HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.15.2024rcn $.,iffiifi'ffifirnitt'i*'"
An Employcc Ovrncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumalusa.com
Office Locations: Denver (IJQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED GARAGE AND ADU
3290 COUNTY ROAD 2IO
GARFIELD COUNTY, COLORADO
PROJECT NO. 23-7-67s
JANUARY 15,2024
PREPARED FOR:
LINDA HANSEN
3290 COUNTY ROAD 2IO
RIFLE, COLORADO 81650
lhansen@sopris.net
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY.....
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION
SUBSURFACE CONDITIONS
DESIGN RECOMMENDATIONS ........
FOUNDATIONS
FLOOR SLABS
I.INDERDRAIN SYSTEM ...
SURFACE DRAINAGE.......
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF DGLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No.23-7.675
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed gamge and ADU to be located at
3290 County Road 210, 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 Linda
Hansen dated November 30, 2023.
A field exploration program consisting of an exploratory boring 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 analyzedto 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
Development plans for the building were conceptual at the time of our study. We understand the
construction will generally consist of a slab-on-grade garage,24'by 26' in plan size, with a
second level ADU separate from the existing residence at the approximate boring location shown
on Figure 1. 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.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject building site was vacant at the time of our field exploration. The property is
occupied with the residence and outbuildings as shown on Figure 1. The ground surface slopes
gently down to the southwest with around %to I foot of elevation difference across the building
area. Vegetation consists of grass and weeds with mafure trees in surrounding areas.
FIELD EXPLORATION
The field exploration for the project was conducted on December 27 ,2023. One exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The
Kumar & Associates, lnc. o Project No.23-7-675
boring was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted
CME-45B drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with l% inch and Z-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-l586.
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 Log of Exploratory Boring, Figure 2. The samples were retumed to our laboratory
for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about Yzfootof topsoil overlying loose/medium stiff to soft, silty sand and
sandy silt down to the maximum drilled depth of 30 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on a relatively undisturbed drive sample of the very silty sand,
presented on Figure 3, indicate low compressibility under existing low moisture condition and
light loading and a low collapse potential (settlement under constant load) when wetted and
relatively high compressibility under additional loading after wetting. The laboratory testing is
summarized in Table 1.
Free water was encountered in the boring at a depth of about 14 feet at the time of drilling and
l2%feetwhen checked 14 days later. The upper soils were slightly moist to moist and wet with
depth.
DESIGN RECOMMENDATIONS
FOI]NDATIONS
The upper sand and silt soils generally have low bearing capacity and possess settlement
potential mainly when wetted. Precautions should be taken to keep the bearing soils dry during
and after construction. The subgrade soils should be fuither evaluated for bearing conditions at
the time of excavation.
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, the building can be founded with spread footings bearing on the upper
natural soils with a risk of settlement mainly if the bearing soils are wetted.
Kumar & Associates, lnc. o Project No.23-f-675
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The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. There could be additional post-construction settlement if
the bearing soils become wetted. The magnitude of the additional movement
would depend on the depth and extent of wetting but could be up to about I inch.
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
atea.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet.
Foundation walls acting as retaining structures (if any) should also be designed to
resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at
least 50 pcf for the onsite soils as backfill.
5) Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the firm natural soils. The exposed soils in footing area
should then be moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. There could be some slab movement if the subgrade soils are wetted. 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 CDOT Class 6 road base, should be placed beneath slabs for subgrade support. This
material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve
and less than I2Yo passing the No. 200 sieve.
Kumar & Associates, lnc, @ Projec-t No. 23-7-675
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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 above optimum. Required fill can consist of the
on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
It is our understanding the proposed ground floor will be slab-on-grade with an elevation near
the surrounding grade. Therefore, a foundation drain system is not recommended. If a basement
level is considered, we recommend an underdrain be provided to protect the lower level from
wetting and hydrostatic pressure buildup.
If the finished floor elevation of the proposed building is revised to have a floor level below the
surrounding grade, we should be contacted to provide recommendations for an underdrain
system. All earth retaining structures (site walls) should be properly drained.
SURFACE DRAINAGE
Providing proper surface grading and drainage will be critical to limiting subsurface wetting
below the foundation and the risk of building movement and distress. The following drainage
precautions should be observed during construction and maintained at all times after the building
has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfitl should be adjusted to near optimum moisture and compacted to
at least 95% 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.
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 l0 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in paved 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
10 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irigation.
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.
Kumar & Associates, lnc. o Project No.23-7-675
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The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory boring drilled at the location indicated on Figure 1, the proposed 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 findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory boring 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 veriry 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 fiIl by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kunrar & Associatesr l
Steven L. Pawlak, P.E.
Reviewed by:
b
Daniel E. Hardin, P.E.
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Kumar & Assoeiates, lnc. @ Projeet No" 23-7-675
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APPROXIMATE SCALE-FEET
23-7-675 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
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BORING 1
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T0PSO|L; 0RGANIC SANDY SILT AND CLAY, FIRM, SLIGHTLY
MOIST, BROWN.
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WC=1,l.0
DD=90
-200=48
SAND AND SILT (SM-ML); FINE TO MEDIUM GRAINED,
Lo0SE/MED|UM STIFF T0 SoFT, SLIGHTLY MoIST T0 WET
WITH DEPTH, LIGHT BROWN.
5
e/12
WC=12.2
DD=97 F
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRTVE SAMPLE, 1 s/8-INCH l.D. SPLIT SP00N STANDARD
PENETRATION TEST.
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3/12
WC=23.7
DD=99
671jDRIVE SAMPLE BL0W COUNT. INDICATES THAT 9 BL0WS 0F"/'.A 140-POUNO HAMMER FALLING 50 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
Q*14DEPTH TO WATER LEVEL AND NUMBER OF DAYS AFTER
= DRILLING MEASUREMENT WAS MADE.-.->-F
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1 /12
.--> DEPTH AT WHICH BORING CAVED WHEN CHECKED ON
JANUARY 10, 2021.
2A
NOTES
t. THE EXPLORATORY BORING WAS DRILLED ON DECEMBER 27,
2023 WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER
AUGER.
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIOED.
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5. THE ELEVATION OF THE EXPLORATORY BORING U/AS NOT
MEASURED AND THE LOG OF THE EXPLORATORY BORING IS
PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATION SHOULD BE CONSIDERED
ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
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5. IHE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN
MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER LEVEL SHOWN ON THE LOG WAS MEASURED AT
THE TIME AND UNDER CONDITIONS INDICATED. FLUCTUATIONS IN
THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSTTY (pcf) (lSrU 0 ZZ1O);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1 1 40).
23-7 -675 Kumar & Associates LOG OF EXPLORATORY BORING Fi1. 2
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SAMPLE OF: Very Silty Sond
FROM:Boringl@4'
WC = 12.2 %, DD = 97 pcf
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ofthcr.lttcnwithoutfull,
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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Fig. 3SWELL_CONSOLIDATION TEST RESULTS23-7 -675 Kumar & Associates
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Very Silty Sand
Very Silty Sand
Very Sandy Silt
{osfl
UNCONFINED
COMPRESSIVE
STRENGTH
PLASTIC
INDEX
lolol
ATTERBERG LIMITS
P/ol
LIQUID LIMIT
PERCENT
PASSING NO.
2()() SIEVE
48
SAND
tf/"\
GRADATION
(:/,1
GRAVEL
NATURAL
DRY
DENS]TY
(pcfl
90
97
99
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NATURAT
MOISTURE
CONTENT
.011
t2.2
23.7
tft)
DEPTH
2
4
9
BORING
1
No.23-7-675