HomeMy WebLinkAboutSubsoils Report for Foundation Designrcrf ffi['ff:]"fffinr*''e**
; i .'i-o'::;:,.,, jrr An Atlp&oWo Owrud Conrparry
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa,com
www.kumarusa,com
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED GARAGE/ADU
LOT 8o HAWK RIDGE
3120 COUNTY ROAD 103
CARBONDALE, COLORADO
PROJECT NO.24-7-616
DECEMBER3t,2024
PREPARED FOR:
STEVE PSALEDAKIS
3120 COUNTY ROAD 103
CARBONDALE, COLORADO
steve@a i axmountain.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS....
FIELD EXPLORATION
SUB SURFACE CONDITIONS .
FOUNDATION BEARING CONDITIONS ......
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING WALLS (IF ANY).
FLOOR SLABS
SURFACE DRAINAGE
LIMITATIONS
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
TABLE 1 _ SUMMARY OF LABORATORY TEST RESULTS
I
1
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1
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2
2
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4
4
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Kumar & Associates, lnc. @ Project No. 24-7-616
1
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed garugelADU to be located on
Lot 8, Hawk Ridge, 3120 County Road 103, Carbondale, 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 Steve Psaledakis dated October 18, 2024.
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
The proposed garagelADU will be a two-story wood-frame structure. Ground floor will be 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.
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 site was developed with a two-story residence and two outbuildings at the time
of our field exploration. The ground surface slopes down to the west at grades between 5 and
20 percent. Vegetation consists ofjuniper trees, pine trees, sage brush, grass, and weeds.
FIELD EXPLORATION
The field exploration for the project was conducted on November 19,2024. 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. The borings were logged by a representative of Kumar & Associates, Inc
Samples of the subsoils were taken with 1%-inch and 2-inch I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 14O-pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
Kumar & Associates, lnc. @ Project No. 24-7-616
a
The penetration resistance values are anindication 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 retumed 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, below about 4 inches ofcompacted aggregate base course, consist ofabout 2to2Yzfeet
of very stiff, sandy to very sandy silt with scattered basalt fragments overlying dense basalt
gravel and cobbles in a calcareous silty sand matrix that extended down to the maximum
explored depth of l2 feet. Drilling in the dense granular soils with auger equipment was difficult
due to the cobbles and boulders and drilling refusal was encountered at depths of 12 feet in
Boring I and 6 feet in Boring 2.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and finer-than-sand (minus No. 200) gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples, presented on Figure 3, indicate
moderate compressibility under conditions of loading and wetting. The laboratory testing is
summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper silty sandy soils encountered in the borings possess a low bearing capacity and
generally low to moderate settlement potential, especially when wetted. Spread footings bearing
on the natural basalt gravel below about 3 feet should be feasible for foundation support of the
building. Provided below are recommendations for a spread footing foundation.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed constructiono we recommend the building be founded with spread footings bearing
on the undisturbed natural soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Footings placed on the undisturbed natural granular soils should be designed for
an allowable bearing pressure of 2,000 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
Kumar & Associates, lnc. @ Project No. 24-7-616
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2)The footings should have a minimum width of 16 inches for continuous walls
and2 feet for isolated pads.
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.
Continuous foundation walls should be well reinforced top and bottom to span
local anomalies and beffer resist any differential movement such as by assuming
an unsupported length of at least l0 feet. Foundation walls acting as retaining
structures (ifany) should also be designed to resist lateral earth pressures as
discussed in the "Foundation and Retaining Walls" section of this report.
All existing fill, topsoil and any loose or disturbed soils should be removed and
the footing bearing level extended down to the relatively dense natural granular
soils. The exposed soils in footing area should then be moistened and compacted.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
3)
4)
FOLINDATION AND RETAINING WALLS (IF ANY)
Foundation walls and retaining structures which are laterally supported and can be expected
to undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting
of the on-site soils. Cantilevered retaining structures which are separate from the building 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 45 pcf for backfill consisting of the on-site soils. Backfill should not contain
vegetation, topsoil, or oversized rocks. .
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 90o/o of lhe maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway
areas should be compacted to at least 95Yo 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.
5)
6)
Kumar & Associates, lnc. o Project No. 24-7-616
-4-
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 0.40. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 37 5 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 4-inch layer of relatively
well graded sand and gravel should be placed beneath interior slabs for support. This material
should consist of minus 2-inch aggregate with at least 50olo retained on the No. 4 sieve and less
than l2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
on-site granular soils devoid of vegetation, topsoil and oversized rock.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the building has been completed:
l) 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 90%o 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 10 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
capped with about 2 feetof the on-site soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Kumar & Associates, lnc. o Project No. 24-7-616
-5-
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this areaat 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
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 concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and exhapolation of the
subsurface conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is perfiormed. If conditions encountered
during conskuction 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 veriSr 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,
Knxr:?*r & Assoeiates, l*#-
.' ,., ,,. r :,. ._
Paul J. Graf, Staff Engineer
Reviewed by
Daniel E. Hardin, P
PJGlkac
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APPROXIMATE SCALE-FEET
24-7 -61 6 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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BORING 1 BORING 2
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WC=10.0
DD=89
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WC=15.0
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24-7-616 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
LEGEND
(4)
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ROAD BASE, THICKNESS IN INCHES SHOWN IN PARENTHESES TO LEFT OF THE LOG.
SILT (SM); SANDY TO VERY SANDY, SCATTERED BASALT FRAGMENTS' VERY STIFF' SLIGHTLY
MOIST, BROWN.
BASALT GRAVEL AND COBBLES (GM); VERY SANDY, SILTY, VERY CALCAREOUS' POSSIBLE
BOULDERS, DENSE, SLIGHTLY MOIST, TAN.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 s/8-INCH l.D. SPLIT SPOoN STANDARD PENETRATION TEST.
W
F
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-l otsrunsED BULK sAMPLE.
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AA I1^ DRIVE SAMPLE BLOW COUNT' INDICATES THAT 19 BLOWS OF A 140'POUND HAMMER
'"/ '' FALLTNG 30 tNcHEs wERE REQUIRED To DRIvE THE SAMPLER 12 lNcHEs.
^ PRACTICAL AUGER REFUSAL. WHERE SHOWN ABOVE BOTTOM OF BORING, INDICATES THAT
T uulrtpu ATTEMpTS WHERE MADE To ADVANcE THE HoLE.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 19,2024 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 PI.AN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF
THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATIONS 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 (pcf) (ASTM D2216);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140).
24-7-616 Kumar & Associates LEGEND AND NOTES Fig. 3
SAMPLE OF: Sondy Silt
FROM:Boringl@2'
WC = 1O.O %, DD = 89 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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24-7-616 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fis. 4
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SAMPLE OF: Sondy Sllt
FROM:Borlng2@2'
WC = 15.O %, DD = 89 pcf
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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24-7-616 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 24-7'61 6
SOIL TYPE
Sandy Silt
Gravelly Silty Sand
Sandy Silt
Gravelly Silty Sand
UNCONFINED
COMPRESS]VE
STRENGTH
{osfl
ATTERBERG LIMITS
PLASTIC
INDEX
lo/"1
LIQUID LIMIT
lo/ol
PERCENT
PASSING NO.
200 stEvE
32
27
GRADATION
SAND
("/.)
I 2 10.0 89
GRAVEL
P/,1
NATURAL
DRY
DENSTTY
(ocfl
89
NATURAL
MOISTURE
CONTENT
Iolol
s.8
15.0
4.8
SAMPLE LOCATION
DEPTH
(ft)
9
2
4
BORING
2