HomeMy WebLinkAboutSubsoils Study for Foundation Designrcnfliffif;ffifffinii:'i'*.
An Employcc olflncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 15, RAPIDS ON THE COLORADO
RAPIDS \TIEW LANE
GARFIELD COUNTY, COLORADO
PROJECT NO.25-7-111
FEBRUARY 5,2025
PREPARED FOR:
KENIA CARDENAS
1534 EAST 12th STREET
RTFLE, COLORADO 81650
keniacardenas@live.com
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION 1
SITE CONDITIONS......1
FIELD EXPLORATION..1
STIBSURFACE CONDITIONS .a
FOIINDATION BEARING CONDITIONS .,,.,.- 2 -
DESIGN RECOMMENDATIONS........,,.. ........- 2 -
FOUNDATIONS 2-
FOUNDATION AND RETAINING WALLS ........,.......,- 3 -
FLOOR SLABS 4-
LIMITATIONS....6-
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - USDA GRADATION TEST RESULTS
FIGURE 7 - GRADATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No.25-7-111
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 15, Rapids on the Colorado, Rapids View Lane, Garfield County, Colorado. The project
site is shown on Figure 1. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to Kenia Cardenas dated January 7,2025.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification, compressibility or
swell and other engineering characteristics. The results of the field exploration and laboratory
testing were analyzed to develop recommendations for foundation types, depths and allowable
pressures for the proposed building foundation. This report summarizes the data obtained during
this study and presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered,
PROPOSED CONSTRUCTION
Plans for the proposed residence were conceptual at the time of our study. The proposed
residence is assumed to be a one- or two-story structure with an attached garage. Ground floors
could be structural over crawlspace or slab-on-grade. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 4 feet. We assume relatively light
foundation loadings, typical of the proposed type of construction.
When building location, grading and loading information have been developed, 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 field exploration. The ground surface was gently
sloping down to the north at grades less than around 5 percent. The Colorado River borders the
lot to the north. Vegetation consists of grass and weeds.
FIELD EXPLORATION
The field exploration for the project was conducted on January 14,2024. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4-inch diameter continuous flight augers powered by a truck-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Kumar & Associates, lnc. @ Project No.25-7-111
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Samples of the subsoils were taken with l%-inch and 2-inch I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 140-pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our
laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered below about 1 foot of topsoil consist of medium dense silty sand to about
4Yz feet deep overlying dense, silty sandy gravel and cobbles to the maximum drilled depth
of 7% feet. Drilling in the coarse granular soils with auger equipment was difficult due to the
cobbles and boulders and drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and. gradation analyses. Results of swell-consolidation testing performed on
relatively undisturbed drive samples of the upper silty sand soil, presented on Figures 4 and 5,
indicate low to moderate compressibility under conditions of loading and wetting. Results of
gradation analyses performed on a small diameter drive sample (minus l%-inch fraction) of the
coarse granular subsoils are shown on Figure 7. The laboratory testing is summarizedin Table 1
No free water was encountered in the borings at the time of drilling and the soils were slightly
moist to moist.
FOT]NDATION BEARING CONDITIONS
The upper silty sand soils encountered in the borings possess low bearing capacity and
low to moderate settlement potential especially when wetted under load. The underlying coarse
granular soils possess moderate bearing capacity and typically low settlement potential. At
assumed excavation depths, we expect the exposed subsoils to consist primarily of silty sand.
The proposed residence can be supported on spread footings bearing on the natural soils with a
risk of differential settlement due to the variable bearing conditions of the silty sand soils and
possibly the gravel subsoils. A lower risk option would be to sub-excavate foundation areas to
expose the underlying gravel soils and extend the bearing level down to the dense gravel or
backfill the sub-excavated depth with compacted structural fill up to design bearing level.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature
of the proposed construction, the building can be founded with spread footings bearing on the
Kumar & Associates, lnc. @ Project No. 25-7-1'11
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dense natural gravel soils or compacted structural fill with a risk of settlement. Placing footings
on the sand soils, encountered down to about 4%feet deep, is not recommended.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Footings placed entirely on the underlying gravel soils or compacted structural fill
can be designed for an allowable bearing pressure of 2,500 psf. Based on
experience, we expect initial settlement of footings designed and constructed as
discussed in this section will be about I inch or less. Post construction settlement
could occur for footings placed on the silty sand soils if the bearing soils become
wetted.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be well reinforced top and bottom to span
local anomalies and resist differential movement such as by assuming an
unsupported length of at least 12 feet. Foundation walls acting as retaining
qtructures should also be designed to resist lateral earth pressures as discussed
in the "Foundation and Retaining Walls" section of this report.
5) Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the firm natural soils. If structural fill is used, footing
areas should be sub-excavated to expose the underlying gravel soils with alateral
distance out from the sides of the footing at least half the depth of fill below the
footing. The exposed soils in footing area should then be moistened and
compacted. Structural fill can consist of the onsite gravel soils devoid of
organics, topsoil and rock larger than about 4 inches or a suitable imported
granular soil, such as CDOT class 6 base course. Structural fill should be
moisture conditioned to near optimum moisture content and compacted to at least
98 percent maximum proctor density.
6) A representative ofthe geotechnical engineer should observe all footing
excavations and test structural fill prior to concrete placement to evaluate bearing
conditions.
