HomeMy WebLinkAboutB15-0021_Soils Test_1423775820.pdf Hepworth-Pawlak Geotechnical,Inc.
5020 County Road 154
GL7I6SteCh Glenwood Springs,Colorado 81601
Phone: 970-945-7988
Fax:970-945-8454
hpgeo@hpgeotech.com
June 30, 1999
Chip Webster
9 Amelia Drive
Nantucket, Massachusetts 02544 Job No. 199 404
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 10,
Block 9, Intermountain Subdivision, 2755 Snowberry Lane, Vail,
Colorado.
Dear Mr. Webster:
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 you dated May 18, 1999. The
data obtained and our recommendations based on the proposed construction and
subsurface conditions encountered are presented in this report. Interim findings were
reporter: to you in a letter dated May 24, 1999.
Proposed Construction: The proposed residence will be a two story wood frame
structure over a walkout level and located on the site as shown on Fig. 1. Ground floor
will be slab-on-grade. Cut depths are assumed to range between about 4 to 8 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 and Geologic Conditions: The proposed building site is located in the
northeastern part of the lot below Snowberry Lane. The terrain is steep sloping
generally down to the northwest. The lot lies within mapped geologic hazard areas by
the Town of Vail. The mapped hazards consist of the potential for debris avalanche
flow and rockfall. The debris avalanche potential appears limited to the relatively deep
drainage in the western part of the lot outside the building area. The rockfall potential
is mapped as medium severity and apparently limited to the upper part of the lot
extending to the building area. Based on observation of the site conditions, we believe
the rockfall potential risk is probably low and mitigation should not be needed. The lot
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Chip Webster
June 30, 1999
Page 2
could also be susceptible to construction induced slope instability due to the steep
natural slopes and very steep fill slope of Snowberry Lane. Cut and fill depths should
be limited as discussed in then "Site Grading" section of this report. Vegetation
consists of an aspen and evergreen mixed forest with grass and weed understory.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits at the approximate locations shown on Fig. 1. The logs
of the pits are presented on Fig. 2. The subsoils encountered, below about 11/2 feet of
topsoil, consist of clayey sand and gravel with cobbles in Pit 1 and clay and sand with
gravel in Pit 2. The clayey sand and gravel was loose to medium dense and extended to
the Pit 2 depth of 10 feet. The sand and clay was medium dense stiff and underlain by
weathered siltstone/sandstone shale to the Pit 1 depth of 9 feet. Results of swell-
consolidation testing performed on relatively undisturbed samples of the sand and clay
soil from Pit 2, presented on Fig. 3, indicate low to moderate compressibility when
loaded and wetted. Results of a gradation analysis performed on a bulk sample of sand
and gravel from Pit 1 (minus 5 inch fraction) are presented on Fig. 4. 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 very moist with depth and the shale was moist.
Foundation Recommendations: Considering the subsurface conditions encountered in
the exploratory pits and the nature of the proposed construction, we recommend spread
footings placed on the undisturbed natural soil or weathered shale and designed for an
allowable bearing pressure of 2,000 psf for support of the proposed residence. The soils
tend to compress when loaded and wetted and there could be some post-construction
foundation settlement, especially at the transition from soil to shale bearing. The
footings should bear at least 5 feet below the natural ground surface. Footings should
be a minimum width of 16 inches for continuous walls and 2 feet for columns. All
topsoil and loose disturbed materials encountered at the foundation bearing level within
the excavation should be removed and the footing bearing level extended down to the
undisturbed natural soils or weathered shale. Exterior footings should be provided with
adequate cover above their bearing elevations for frost protection. Placement of
footings at least 48 inches below the exterior grade is typically used in this area.
Continuous foundation walls should be well 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
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Chip Webster
June 30, 1999
Page 3
based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil or well
broken shale as backfill.
Floor Slabs: The natural on-site soils, exclusive of topsoil, and the weathered shale 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 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 soils and well broken shale devoid of vegetation, topsoil and
oversized rock.
Underdrain System: Although free water was not encountered during our exploration,
it has been our experience in the area and where bedrock is shallow 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 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 feet deep.
Site Grading: The risk of construction-induced slope instability at the site generally
appears low provided the building is located in the less steep part of the lot as planned
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Chip Webster
June 30, 1999
Page 4
and cut and dill depths are limited. Cuts that extend into the weathered shale could
have a potential to fail on the bedding layers and fractures. We assume the cut depths
for the basement level will not exceed one level, about 8 to 10 feet. Fills should be
limited to about 8 to 10 feet deep and not be placed on natural slopes steeper than 40%.
Embankment fills should be compacted to at least 95% of the maximum standard
Proctor density near optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing all vegetation and topsoil and compacting to
95% standard Proctor density. The fill should be benched into the portions of the
hillside exceeding 20% grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1
vertical or flatter and protected against erosion by revegetation or other means. The
risk of slope instability will be increased if seepage is encountered in cuts and flatter
slopes may be necessary. If seepage is encountered in permanent cuts, an investigation
should be conducted to determine if the seepage will adversely affect the cut stability.
We should observe the building excavations to evaluate slope stability conditions.
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 of the 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.
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
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Chip Webster
June 30, 1999
Page 5
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.
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.
Sincerely,
.0
1.
,10 RL Q,I isms
HEPWORTH - • W, vow:" INC.
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David A. Young, P.E.®31O
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Reviewed By: '�,,'Os s L tN a�o�
Steven L. Pawlak, P.E.
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APPROXIMATE SCALE
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LEGEND:
TOPSOIL; sandy silty clay, gravelly, organic, very moist, black.
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SAND AND CLAY (SC-CL); gravelly, scattered cobbles, medium dense/stiff, moist, brown.
o GRAVEL AND SAND (GC-SC); clayey, with cobbles and small boulders, loose to medium dense
eda with depth, moist to very moist, brown.
SILTSTONE/SANDSTONE; weatherd to medium hard, moist, gray-brown. Minturn Formation.
ILg 2" Diameter hand driven liner sample, f
jI Disturbed bulk sample.
NOTES:
1. Exploratory pits were excavated on May 19, 1999 with a backhoe.
2. Locations of exploratory pits were measured approximately by pacing from features on the site
plan provided.
3. Elevations of the 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.
Fluctuations in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content ( % )
DD = Dry Density ( pcf )
+4 = Percent retained on No. 4 sieve
-200 = Percent passing No. 200 sieve
199 404 HEPWORTH - PAWLAK LOGS OF EXPLORATORY PITS Fig. 2
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Moisture Content = 18.3 percent
Dry Density = 102 pcf
1 . Sample of: Sand and Clay
From: Pit 2 at 2 Feet
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APPLIED PRESSURE — ksf
Moisture Content = 26.7 percent
Dry Density = 105 pcf
Sample of: Sandy Clay
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1199 404 FiEPWORTH — PAWLAK SWELL—CONSOLIDATION TEST RESULTS Fig. 3
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199 404 HEPWORTH — PAWLAK GRADATION TEST RESULTS Fig. 4
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