HomeMy WebLinkAboutSubsoils Study for Foundation Design 08.26.16H-P~l<UMAR 5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
August 26, 2016
Crawford Design Build, LLC
Attn: Simon Bentley
P .0. Box 1236
Carbondale, Colorado 81623
( cdbsimon@comcost.net)
Offtce locations· Parter, Glenwood Springs, and Silverthome, Colorado
Project No.16-7-143
Subject: Subsoil Study for Foundation Design, Proposed Residence and Studios, Lot 2,
Roaring Fork Preserve, 20 Silver Spruce Drive, Garfield County, Colorado
Dear Mr. Bentley:
As requested, H-P/Kumar performed a subsoil study for design of foundations at the subject site.
The study was conducted in accordance with our agreement for geotcchnical engineering
services to Crawford Design Build, LLC dated June 27, 2016 . The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report . Hepworth-Pawlak Geotechnical (now H-P/Kumar) previously
perfonned a preliminary geotechnical study for the subdivision development and reported their
findings November 30, 2003, Job No . 103 201.
Proposed Construction: The proposed residence will be two story wood frame construct ion .
Two studio buildings nre proposed and will be two story wood frame structures. The buildings
will be located on the site as shown on Figure 1. Ground floors will be slab-on-grade. Cut
depths are expected to range between about 2 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 property is vacant and vegetated with scattered evergreen nnd cottonwood
trees, brush, grass and weeds . The ground surface is relatively flat with a slight slope down to
the northwest. There is about 2 feet of elevation difference across the building envelope. An
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active irrigation ditch crosses the front part of the site and the Roaring Fork River borders the
rear of lhe site.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits at the approximate locations shown on Figure t. The logs of the pits are
presented on Figure 2 . The subsoils encountered, below about 6 inches of topsoil, consist of silty
sandy gravel with cobbles and small boulders. Results of a gradation analysis performed on a
sample of sandy gravel (minus 5-inch fraction) obtained from the site are presented on Figure 3.
Free water was observed in Pits1and2 at depths of9 and 61/l feet, respectively at the time of our
s ite visit and the upper soils were slightly moist to moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction. "fie recommend spread footings
placed on the und isturbed natural gravel soil designed for an allowable soil bearing pressure of
2,500 psf for support of the proposed residence and studios. The soils should have relatively low
compressibility and post-construction foundation settlement should be relatively minor.
Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns.
The topsoil and loose disturbed soils encountered at the foundation bearing level within the
excavations should be removed and the footing bearing level extended down to the undisturbed
natural gravel soils. Exterior footings should be provided with adequate cover above their
bearing elevat ions for frost protection. Placement of footings at Jeast 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 I 0
feet. Foundation walls acting as retaining structures (if any) should be designed to resist a lateral
earth pressure based on an equivalent fluid unit weight of at least 45 pcf for the onsite gravel 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-dra ining gravel should be placed beneath interior 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.
HP. KUMAR
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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 gravel soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: It is our understanding the finished floor elevation at the lowest level of
the buildings will be at or above the surrounding grade. Therefore, a foundation drain system is
not required. It has been our experience in the area that the water level can rise and 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 or crawlspace areas, be protected from wetting and
hydrostatic pressure buildup by an underdrain and wall drain system.
If the finished floor elevation of the proposed structures has a floor level below the surrounding
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence and studios have been completed:
I) 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
al least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the ma.-ximum 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 I 0 feet in unpaved areas and a minimum slope of 3
inches in the first I 0 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
feel 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 detennining the presence, prevention or possibility of mold
or other biological contaminants (MOBC) developing in the future. If the client is concerned
H-P:. KUMAR
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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 perfonned. 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 infonnatfon. 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 off urther assistance, please let us know.
Respectfully Submitted,
H-P~KUMAR
Louis Eller
Reviewed by:
Attachments: Figure I -Locution of Exploratory Pits
Figure 2 -Logs of Exploratory Pits
Figure 3 Gradation Test Results
H-P-KUMAR
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ff 16-7-143 H-P~ KUMAR LOCATION Of' EXPLORATORY PITS f'lg. 1 ,,
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~TOPSOIL: ORGANIC SANDY SILT WITH GRAVEL, FIRM, SLIGHTLY MOIST, DARK BROWN.
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GRAVEL, COBBLES AND BOULDERS (CP-GM); SANDY, SLIGHTLY SILTY, DENSE, SLIGHTLY MOIST
TO WET WITH DEPTH, LIGHT BROWN, ROUNDED ROCKS.
t] DISTURBED BULK SAMPLE .
~ DEPTH TO WATER LEVEL AT TIME OF' OBSERVATION.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON AUGUST 12, 2016.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3 . THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED FROM INTERPOLATION or
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.
S. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MAT£RIAL TYPES ANO THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS ENCOUNT£R£0 IN THE PITS AT THE TIME or OUR OBSERVATION.
FLUCTUATION IN WATER LEVELS COULD OCCUR WITH TIME.
7. LABORATORY TEST RESULTS :
+4 :;;:: PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
-200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
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