HomeMy WebLinkAboutSubsoil Study for Foundation Design 06.24.2021lGrtiffiïffiffirx'å*'-
An Empknpc Cn¡nod CorçcnY
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
June24,202l
Dave and Sheryl Babiarz
6626BigHorn Trail
Littleton, Colorado 80 125
dbabiarz_@pqlsinelltçom
Project No. 2l-7-481
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 35, Filing 9,
Elk Springs, Grosbeak Place, Garfield County, Colorado
Dear Dave and Sheryl:
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 proposal for geotechnical
engineering services to you dated May 2I,202I. 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 residence were not available at the time of our held
exploration. The proposed residence is assumed to be a one- or two-story wood-frame structure
over a lower walkout level with attached garuge located roughly between the exploratory pits
shown on Figure 1. Ground floor could be slab-on-grade or structural above crawlspace. Cut
depths are expected to range between about 2 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 Conditions: The subject site was vacant at the time of our flreld exploration. The ground
surface is sloping down to the south with a grade estimated at about l0 to 15 percent. Vegetation
consisted of juniper trees, sagebrush, grass and weeds. The juniper was primarily growing in
two shallow, dry, drainage swales that merge near the lower portion of the lot. Basalt cobbles
and boulders were exposed on the ground surface.
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, below aboutVzto l% feet of topsoil, consist of
dense, basalt gravel, cobbles and boulders in a hard, highly calcareous sandy silt matrix. Results
-2
of a gradation analysis performed on a sample of the sandy silt and gravel matrix (minus 3-inch
fraction) obtained from the site are presented on Figure 3. No free water was observed in the pits
at the time of excavation and the soils were slightly moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction' we recommend
placed on the undisturbed natural soil designed for an allowable bearing of 2,000 psf
for support of the proposed residence. The matrix soils tend to compress
load and there could be 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 undisturbed natural soils. Exterior
footings should be provided with adequate cover above their bearing elevations for frost
protection. Placement of footings at least 36 the exterior grade is typically used in
this area. Continuous foundation walls be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least t2 feet. Foundation walls
acting as retaining structures should be designed to resist alaterul earth pressure based on an
equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill excluding organics
and rock fragments larger than 6 inches.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded
slab-on-grade construction. Slab subgrade can be re-established with suitable onsite soils or
imported 3/o-inch road base sand and gravel. 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 relatively well graded sand and gravel such as road base should
be placed beneath slabs for support. This material should consist of minus 2-inch aggregate with
less than 50%o passing the No. 4 sieve and less than l2%o passing the No. 200 sieve.
All fîll materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils or imported granular soils 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 that local perched groundwater can develop during times of
Kumar & Associates, lnc. @ Project No. 21-7-481
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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 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 lYoto
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2o/o 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.
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 95o/o 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 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the flrrst 10 feet in pavement and walkway areas. A swale should be
provided uphill to direct surface runoff around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy inigation 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 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
Kumar & Associates, lnc. @ Project No. 2'l-7-481
4
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 ofpractice 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 c,onstruction 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 prçpared 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 veriff 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.
Respectfully Submitted,
Kumar &,4"ssoeintes,
James H. Parsons, P.E.
Reviewed by:
ffi.-/.
Steven L. Pawlak, P.E.
JHPlkac
attachments Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory pits
Figure 3 - Gradation Test Results
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LOT 36
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APPROXIMATE SCALE-FEET
21 -7 -481 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
PIT 1 Ptl 2 PIT 3
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LEGEND
TOPSOIL; SILT AND SAND, COBBLES, BOULDERS, 0RGANICS, FIRM, SLIGHTLY MOIST, MIXED
BROWN.
BASALT GRAVEL (GM); COBBLES, BOULDERS, HIGHLY CALCAREOUS SANDY SILT MATRIX,
DENSE, SLIGHTLY MOIST, PALE_TAN.
1
Ij
I
DISTURBED BULK SAMPLE.
PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A MINI EXCAVATOR ON MAY 26,2021.
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 ÏYPES 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:
wc = wATER CONTENT (Z) (¡SrV D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ISTU O +ZZ);
-200= PERCENTAGE PASSING No. 200 SIEVE (ASTM D 1 1 40).
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21 -7 -481 Kumar & Associates LOGS OF EXPLORATORY PITS Fis. 2
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DIAMETER OF PARTICLES IN MI RS
CLAY TO SILT COBBLES
GRAVEL 42 % SAND 36 %
LIQUID LIMIT - PLASTICITY INDEX
SAMPLE OF: Colcoroôus Sondy Sill ônd Grovel Mdlrlx
SILT AND CLAY 22 %
FROM: Plt 1 O 1.5' to 2'
Th6t6 lôsl ræull. dpply only lô lh.
3ompl.s whlch wrr. l.8l.d. Th6
lcallhg r.porl shôll nol b6 rcproduccd,
6xc€pl ln full, wllhoul lhê vrltl€n
opprcvql ot Kumor & Acroclolor, lnc.
Sld6 onolyll¡ l.sllng l. pÊrform.d ln
occôrdonc6 wlth ASTV 06913, ASTV 07928,
ASIM Cl36 ondlor ASTM Oll,ao.
HYDROMETÉR ÀNÀLYSIS SIEVE ÀNALYSIS
u.s. sl ND^Ro sERtEs CLEAR SOUARE OPEHIXOS
tlre .r^ü I tltq24 HRS 7 HRS
!MIN 11
frvÊ R€AD|NôS
aôutN tautl ¡MlN
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SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
21 -7 -481 Kumar & Associates GRADATION TEST RESULTS Fig. 3