HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.15.2020l$l[ffiå'ffi*ffin*ü--5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.conrÅn façlaVca ûrnød Compaly
Oflice Locations: Denver (HQ), Parke¡ Colorado Springs, Fort Collins, Glenwood Springs, and Sunrmit County, Colorado
December 15,2020
Amanda Poindexter
P.O. Box 3653
Aspen, Colorado 81612
arnantla@ fu llclrcl cconstructi on. org
Project No. 20-7-658
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 62, Filing 5,
Elk Springs, 154 Crescent Lane, Garfield County, Colorado
Dear Amanda:
As requested, Kumar & Associates 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 Octob er 28, 2020 . The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report. We previously provided a summary soils report to you in a letter report
dated November 10, 2020.
Proposed Construction: Building plans for the proposed residence had not been developed at
the time of our study. In general, the residence will be a single-story structure with a walkout
lower level and slab-on-grade floors. The residence will be located in the eastern part of the
building envelope shown on Figure 1. Cut depths are assumed to range up to about 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 lot was vacant at the time of our field exploration. Vegetation consists of
a pinon and juniper forest with sage brush and sparse grass and rveeds. The ground surface is
gently sloping down to the south-southwest at a grade of around 5 percent. Numerous basalt
cobbles and boulders were exposed on the ground surface.
Subsurface Conditions: The subsurface conditions at the site were bvaluated by observing
three explorctory 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 foot of topsoil, consist of basalt
cobbles and boulders in a sand and silt matrix down to pracfical digging refusal at the depths
explored of 2 to SYz feet. No free water was observed in the pits at the time of excavation and
the soils were slightly moist.
1
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 basalt rock soil designed for an allowable soil bearing pressure
of 2,500 psfjor support of the proposed residence. Footings should be a minimum width of
16 inches for continuous walls and2 feet for columns. Loose disturbed soils encountered at the
foundation bearing level within the excavation should be removed to expose the undisturbed
nattnal soils. Our experience in this area is that a conventional heavy-duty trackhoe used for
excavating for the foundation should be able to get down about 2 feet deeper than our pits.
Deeper excavating may require chipping or blasting. Voids created by boulder removal should
be backfilled with well-graded granular material such as road base compacted to at least 98% of
standard Proctor density atnear optimum moisture content. 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 alateral earth pressure based on an equivalent fluid unit weight of at least 50
pcffor granular soil as backfill excluding organics and rock larger than 6 inches. A sliding
coefficient of 0.45 and passive earth pressure of 400 pcf equivalent fluid unit weight can be used
to resist lateral loading on foundation walls.
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 basement level slabs. This material
should consist of minus 2-inch aggregate with less than 50Yo passing the No' 4 sieve and less
than2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill should consist of
granular soils devoid ofvegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the areathat local perched groundwater can develop during times of
Kumar & Associates, lnc. @ Project No.20-7-658
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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 matenal. 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 Io/oto
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yopassingthe 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 I1/z 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 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 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90%o of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill should be capped with about 2 feeL 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.
Limitations: This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area atthis 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,theproposed type of construction, and our experience in
the area. Our services do not include determining the presence, prevention or possibility of mold
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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 inelude interpolation and extrapolation of the subsurface conditions identified at the
exploratory pits and variations in the subsurface conditions may not become evident until
excavation isryerformed. If conditions encountered during construction op¡rear different fmrn
those describcd in this report, we should be riotilied at once so re-e-valuation of the
recommendations may be made.
This report has been prepared for the cxclusive use by our client fbr desigrr purposes. rff'e are not
responsible for technical interpretations by others of our information. As the project evolves, we
shoutd provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to veriff that the recommendatìons
have been appropriately inrerpreted. Significant design changes may require additional analysis
or modi{ìcations to the recammendations presented herein. We recommend on-site obseruation
of excavations and foundation beming strata and testing of struch¡ral fill by a representative of
the geotechnieal engineer,
lf you have any questions or if we may be of tì¡rther assistance, please let us know'
Respectfu I ly Subrnitted,
Kumar& Assotri*tes,
Daniel E. Hardin, P.E,
Reviewed by:
Ëe*;{-6?.-*r{.
Steven L. Pawlak, P.Ë.
Attachments: Figure 1 * Location of Exploratory Pits
Figure 2 - Logs of ExPloratory Pits
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20-7 -658 Kumar & Associates LOCATION OF IXPLORATORY PITS Fig. 1
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LEGEND
TOPSOIL; ORGANIC SANDY SILTY CLAY WITH ROOTS, GRAVEL SIZE BASALT ROCK' SOFT'
MOIST, DARK BROWN.
cLAy (cL); sANDy, StLTy wtTH CoBBLE--SIZE BASALT ROCKS, MEDIUM DENSE' SLIGHTLY
MOIST, LIGHT BROWN.
F.1|rts¡tT RocKS (cM): UP TO 2 FEET lN SlzE lN SANDY SILT MATRIX, CALCAREOUS, MEDIUM
Kl ot*.t, sLtcHTLi Mblsr, wHlrlsH-GRAY.
I enlcrrcAl DrcctNG REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON NOVEMBER 4' 2O2O'
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 VITENI¡I 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.
20-7 -658 Kumar & Associates LOGS OF' EXPLORATORY PITS fig. 2
