HomeMy WebLinkAboutSubsoils Report for Foundation DesigngKFov4€- %97
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An Employcc Owncd Compony
5020 County Road 154
Glenwood Splings, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenr.vood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 79, PINYON MESA
PINYON MESA DRIVE
GARFTELD COUNTY, COLORADO
PROJECT NO.22-7-292
JUNE 8,2022
PREPARED FOR:
JORDAN ARCHITECTURE
ATTN: BRAD JORDAN
P.O. BOX 1031
GLENWOOD SPRTNGS, COLORADO 81602
brad iordanarchitect@smail.com
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ...............
PROPOSED CONSTRUCTION
SITE CONDITIONS
SUB SIDENCE POTENTIAL ......
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOLTNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FLOOR SLABS
LINDERDRAIN SYSTEM..
SURFACE DRAINAGE
LIMITATIONS
FICURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 . LOG OF EXPLORATORY BORTNG
FICURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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FOLINDATION AND RETAINING WALLS ........,...,,,- 4 -
......- 3 -
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Kumar & Associates, lnc. @ Project No.21-7-292
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot79, Pinyon Mesa, Pinyon Mesa 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 April 14, 2022.
An exploratory boring was drilled 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 in progress at the time of our study. The proposed
residence will generally be a one- and two-story structure with attached garage at the main level.
Ground floors could be a combination of structural over crawlspace and slab-on-grade. Grading
for the structure is assumed to be relatively minor with cut depths between about 3 to
12 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 vacant at the time of our field exploration. The ground surface is relatively
flat in the building area of the lot and slopes mainly down to the west. Pinyon Mesa Drive
borders the north side of the lot and is approximately 5 feet below the center of the building site.
The ground surface is mostly barren with scattered gravel. Vegetation consists of sparse grass
with sage brush at the rear of the lot.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies the Pinyon Mesa
Development. These rocks are a sequence of gypsiferious shale, fine-grained sandstone/siltstone
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and limestone with some massive beds of gypsum. There is a possibility that massive gypsum
deposits associated with the Eagle Valley Evaporite underlie portions of the property.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. During previous work in the area, sinkholes have been
observed scattered throughout the lower Roaring Fork River valley.
No evidence of subsidence or sinkholes was observed on the property or encountered in the
subsurface materials, however, the exploratory boring was relatively shallow, for foundation
design only. Based on our present knowledge of the subsurface conditions at the site, it cannot
be said for certain that sinkholes will not develop. The risk of future ground subsidence at the
site thtoughout the serviue lil'e uf the propt-rsed struc[ure, in our opinion is low, however the
owner should be aware of the potential for sinkhole development. lf t'urther investigation of
possible cavities in the bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on Apri|26,2022. One exploratory boring
was drilled at the location shown on Figure I to evaluate the subsurface conditions. The 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 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.
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 returned to our laboratory
C^----^-":^--.1--. Ll-- ,-,--l--L -,--r,,--, t L Lrior revlgw Dy rne pi-ojecl engineer aiiq testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about 8 feet of very stiff, sandy silt and clay underlain by very stiff to hard,
sandy clay with scattered gravel overlying medium dense, silty sand and gravel with scattered
cobbles below about l7 feet down to the maximum explored depth of 31 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples of the upper silt and clay soil and the
underlying sandy clay soil, presented on Figure 3, indicate low compressibility under natural low
moisture conditions. The upper silt and clay soil showed low collapse potential (settlement
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under constant load) and the underlying clay soil showed low expansion potential when wetted.
The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist.
FOUNDATION BEARING CONDITIONS
The natural sandy silt and clay soils possess relatively low bearing capacity and variable
compressibility or expansion potential when wetted. A shallow foundation placed on these soils
will have a risk of movement if the soils become wetted and care should be taken in the surface
and subsurface drainage around the house to prevent the soils from becoming wet. It will be
critical to the long-term performance of the structure that the recommendations for surface
grading and drainage contained in this report be followed. The amount of movement will mainly
be related to the depth and extent of subsurface wetting but may result in settlements of around
I to 2 inches which could cause building distress. Mitigation methods such as removing and
replacing the bearing soils as compacted structural fill or micro-piles down into the gravelly soils
could be used to support the proposed house with a lower risk of movement.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on a minimum of 3 feet of compacted structural fill below garage and crawlspace footings. We
should observe the soils for use of compacted structural fill below basement level footings. We
should be contacted for additional recommendations if a deep foundation is desired.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the compacted structural fill or the native deeper sandy clay
soils should be designed for an allowable bearing pressure of 2,000 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. Additional differential
movements of about %to I inch could occur if the bearing soils are wetted.
2) The footings should have a minimum width of l6 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.
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4) Continuous foundation walls should be heavily reinforced top and bottom to span
localanomalies such as by assuming an unsupported length of at least l4 feet.
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 report.
5) The topsoil, sub-excavated silt and clay soil depth and any loose disturbed soils
should be removed below the foundation area. The exposed soils in footing areas
after sub-excavation should then be moistened and compacted. Structural fill
should consist of low permeable soil (such as the on-site sandy silt and clay soils)
compacted to at least 98%o of standard Proctor density within 2olo of optimum
moisture content. The structural fill should extend laterally beyond the footing
edges equal to at least %the fill depth below the footing.
