HomeMy WebLinkAboutSubsoils Study for Foundation DesignI Crf iiçh:#flfËtrn[,Í å *' "
An Employcc Owncd Gompony
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
March 16,2023
Michael Jelks
1551 South Gilpin Street
Denver, Colorado 80210
mjelks@comcast.net
Project No. 23-7-166
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot36, Filing 9,
Elk Springs, 57 Grosbeak Place, Garfield County, Colorado
Dear Mr. Jelks:
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 you dated February 22,2023. 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 residence were not available at the time of our f,reld
exploration. The proposed residence is assumed to be a one- or two-story wood-frame structure
over a lower walkout level with atiached garcge 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 and covered with about 2 feet of snow at the time
of our field exploration. The ground surface is sloping down to the south with a grade estimated
at about 15 percent. Vegetation consisted ofjuniper trees, sagebrush, grass and weeds.
F-#
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 about I to l% feet of topsoil, consist of
dense basalt gravel, cobbles and boulders in a hard, highly calcareous sandy silt with gravel
matrix. Results of a gradation analysis performed on a sample of the sandy silt and gravel matrix
.,
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(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 moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placedontheundisturbednaturalsoildesignedforanallowab1ebearingpresSureof€
for support of the proposed residence. The matrix soils tend to compress after wetting under
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_]flirylgþlow 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 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 supporl lightly loaded
slab-on-grade construction. Slab subgrade can be re-established with suitable onsite soils or
imported %-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% passing the No. 4 sieve and less than I2%o passing the No. 200 sieve. Gravel
below the basement floor slab should be free draining with less thanTo/o passing the No. 200
sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/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 areathat local perched groundwater can develop during times of
Kumar & Associates, lnc. @ Project No. 23-7-166
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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, crawfspace 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 a$acent f,rnish grade and sloped at a minimum l%o fo
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 Ilz 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 95o/o 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 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 sunounding 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 f,rrst 10 feet in unpaved areas and a minimum slope of
3 inches in the first 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 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 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
Kumar & Associates, lnc, @ Project No. 23-7-166
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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
findlngs 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
recofirmendations may be made.
This report has been prepared for the exclusive use by our client for design pu{poses. 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 recoflrmendations, 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 assistance, please let us know.
Respectfully Submitted,
Kumar & Associates,
Daniel E. Hardin, P.E.
Reviewed by:
Steven L. Pawlak, P.E.
DEH/kac
attachments
cc
Figure 1 - Location of Exploratory Pits
Figure 2 -Logs of Exploratory Pits
Figure 3 - Gradation Test Results
Table 1 - Summary of Laboratory Test Results
S tephanie H ernandez ( shernandez2 00 1 f¿D.hotmai L com)
Cassondra Westerlind (cassondra@olaÛ ean. com)
Kumar & Associates, lnc. ft Project No, 23-7-166
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APPROXIMATE SCALE-FEET
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23-7 -1 66 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
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PIT 1 PIT 2 PIT 5
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TOPSOIL; ORGANIC SANDY SILT AND CLAY, MOIST/FROZEN, BROWN.
BASALT COBBLES AND BOULDERS (GM); CALCAREOUS SILT, SAND AND GRAVEL MATRIX,
DENSE, MOtST, LIGHT BROWN/WHITE AND GRAY.
DISTURBED BULK SAMPLE
I PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON FEBRUARY 23, 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
7. LABORATORY TEST RESULTS:
Wc = WATER OONTENT (%) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D a22);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 1 4O).
23-7 - 1 66 Kumar & Associates LOGS OF EXPLORAÏORY PITS Fig. 2
SIEVE ANALYSISHYDROMETER ANALYSIS
CLEAR SQUARE OPENINGSSTANDARD SÊRIES
I MIN T'2¿ HRS 7 HRS¡5 UtN tl vlN
TIME READINCS
60vtN lgMtN 4MtN
11
-+--
100
90
ao
70
60
50
/ao
30
20
10
0
l0
20
30
10
50
60
70
80
90
loo
=
.t50 152
ROF IN MI RS
CLAY TO SILT COBBLES
GRAVEL 54 % SAND
LIQUIO LIMIT
SAMPLE OF: Sîlly Sondy Grovel (Motrix)
34%
PLASTICITY INDEX
SILT AND CLAY 12 %
FROM: Pit 1 @ 1.5'-2'
Thos6 losl rosulls opply only lo lh6
sqmplss wh¡ch wqro losl6d, lh€
losllng roporl sholl not be raproducod,
excopl ln full, wlthoul lho wr¡tl€n
opprovol of Kumor & Assoçiqles, lnc.
Slovô qnolysls losllng ls performed ln
occordqnce wlth ASTM 06913, ASTM D7928,
ASTM c136 ond/or ASTM Dl140,
GRAVELSAND
FINE COARSEFI NE MEDIUM COARSE
23-7 -166 Kumar & Associates GRADATION TEST RESULTS Fig. 3
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Silty Sandy Gravel
(Matrix)
SOIL TYPE
21345431.5lYzto 2I
AÏTERBERG LIMITSGRADATIONSAMPLE LOCATION
DEPTHPIT LIQUID LIMIT
UNCONFINED
COMPRESSIVE
SÏRENGTH
PERCENT
PASSING NO.
200 stEVE
NATURAL
DRY
DENSITY
NATURAL
MOISTURE
CONTENT
SAND
(%)
GRAVEL
(%l
PLASTIC
INDEX
Pro No.23-7-166