HomeMy WebLinkAboutSubsoil StudyI(t'T l(unw & AssoclatEs, lnc.
Geotechnical and Materials Engineers
and Environmental Scientists
5020 County Road 154
Glenwood Springs, CO 81601
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945-7988
945-8454
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An Employcc Orncd Compony
marusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 3, THE RAPIDS SUBDIVISION
RAPIDS VIEW LANE (MID VALLEY DRIVE)
GARFTELD COUNTY, COLORADO
PROJECT NO. 21-7-636
SEPTEMBER 280 2021
PREPARED FOR:
JORDÄN ARCHITECTURE
ATTN: BRAD JORDAN
P.O. BOX 1031
GLENWOOD SPRINGS, COLORADO 81602
b radi ordanarchitect@ gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION
SUBSURFACE CONDITIONS .
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ............
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM .............
SURFACE DRAINAGE...
LIMITATIONS..
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
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FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
Kumar & Associates, lnc,Project No.2l-7-636
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot3, The Rapids Subdivision, Rapids View Lane (Mid Valley Drive) 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 Jordan Architecture dated July 28, 202I.
A field exploration program consisting of three exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classification,
compressibility or swell potential, 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
At the time of our study, design plans for the residence had not been developed. The residence
and garage are proposed within the building envelope shown on Figure 1. In general, the
residence will be a two- story wood-frame structure over a crawlspace with a slab-on-grade
garage. 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. A swimming pool may also be constructed.
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 lot was vacant and appeared to have had minor cut and fill grading, likely during the
subdivision development. The ground surface is mostly relatively flat within the building
envelope steepening at the lot rear to about 15 to 20 degrees on the west downhill slope, and then
decreasing to about 2 to 4 degrees down to the west and southwest toward the Colorado River.
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A dry drainage traverses approximately along the southem property line. Vegetation consists of
sparse grasses and weeds within the building envelope and relatively small to large trees and
shrubs adjacent the river. A well is located near the southwest building envelope corner.
FIELD EXPLORATION
The field exploration for the project was conducted on August 30,2021. Three exploratory
borings were drilled at the approximate 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-458 drill rig. The borings were logged by a representative of Kumar
& Associates, Inc.
Samples of the subsoils and bedrock were taken with 1%-inch and 2-inch I.D. California or split-
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 aro an indication of the
relative density or consistency of the subsoils and hardness of bedrock. Depths at which the
samples were taken and the penetration resistance values are shown on the Logs of Exploratory
Borings, Figure 2. The samples \Ã/ere 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. Below
the topsoil or fill soils, the subsoils consist of about 3/zto 7% feet of very stiff/medium dense silt
and sand overlying very dense, slightly silty sand, gravel and cobbles with probable small
boulders underlain in Boring 1 at a depth of 12 feet by very hard siltstone bedrock. A 3%-foot
thick layer of very silty, clayey sand fill with organics was encountered below the topsoil of
Boring 3. This fill was probably placed as part of initial subdivision development.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, swell-consolidation and percent silt and clay-sized particles. Results of
swell-consolidation testing performed on relatively undisturbed drive samples of very silty sand
soils, prescnted on Figurc 4, indicate low compressibility under existing low moisture conditions
and light loading and minor to low collapse potential on the samples when wetted under loading.
The laboratory testing is summarizedinTable 1.
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No free water was encountered in the borings at the time of drilling and the subsoils and bedrock
were slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The upper silt and sand soils encountered in the borings possess low bearing capacity and
typically a low settlement potential if wetted. Existing filIpresents risk of post-construction
movement and is unsuitable for support of new foundations. Shallow spread footings placed on
the natural silt and sand soils can be used for support of the proposed residence with a risk of
foundation movement mainly if the bearing soils become wetted. V/e should observe the soil
conditions exposed at the time of excavation and evaluate them for swell-compression potential
and possible mitigation. Proper surface drainage as described in this report will be critical to the
long-term performance of the structure.
A low settlement risk can be achieved by extending the bearing level down to the relatively
dense, coarse granular soils. Although groundwater was not encountered in our borings, the
groundwater level could rise to near the top of the sand, gravel and cobble layer during seasonal
high water of the river and be encountered in deep excavations. In general, below-grade
construction should be limited to a floor or crawlspace level at least 2 feet above the anticipated
maximum groundwater level (typically assumed as 100 year river flood elevation).
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 upper
natural silt and sand soils with a settlement risk or on the deeper, coarse granular soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural silt and sand soils should be designed
for an allowable bearing pressure of 1,500 psf. Footings placed entirely on the
underlying, relatively dense, coarso granular materials can be designed for an
allowable bearing pressure of 3,000 psf. Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. Additional differential settlement up to about I inch could
occur if the silt and sand bearing soils are wetted.
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2)The footings should have a minimum width of 18 inches for continuous walls and
2 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 heavily 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 also be designed to resist a
lateral earth pressure corresponding to an equivalent fluid unit weight of at least
55 pcf for the on-site silt and sand soil as backfill.
