed a preliminary geotechnical
study for the Callicotte Ranch Subdivision and reported our findings on April 19, 2002,
Job No. 101 821.
Proposed Construction: The proposed residence will be one story wood frame
construction with an attached garage. The residence will be located on the site as shown
on Figure 1. Garage floor will be slab -on -grade. Cut depths are expected to range
between about 3 to 6 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 vacant lot is vegetated by a pinion and juniper forest with a ground
cover of sparse grass, weeds and cactus. The ground surface in the building area slopes
down to the west at a grade of about 15 percent. Scattered basalt cobbles and boulders
are visible on the ground surface.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
observing three exploratory pits at the approximate locations shown on Figure 1. The
Parker 303-841-7119 e Colorado Springs 719-633-5562 ® Silverthorne 970-468-1989
-2-
Iogs ofthe pits are presented on Figure 2. The subsoils encountered, below about 1.2 to
18 inches of topsoil, consist of basalt cobbles and boulders in a gravelly sand and silt
matrix. Digging in the basalt rock was reportedly difficult and practical refusal to
excavating with a mini -excavator encountered in the pits. Results of swell -consolidation
testing performed on relatively undisturbed samples ofthe matrix soils, presented on
Figures 3 and 4, indicate low compressibility under existing moisture conditions and Light
loading and a moderate collapse potential when wetted. 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 2,000 psf for support of the proposed residence. Footings should be a
minimum width of 16 inches for continuous walls and 2 feet for columns. Utility
trenches and deep cut areas below about 3 feet may require rock excavating techniques
such as chipping or blasting. Loose and 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. Voids created from boulder removal at
footing grade should be filled with concrete or a structural material such as road base
compacted to 98 percent standard Proctor density at a moisture content near optimum.
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 wails 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. 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 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.
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
create a perched condition. We recommend below -grade construction, such as retaining
Job No.113 278A
-3 -
walls, crawlspace and basement areas, be protected from wetting and hydrostatic pressure
buildup by an underdrain system. Where interior slab -on -grade floors are near exterior
grade elevation, an underdrain should not be needed.
Where installed, 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 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. 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 ofthe building should be
sloped to drain away from the foundation in all directions. We
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 may be needed uphill to direct surface runoff
around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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 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. 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
Job No.1 I3 278A
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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
bearing strata and testing of structural fill by a representative ofthe 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.
Louis E. Eller
Reviewed by:
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Daniel E. Hardin, P.E. • S
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attachments: Figure 1— Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figures 3 and 4 — Swell -Consolidation Test Results
Table 1 — Summary of Laboratory Testing
Job No.113 278A
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APPROXIMATE SCALE
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113 278A
H
Hepworth—Pc:Wok Geotechnical
LOCATION OF EXPLORATORY PITS
Figure 1
0
10
LEGEND:
PIT 1
E LEV. = 6773'
WC=14.6
DD=62
-200=74
WC=24.9
DD=63
PIT 2
ELEV.= 6777'
PIT 3
ELEV.� 6780'
Tv,
f
0
5
10 —
pTOPSOIL; organic sandy silt and clay with scattered cobbles and boulders, firm, slightly moist, dark brown.
NOTES:
BASALT COBBLES AND BOULDERS (GM -GC); in a gravelly sandy silt and clay matrix, dense, moist, light
brown, calcareous.
2" Diameter hand driven liner sample.
1. Exploratory pits were observed on July 31, 2013.
2. Locations of exploratory pits were measured approximately by pacing from features shown an the site plan
provided.
3. Elevations of exploratory pits were obtained by Interpolation between contours shown 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 Togs 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)
-200 - Percent passing No. 200 sieve
113 278A
Hepworth—Pawlak Geotechnical
LOGS OF EXPLORATORY PITS
Figure 2
Compression %
1
0
1
2
3
4
5
6
7
8
9
10
11
Moisture Content = 14.6 percent
Dry Density = 62 loaf
Sample of: Sandy Silt and Clay Matrix
From: Pit 1 at 1 a Feet
,
.�v
Compression
upon
wetting
0.1 1.0 10 100
APPLIED PRESSURE - ksf
113 278A
eft F -I Ch
Hepworth—Port k Oeotechnioal
SWELL -CONSOLIDATION TEST RESULTS
Figure 3
Compression °
0
1
2
3
4
5
6
7
8
9
10
11
Moisture Content = 24.9 percent
Dry Density = 63 pcf
Sample of: Sandy Silt and Clay Matrix
From: Pit 1 at 2 Feet
Compression
upon
wetting
,
0.1
1.0
APPLIED PRESSURE - ksr
10 100
113 278A
1"I
Hepworth—Pawlak Geotechnleal
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 113 278A
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
MI) _
!NATURAL
DRY
DENSITY
(Pc)
GRADATION
PERCENT
PASSING
SIEVE 200
SIEVE
-.
ATTERRERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
Ste)
SOIL TYPE
PIT
DEPTH
(ft)
GRAVEL
(%)
SAND
(%)
-
LIQUID
LIMIT
06)
PLASTIC
INDEX
(%)
1
1 1/2
14.6
62
74
Sandy Silt and Clay Matrix
2
24.9
63
Sandy Silt and Clay Matrix