HomeMy WebLinkAboutSubsoils Study for Foundation DesignlGrtfj|,çl[,åi':'fËtrr':nÊü'**
An Employcc Ownad Conrpcny
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
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Office Locations: Denver (HQ), Parkea Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
November 15,2022
Hari and Christy Dallakoti
2457 West Sunset Drive
Littleton, Colorado 80120
hdal lakoti @hotrrrai l.com
Job No. 22-7-581
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 37 , Filing 9,
Elk Springs, 0095 Vista Place, Garfield County, Colorado
Hari and Christy:
As requested, Kumar and 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 August 16,2022. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report.
Proposed Construction: The proposed residence will be a single-story, wood frame structure
over crawlspace with an attached slab-on-grade garage located as shown on Figure l. Cut depths
could 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 described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The site is vacant and vegetated with grass, weeds and juniper trees. The lot is
bordered on the east by Vista Place, and on the west by a common area. The lot is near a natural
hilltop and slopes moderately down to the south. The ground surface is natural with basalt
cobbles and boulders visible on the surface.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits at the approximate locations shown on Figure l. The logs of the pits are
presented on Figure 2. Below about I to l% feet of topsoil, dense basalt gravel, cobbles and
boulders in a calcareous sandy silt matrix was encountered down to the maximum explored depth
of 3/z feet. Digging in the basalt rock was difficult due to the cobbles and boulders and practical
digging refusal was encountered in the deposit in all three pits at depths of 2% to 3/z feet.
Results of a gradation analysis performed on a sample of the matrix soils (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.
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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 2,000 psf for support of the proposed residence. Footings should be a minimum width of
l6 inches for continuous walls and 2 feet for columns. The topsoil, and any 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. Utility trenches and deep
cut areas may require rock excavating techniques such as chipping and blasting. Voids created
from boulder removal should be backfilled with road base compacted to at least 95olo of standard
Proctor density at a moisture content near optimum or with concrete. We should observe the
completed foundation excavation for bearing conditions. Exterior footings should be provided
with adequate soil 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 heavily reinforced top and bottom to span local anomalies such as by
assuming an unsupported length of at least l0 feet. Foundation walls acting as retaining
structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit
weight of at least 45 pcf for the on-site soil or imported gravel as backfill, excluding organics
and 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 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 to facilitate drainage.
This material should consist of minus 2-inch aggregate with less than 50%o passing the No. 4
sieve and less than 2o/o passing the No. 200 sieve. The 4-inch gravel layer placed below slab-at-
grade such as the garuge can consist of t/¿-inch road base.
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 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 mountainous areas that 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, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
Kumar & Associates, lnc. @ Project No. 22-7-58'l
-3-
The drains should consist of drainpipe placed in the bottom of the wall backfill sumounded 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 frnish grade and sloped at a minimum lYoto
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2%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 lt/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 95%o 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 l2 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in pavement and walkway areas.
4) Roof downspouts and drains should discharge wellbeyond 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 assumed 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 fìndings
include interpolation and extrapolation of the subsurface conditions identihed 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.
Kumar & Associates, lnc. @ Project No, 22-7-581
4
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 verifu 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 fuither assistance, please let us know
Respectfully Submitted,
Kunrar'& .4ssociates,
Steven L. Pawlak, P
Reviewed by:
Daniel E. Hardin, P.E.
Attachments: Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Gradation Test Results
Cc; Focal Studio - Tim Owen (tim{*ìfc¡çSlstu¡1ìearch.çc.¡ur)
Kumar & Associates, lnc. "Froject No, 22-7-58',1
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APPROXIMATE SCALE-FEET
22-7 -581 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
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PIT 1
EL. 7143'
PIT 2
EL. 7144'
PIT 3
EL. 71 50'
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LEGEND
TOPSOIL; oRGANlc SANDY SILT AND CLAY, SCATTERED ROCK, FIRM, SLIGHTLY MOIST, BROWN.
BASALT COBBLES AND BOULDERS (GM); HIGHLY CALCAREOUS SAND AND SILT MATRIX, VERY
HARD, SLIGHTLY MOIST, WHITE AND GRAY.
DISÏURBED BULK SAMPLE
I PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON NOVEMBER 10, 2022.
2. THE LOCATIONS OF THE EXPL'ORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SIÏE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
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.
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:+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 1 4O).
22-7 -581 Kumar & Associates LOGS OF EXPLORATORY PITS Fis. 2
3
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,o75 t50 .30O i .AOO l.1a i 2.36.123 2,O
PARTICLES IN MILLIMETERSDIAMETER OF
CLAY TO SILT COBBLES
GRAVEL 30 % SANO 51 %
LIQUID LIMIT - PLASTICITY INDEX
SAMPLE 0F: Hlghly Colcoreous Silly Sond Mdtrlx
SILT AND CLAY 19 %
FROM: Pil 3 O 3' -3.5'
Th!s. l.sl rcsults opply only lo lho
sompl.3 whlch w!ru l.slrd. Th.
losllng r.pori 3holl not b. r.produc.d,.xc!pì ln full, wllhoul lho wrlll.n
opprovol of Kumqr & Aagoclqhc, lnc.
Slav. oñdlysls l!3llng b parformad ln
occordonce wlth ASTM 059t5, ASÍM D7928,
ASTM Cl36 ond/or ASÍM Dll¡10.
I{YDROMElER ANALYSIS SIEVE ANALYSIS
TIME RE OINOS
24 HRS 7 HRS
U.S. SIANDARD SERIES CLAR SOUARE OPENI¡GS
atÀ6 a/t¡ t 1/.â
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SAND GRAVEL
FINE MEDIUM IcoARSE FINE COARSE
22-7 -581 Kumar & Associates GRADATION TEST RESULTS Fig. 3