6) A representative of the geotechnical engineer should evaluate the fill placement
for compaction and observe all footing excavations prior to concrete placement to
eva hlate hearing conditions.
FOLINDATION 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 pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site fine-grained soils. Cantilevered retaining structures which are separate from the
t'esitlctruc antl uan bc expecl.ecl Lo deflect sufficiently to mobilize the full active eafth 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 fine-grained soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
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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 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 he expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger
than about 6 inches.
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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. 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 limitthe 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, can be used to support lightly loaded slab-on-grade
construction with a movement risk similar to the foundation if the underlying 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 free-draining gravel
should be placed beneath basement level slabs to facilitate drainage. This material should
consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve and lessthan2Yo
passing the No. 200 sieve.
All nll materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils or imported granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where clay soils are present that 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,
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system. An underdrain should not be provided around slab-at-grade garage and
shallow crawlspace areas to help limit potential wetting of bearing soils from shallow water
sources.
The drains should consist of drainpipe placed in the bottom of the wall backfill sunounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum lo/oto
a suitable gravity outlet or sump and pump. Free-draining granular material used in the
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underdrain system should contain less than 2o/opassingthe 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 I %feet deep. An impervious membrane such as 20 mil 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
Proper surface grading and drainage will be critical to keeping the bearing soils dry and limiting
building movement. The following drainage precautions should be observed during construction
and maintained at all times after the residence 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% of the maximum standard Proctor density in landscape areas.
3) The ground surface sumounding the exterior of the building should be sloped to
drain away from the fbundation 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. 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which rcquircs regular heavy irrigation should be located at least
5 feet from foundation walls. Consideration should bc givcn to usc 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 atthis 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 boring drilled at the location indicated on Figure 1, the proposed type of
construction and our cxperience in the area. Our seryices do not include detemrining 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
suhsurfacc conditions identified at the cxploratory boring aud variatiuns in thc subsurface
conditions may not become evident until excavation is performed. If conditions encountered
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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 engineqr.
Respectfully Submitted,
Kumar & Associates, Inc
Steven L. Pawlak, P.E.
Reviewed by:
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Daniel E. Hardin, P.E.
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Kumar & Associates, lnc. @ Project No. 21-7-292
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22-7 -292 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
3
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BORING 1
EL. 6201'
LEGEND
0
SILT AND CLAY (ML-CL); SANDY, VERY STIFF, SLIGHTLY
MOIST, LIGHT BROWN, CALCAREOUS TRACES,
28/ 12
WC=4.1
DD=101
CLAY
LIGHT
(cr); slN0v, SCATTERED GRAVEL, SLTGHTLY M0|ST,
BROWN, SLIGHTLY CALCAREOUS.
5
26/12
SAND AND GRAVEL (SM-GM); SILTY, SCATTERED CoBBLES,
MEDIUM DENSE TO DENSE, SLIGHTLY MOIST, MIXED BROWN
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
10 2e/12
WC=8.6
DD= 1 08
DR|VE SAMPLE, 1 3/8-INCH l.D. SPLIT SP00N STANDARD
PENETRATION TEST.
"",,"DR|VE SAMPLE BLOW COUNT. INDICATES THAT 28 BLOWS 0Fzo/ tL A 14o-pouND HAMMER FALLTNG J0 TNcHES wERE REQUTRED
TO DRIVE THE SAMPLER 12 INCHES.
15
40/12
WC=9.3
DD=1 10
-200=89
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NOTES
THE EXPLORATORY BORING WAS DRILLED ON APRIL 26, 2022
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER
20
34/ 12
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIDED.
5. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PI.AN
PROVIDED.
25
28/ 12
4, THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
30
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE
GRADUAL.
50/ 1 6. GROUNDWATER WAS NOT ENCOUNTTRED IN THE BORING AT THE
TIME OF DRILLING.
30
7, LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSTTV (pcr) (lSiU D 2216);
-2OO = PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 1 40).
22-7 -292 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
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SAMPLE OF: Sondy Silt ond Cloy
FROM:Boringl@2.5'
WC = 4.1 %, DD = 101 pcf
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
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SAMPLE OF: Sond Cloy
FROM:Boringl@10'
WC = 8.6 %, DD = 108 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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22-7-292 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fis. 3
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
ATI LIMITSSAMPTLOCATIONGRADATION
sAt'tD
(%)
PERCENT
PASSING NO.
200 stEvE
LIQUID LIMIT
(ol
PLASTIC
INDEX
to\(os0
UNCONFINED
COMPRESSIVE
STRENGTH SOILTYPESORING
tftt
DEPl}I
to\
NATURAL
MOISTURE
CONIENT
NATURAL
DRY
DENSIW
(ocf)
GRAVEL
l"kl
Sandy Silt andClayl0lIat/4.1
Sandy Clayt08.6 108
89 Sandy Clayl59.3 110
No.22-7-292