HomeMy WebLinkAboutSubsoils Report for Foundation DesignI (3rt [iffi],::#trf ffin$ri' *"5020 County Road 154
Glenwood Spdngs, CO 81601
plrone: (970) 945-7988
lax: (970) 945-8454
email : kaglenlvood@kumarusa.com
An Emplnycc *lvrild Cor*pcny wwrv.kumatusa.com
Office Locations: Denver (l{Q), Parkel Colorado Springs, Fort Collins, Glenwood Springs. and Sr.rmrnit Count_v, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 74, SPRING RIDGE RESERVE PT]D
285 HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO.22-7-501
SEPTEMBER 8,2022
PREPARED FOR:
JOHN CURRIER
357 HIGTI DESERT ROAD
GRAND JUNCTION, COLORADO 81507
i ohn@currierwater.com
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......
PROPOSED CONSTRUCTION
SITE CONDITIONS
ROCKFALL.
FIELD EXPLORATION
SUBSURFACE CONDITIONS
DESIGN RECOMMENDATIONS .....
FO{.INDATIONS
FOTINDATION AND RETAINING WALLS
FLOOR SLABS
LINDERDRAIN SYSTEM ...........
SURFACE DRAINAGE......,.........
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
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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Kumar & Associates, lnc. o
Project No. 22-7-501
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located at
Lot74, Spring Ridge Reserve, 285 Hidden 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 John Currier dated July 1I,2022.
A field exploration program consisting of 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 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
The proposed residence will be a single-story structure above crawlspace with a slab-on-grade
garcge floor. Grading tbr the structure is assumed to be relatively minor with cut depths befween
about 3 to 8 feet. We assume relatively light foundation loadings, typical of the proposed type of
construction.
If building location, grading or loading information is significantly different than described
above, we should be notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The building site was vacant and vegetated with grass and weeds with scrub oak and juniper
trees uphill to the northeast. The lot slopes down to the southwest at a grade of about l0% in the
building site to about 20Yo inthe uphill part of the lot. Red sandstone bedrock outcrops on the
hillside above the lot. Nearby lots are developed with 1 to 2 story residences. An abandoned
irrigation ditch crosses the site near the uphill side of the building envelope as shown on
Figure i ' The hillside slope above the property steepens to around 40 to 45%o down to the
southwest.
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Project No. 22-7-501
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ROCKFALL
The hillside above the irrigation ditch consists of shallow, rocky colluvium above sandstone
bedrock of the Maroon Formation. The ground surface slopes at about 40 to 45Yo (22. to 24')
which roughly coincides with the bedding dip of the formation rock. The colluvium thickens at
the base of the hillside where the ditch is located. Scattered, mostly flat shaped rock fragments
typically up to 1 to 2 feet in size are exposed on the hillside to the top of ridge about 150 feet
above the building area.
No evidence of rockfall into the building area was observed and mitigation of rockfall potential
in the proposed building areao in our opinion, is not warranted. The existing abandoned
irrigation ditch provides a catchment area for small rock fragments that may slide down the
hillside by erosion and should be kept above the residence for a width of at least l0 feet.
FIELD EXPLORATION
The field exploration for the project was conducted on July 20, 2022. 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-458 drill rig. The borings were logged by a representative of Kumar &
Associates.
Samples of the subsoils were taken with I%-inch and 2-inchl.D. spoon samplers. The samplers
were driven into the subsurface materials 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-i586. The penetration resistance values are an indication of the relative
density of the subsoils and hardness of the 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 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 consist of about 1 foot of topsoil overlying about l0 to l2 feet of medium dense/very
stiff, silty clayey sand and sandy silty clay and about 4 feetof medium dense, clayey sand and
gravel at Boring 1. Medium hard to hard siltstone/sandstone bedrock of the Maroon Formation
was encountered at depths of about 14 and 17 feet down to the rnaximum depth explored of
2l feet- Drilling in the bedrock with auger equipment was difficult due to increasing hardness
with depth.
Kumar & Associates, lns. @
Project No. 22-7-501
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Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, and percent finer than sand size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the sand and clay
soils, presented on Figure 4, indicate low to moderate compressibility under conditions of
loading and wetting with a minor expansion potential when wetted under light load. The
laboratory testing is summarizedin Table l.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
DESIGN RECOMMENDATIONS
FOIINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed consfruction, we recommend the building be founded with spread footings bearing
on the natural sand and clay soils.
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 1 inch or less. There could be additional settlement of around % to
1 inch if the bearing soils are 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 reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 12 tbet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this reporf.
5) The topsoil and any loose or 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.