FOTINDATION AND RETAINING WALLS
Foundation walls and retaining structures which arelaterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
Kumar & Associates, lnc. @ Project No.25-7-111
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computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting
of the on-site soils. Cantilevered retaining structures which are separate from the residence and
can be expected to deflect sufficiently to mobilize the full active earth pressure condition should
be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight
of at least 35 pcf for backfill consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffrc, construction materials and equipment.
The pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90oh of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95%o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. 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 325 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
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95o/o of the
maximum standard Proctor density at a moisture content near optimum.
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 6-inch layer of
relatively well graded sand and gravel should be placed beneath floor slabs on grade to facilitate
Kumar & Associates, lnc. @ Project No.25-7-111
5
drainage. This material should consist of minus 2-inch aggregate with at least 50% retained
on the No. 4 sieve and less than l2Yo passing the No. 200 sieve.
All filI materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisfure content near optimum. Required fill can consist of the
on-site soils devoid of vegetation, topsoil and oversized rock.
TINDERDRAIN SYSTEM
Although free water was not encountered in the exploratory borings, it has been our experience
in the areathat the groundwater level will seasonally fluctuate, and 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 and basement areas, be protected from wetting and hydrostatic pressure buildup
by an underdrain system. Typical shallow crawlspace may not need an underdrain to help
protect the bearing soils from wetting.
The drains, if installed, 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 ofexcavation and at least I foot below lowest adjacent finish grade and
sloped at a minimum lo/o to 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 1Yz feet deep.
SURFACE DRAINAGE
Providing proper surface grading and drainage will be critical to help keep the bearing soils dry
and limit potential settlement and distress of the residence. 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 andunderslab 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 90Yo of the maximum standard Proctor density in landscape areas.
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 l0 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
Kumar & Associates, lnc. @ Project No.25-7-111
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4)Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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 borings drilled at the locations indicated on Figure 1, the proposed type of
conskuction 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 concemed 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 borings 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
so that 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 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.
Respectfully Submitted,
Kumar & Associates,
Robert L. Duran, P.
Reviewed by:
Daniel E, Hardin, P.E.
RLDlkac
Kumar & Assoclateo, ln6. o
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Prciect No.25-7-111
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APPROXIMATE SCALE-FEET
25-7 -1 1 1 Kumar & Associates LOCATION OF EXPLORATORY BORINGS 1Fig.
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BORING 1
EL. 5458'
BORING 2
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25-7 -1 1 1 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
TOPSOIL; SANDY CLAY, FIRM, MO|ST, DARK BROWN, ROOTS
SAND (SM), SILTY TO VERY SILTY, CLAYEY, MEDIUM DENSE, SLIGHTLY MOIST, BROWN
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GRAVEL AND COBBLES (GM), SANDY, CLAYEY, DENSE, SLIGHTLY MOIST, BROWN
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
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DRrvE sAMpLE, 1 s/B-tNcH t.D. spLtr spooN STANDARD pENETRAT|oN TEST.
)^/'t, DRIVE SAMPLE BLOW COUNT. INDICATES THAT 20 BLOWS OF A 14O-POUND HAMMER
FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
PRACTICAL AUGER REFUSAL. WHERE SHOWN ABOVE BOTTOM OF BORING, INDICATES THAT
MULTIPLE ATTEMPTS WHERE MADE TO ADVANCE THE HOLE.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 14,2025 WITH A 4-INCH_DIAMETER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING 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 BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCt) (ASTV D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ISTU OOSIS);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D11AO);
GRAVEL = PERCENT RETAINED 0N NO. 10 SIEVE;
SAND= PERCENT PASSING NO.10 SIEVE AND RETAINED ON N0.325 SIEVE
SILT = PERCENT PASSING NO. 325 SIEVE TO PARTICLE SIZE .002MM;
CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002MM.
DISTURBED BULK SAMPLE.
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25-7 -1 1 1 Kumar & Associates LEGEND AND NOTES Fig. 3
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WC = 5.9 %, DD = 91 pcf
)ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
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25-7 -1 1 1 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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HYDROMETER ANALYSIS SIEVE ANALYSIS
OPENINGS
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DIAMETER OF PARTICLES IN MILLIIVETERS
CLAY COBBLES
GRAVEL 0 o/o SAND 48 %SILT 38 %CLAY 14 %
USDA SOIL TYPE: Loam FROM: Boringl@4'
SILT V FINE Fil I GFGE
25-7 -1 1 1 Kumar & Associates USDA GRADATION TEST RESULTS Fig.6
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DIAMETER OF PARTICLES
CLAY TO SILT COBBLES
GRAVEL 4A % SAND
LIQUID LIMIT
SAMPLE OF: Sllly Sondy Grovel
42%
PLASTICITY INDEX
SILT AND CLAY 10 %
FROM:Boring2O6.5'
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HYOROMETER ANALYSIS SIEVE ANALYSIS
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2s-7 -1 1 1 Kumar & Associates GRADATION TEST RESULTS Fig. 7
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No.25-7.111
2
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NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITY
(pc0
BORING DEPTH
{ft)
6%
4
2
4
2
1.5
5 1
5.9
4.5
6.7
99
9 1
98
48
0
GRAVEL
l'/"1
GRADATION
42
54
SAND
f/"1 $t
I 0
46 0
TEXTURE
GRAVEL
f/t
SAND
vt
SILT
(^)
CLAY
tfl
SOIL TYPE
48 38 I 4
Silty Sandy Gravel
Very Silty Sand
Very Silty Sand
Very Silty Sand
Very Silty Sand