HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.22.2023I (an fl{ffi fi.ffifffinl,rn3;,''
**5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.comAn Employcc O\,vncd Compony
Office Locations: Denver (I{Q), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
December 22,2023
RC Schneider Construction
Attn; Bob Schneider
218 East Valley Road
Carbondale, Colorado 81 623
bhs. austinschneider@icloud. com
Project No.23-7-672
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot22, Filing 9, Elk
Springs, 182 Kingbird Court, Garfield County, Colorado
Dear Bob:
As requested, Kumar & Associates, 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 RC Schneider Construction dated November 27,2023. The data
obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report.
Proposed Construction: Plans for the proposed residence had not been developed at the time of
our study. We assume the proposed residence will be a one- or two-story wood-frame structure
with attached garage located on the site within the building envelope shown on Figure 1.
Ground floors could be structural over crawlspace or slab-on-grade. 'Cut depths are assumed to
range between about 2 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 descibed
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The subject site was vacant with patchy snow cover at the time of our field
exploration. The ground swface was gently sloping down to the southeast at a grade estimated at
between 5 and l0 percent. Vegetation consists ofjuniper trees, sage brush and weeds. There
were basalt cobbles and boulders visible on the surface of the lot.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered in Pits 1 and 3, below about I foot of topsoil,
consist of dense, basalt cobbles and boulders in a calcareous sandy silt matrix to practical
digging refusal depths of 2 and3%feet. Pit 2 encountered I foot of topsoil over hard, highly
calcareous sandy silt to the maximum excavated depth of 9 feet. Results of a gradation analysis
performed on a sample of gravel matrix (minus 3-inch fraction) obtained from the site are
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presented on Figure 3. No free water was observed in the pits at the time of excavation and
the soils were slight$ 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
2,500 psf for support of the proposed residence. The calcareous silt and matrix soils could tend
to compress when wetted under load and result in some post-construction foundation settlement.
Footings should be a minimum width of 16 inches for continuous walls and2 feet for columns.
The topsoil and loose disturbed soils encountered at the foundation bearing level within the
excavation should be removed and the footing bearing level extended down to the undistwbed
natural soils. Cavities caused by the removal of large boulders at foundation bearing level
should be backfilled with a suitable granular material such as CDOT Class 6 base course or
concrete. We should observe the completed excavation for bearing conditions. Exterior footings
should be provided with adequate cover above their bearing elevations for frost protection.
Placement of footings at least 3gg"h"rlelow 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 also be designed to resist alateral earth pressure based on an equivalent fluid
unit weight of at least 50 pcf for the on-site soil as backfill. Cantilevered retaining structures
which are separate from the residence and can be expected to deflect sufficiently to mobilize the
full active earth presswe condition should be designed for a lateral earth pressure computed on
the basis of an equivalent fluid unit weight of at least 40 pcf for backfill consisting of the on-site
soils. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient
of friction of 0.45. Passive pressure of compacted backfill against the sides of the footings can
be calculated using an equivalent fluid unit weight of 400 pcf. The coefficient of friction and
passive pressure values recommended above assume ultimate soil strength. Suitable factors of
safety should be included in the design to limit the strain which will occw at the ultimate
strength, particularly in the case of passive resistance. Backfill should not contain organics or
rock larger than 6 inches.
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 expansionjoints 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 relatively well graded sand and gravel should be placed beneath slabs for support and to
facilitate drainage. This material should consist of minus 2-inch aggregate with less than 50%o
Kumar & Associates, lnc. @ Project No. 23-7-672
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passing the No. 4 sieve and less than l2o/o passing the No. 200 sieve. The gravel layer below
basement slabs (if any) should be free draining with less than2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo 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, or imported granular material such
as road base.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the arcalhat 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 (if any), be protected from wetting and hydrostatic pressure buildup by an
underdrain system.
The drains should consist of 4-inch diameter rigid PVC 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 I foot below lowest adjacent finish
grade and sloped at a minimum lVoto a suitable gravity outlet. Free-draining granular material
used in the underdrain system should contain less than 2olo 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 llzfeet deep and be covered by a filter fabric such as Mirafi 140N or 160N.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
1) Inundation ofthe 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 90Yo of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill should be covered with filter fabric and 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 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.
Kumar & Associates, lnc. o Ptoject No, 23-7-672
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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 I
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 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 verif 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 assistanceo please let us know.
Respectfully Submitted,
Kumar & Associates, lnc.
}r'r?rtp*.P@
James H. Parsons, P.E.
Reviewed by:
Steven L. Pawlak, P.
JHP/kac
attachments Figure 1-Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Gradation Test Results
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Kumar & Associates, lnc. o Project No. 23-7-672
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InIIPR O VEMENT IO CA TION CER TIFICA TEELK SPRINGS, FITING 9
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LOT 24
LOT 25
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23-7-672 Kumar & Associates LOCATION OF EXPLORATORY PITS 1Fig
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LEGEND
TOPSOIL; ORGANIC SANDY SILT AND CI-AY, DARK BROWN, ROOT ZONE.
HIGHLY CALCAREOUS SAND AND SILT (SM-ML); HAn0, SLIGHTLY MOIST, LIGHT BROWN.
BASALT FRAGMENTS lN CALCAREOUS SILT MATRIX (GM); VERY HARD, SLIGHTLY MOIST, LIGHT
BROWN AND BLACK.
DISTURBED BULK SAMPLE.
t PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON DECEMBER 15,2023.
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 NOT MEASURED AND THE LOGS OF THE
EXPLORATORY PITS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY PIT LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTV O +ZZ);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1140);
NP = NON-PLASTIC (ASTM D 4518);
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23-7-672 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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23-7-672 Kumar & Associates GRADATION TEST RESULTS Fig. 3