Kumar & Associates, lnc. @
Project No. 22-7.501
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6) A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOI'NDATION AND RETAINING WALLS
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 presswe
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 residence and
can be expected to deflect sufficiently to mobilize the fulI 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 soils.
A1l foundation and retaining strucfures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, 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 90Yo 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 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 fo*ndation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
tacilities constructed on the backfill- Backfrll should not contain organics, debris or rock larger
than about 6 inches.
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 fboting. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. 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
Kumar & Associates, lnc. o Project No, 22-7.501
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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 differentiat 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
free-draining gravel should be placed beneath basement level/depressed slabs (if any) to facilitate
drainage. This rnaterial should consist of minus 2-inch aggregate with at least 50% retained on
the No. 4 sieve and less than2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maxLmum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils devoid of vegetation, topsoil and oversized rock.
We recommend vapor retarders conform to at least the minimum requirements of ASTMEI745
Class C material. Certain floor types are more sensitive to water vapor transmission than others.
For floor slabs bearing on angular gravel or where flooring system sensitive to water vapor
transmission are utilized, we recommend a vapor barrier be utilized conforming to the minimum
requirements of ASTM 8I745 Class A material. The vapor retarder should be installed in
accordance with the manufachrrers' recommendations and ASTM 81643.
T.INDERDRAIN 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 also 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.
The drains 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 Ievel of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimu m lyo to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
Kumar & Associales, lnc. o
Project No, 22-7.501
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contain less than 2Yo passingthe 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 l% feet deep. An
impervious membrane such as 20 mll PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.
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 95o/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
covered with fiiter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
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
5 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 irrigation.
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
construction and our experience in the arca. 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 borings and variations in the subsurface
Kumar & Associates, lnc. @ Project No, 22.7-501
conditions may not become evident unfil 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 evolveso we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify 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 founiation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associafesu
Steven L. Pawlak,
SLPlkac
Xumar & Aesociales, lnc, r'
Proje6{ l{o" 22-F501
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22-7 -5A1 Kumar & Associates LOGS OF IXPLORATORY BORINGS Fig. 2
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LEGEND
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TOPSOIL; ORGANIC CLAYEY SILT AND SAND, FIRM, RED-BROWN.
:.4[D- AI9_ CLAY (SC-CL); SILTY, SCATTERED GRAVEL, MEDIUM DENSE/VERY STIFF, SLIGHTLY
MOIST, RED.
SAND AND GRAVEL (SC-CC); SILTY, COBBLES, MEDIUM DENSE, RED, STLTSTONE FRAGMENTS.
WEATHERED SILTSTONE/SANDSTONE; MEDIUM HARD, SLIGHTLY MOIST, RED.
STLTSTONE/SANDSTONE BEDROCK; HARD TO VERY HARD, SLtcHTLy MO|ST, MAROON
FORMATION.
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE.
i DRIVE SAMPLE, 1 S/9-|NCH t.D. SpLtT SPOON STANDARD PENETRATTON TEST
\a/1t DRIVE SAMPLE BLOW COUNT. INDICATES THAT 59 BLOWS OF A'140-P0UND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
'I THE EXPLORATORY BORINGS WERE DRILLED ON JULY 20, 2022 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.
3. 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 (pcr) (lsru D2216);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1 1 40)
22-7 -501 Kumar & Associates LTGEND AND NOTES Fig.3
I
SAMPLE OF: Very Silty Cloyey Sond
FROM:Boringl@2.5'
WC = 5.0 %, DD = 114 pcf
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NO MOVEMENT UPO
WETTING
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PLIED PRESSURE - KSF 10
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1.0 APPLIED PRESSURE - KSF r0
SAMPLE OF: Very Silty Sondy Cloy
FROM:Borin12@5'
WC = 11,3 %, DD = 115 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
22-7 -501 Kumar & Associates SWTLL_CONSOLIDATION IEST RESULTS Fig. 4
lcn lfunnr & lssod&s,lne.o
Geote$nical and Maledals Engineers
and Environmentai $cientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOITTYPELIQUID LIMlT
A
UNCONFINED
COMPRESSIVE
STRENGTH
PLASTIC
INDEX
Very Silty Clayey Sand
Very Silty Clayey Sand
Very Silty Clayey Sand
Very Silty Sandy Clay
PERCENT
PASS$|G NO.
200 stEvE
49
SAND
fl"l
GRADATION
w
GRAVEL
{ocfl
NATURAL
DRY
DENSITY
t07
r07
5I1
(%l
NATURAL
MOISTURE
CONTENT
6.2
12.0
a.-)11
(ft)
DEPTH
tt45.02y,I
5
10
5
SAMPLE LOCATION
BORING
2
No.22-7-501