Topsoil, filI 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.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
3)
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
computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting
of the on-site fine-grained 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 40 pcf for backfill consisting of the on-site flrne-grained 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 prossure 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 the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement or
structural areas should be compacted to at least 95Yo of the maximum standard Proctor density
4)
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6)
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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 0.35 for on-site fine-grained materials. Passive pressure of
compacted backfill against the sides of the footings can be calculated using an equivalent fluid
unit weight of 325 pcf for on-site fine-grained materials. 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.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil and fill, are suitable to support lightly loaded slab-
on-grade construction with a risk of settlement if the bearing 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
road base should be placed beneath interior slabs for support. This material should consist of
minus 2-inch aggregate with at least 50o/o retained on the No. 4 sieve and less than l2o/o passing
the No. 200 sieve.
Required fill beneath slabs can consist of the onsite soils, excluding topsoil and oversized rocks.
The fiIl should be spread in thin horizontal lifts, adjusted to near optimum moisture content, and
compacted to at least 95%o of the maximum standard Proctor density. All vegetation, topsoil and
loose or disfurbed soil should be removed prior to fill placement.
UNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
the area that local perched groundwater can develop during times of heavy precipitation or
seasonal runoff. Frozen ground during spring runoff can create a perched condition. Therefore,
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we recommend below-grade construction, such as deep crawlspace and basement areas, be
protected from wetting by an underdrain system. The drain should also act to prevent buildup of
hydrostatic pressures behind foundation walls. Typical depth crawlspace of 4 feet or less and
slabs-at-grade, such as a garugq should not need an underdrain with appropriate foundation wall
backfill construction as recommended below in the "surface Drainage Section".
Where provided, the underdrain system should consist of a drainpipe surrounded by free-
draining granular materialplaced at the bottom of the wallbackfill. The drain lines should be
placed at each level of excavation and at least 1 foot below lowest adjacent finish grade, and
sloped at a minimvm lo/o grade to a suitable gravity outlet. Free-draining granular material used
in the drain system should consist of minus 2-inch aggregate with less than 50Yo passing the No.
4 sieve and less than2Yo passing the No. 200 sieve. The drain gravel should be at least I%feet
deep. An impervious liner, such as 20 mil PVC, should be placed below the drain gravel in a
trough shape and attached to the foundation wall with mastic to keep drain water from flowing
beneath the wall and to other areas of the building.
SURFACE DRAINAGE
It will be critical to the building performance to keep the bearing soils dry. The following
drainage precautions should be observed during construction and maintained at all times after the
residence has been completed:
1) Inundation of the 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 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/o of the maximum standard Proctor density in landscape aroas.
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. Graded swales should have a
minimum slope of 3%.
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 inigation.
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LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. \Ve 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 concerned 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 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. Vy'e 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.
Respectfu lly Submitted,
Kumar & Associates, Inc.
Mark , E.I.T.
Reviewed by:
Steven L. Pawlak,
SLP/kac
(t,15222
Kumar & Associates, lnc.Projec{ No,21-7-636
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BORING 1
EL. 5502.5
BORING 2
EL. 5502.5'
BORING 3
EL. 5502'
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17/12
WC=4.0
DD= 1 00 17/12
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WC=9.2
DD=112
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21 -7 -636 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2
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LEGEND
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TOPSOIL; ORGANIC SANDY SILT, SLIGHTLY MO|ST, BROWN.
FILL; VERY SILTY, CLAYEY SAND WITH ORGANICS, SCATTERED GRAVEL, MEDIUM DENSE, MOIST,
DARK BROWN.
stLT AND SAND (ML-SM); VERY STIFF/MEDIUM DENSE, SLIGHTLY Mo|ST, LIGHT BROWN.
SAND, GRAVEL AND COBBLES (SM-GM); SLIGHTLY SILTY, PROBABLE SMALL BOULDERS, VERY
DENSE, SLIGHTLY MOIST TO MOIST, BROWN AND TAN, ROUNDED ROCK.
SILTSTONE BEDROCK; VERY HARD, SLIGHTLY MOIST, GRAY.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
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DR|VE SAMPLE, 1 3/1-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST.
4it.ı DRIVE SAMPLE BLOW COUNT. INDICATES THAT 17 BLOWS 0F A 140-P0UND HAMMERtt/ t¿ FALLTNc Jo TNcHES wERE REQUTRED To DRtvE THE sAMpLER 12 tNcHES.
PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 30, 2021 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) (nsrv D2216);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM Dl140).
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LEGEND AND NOTES Fig. 521 -7 -636 Kumar & Associates
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SAMPLE OF: Very Silty Sond
FROM:Boringl@2.5'
WC = 4.0 %, DD = 'l 00 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WEÏTING
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APPLIED PRESSURE - KSF
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SAMPLE OF: Very Silty Sond
FROM:BoringS@5'
WC = 5.5 %, DÐ = 99 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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Fig. 4Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS21 -7 -636
lGrtffilfiffi:ffi*rïËü**TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.2l-7-636Very Silty SandVery Silty SandVery Silty Clayey Sandßil1)4l47SOILTYPEVery Siþ SandLIQUID LIMIT1004.02Y2A1GRADATIONSAMPLE LOCATIONDEPTHBORINGUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEXPERCENTPASSING NO.200 stEvENATURALDRYDENSITYNATURALMOISTURECONTENTSANDl:/"1GRAVEL(%)1007.852JIT2992y,59.25.3