HomeMy WebLinkAboutSubsoil Study for Foundation Design 09.13.2021rcrf mlm.miiifú-"'
An Employcc onrîcd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945:1988
fax: (97A)945-M54
email : kaglenwood@kumaru sa.com
www.kuman¡sa.com
Office Locations: Denver Q{Q), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 16, PINYON MESA
TBD SAGE MEADOW ROAD
GARFIELD COUNTY, COLORADO
PROJECT NO.2l-7-s6s
SEPTEMBER 13,2021
PREPARED FOR:
ERIC AANONSEN
C/O BRIKOR ASSOCIATES
20 SUNSET DRIVE, UNIT #1
BASALT, COLORADO 81621
ea2@brikor.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY....
PROPOSED CONSTRUCTION
SITE CONDITIONS......
SUBSIDENCE POTENTIAL..................
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS .
FOUNDATIONS .........
FOUNDATION AND RETAINING WALLS ..
FLOOR SLABS.......
I.]NDERDRAIN SYSTEM
SURFACE DRAINAGE
LIMrrATIONS.................
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 . LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I . SUMMARY OF LABORATORY TEST RESULTS
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Kumal &Associates, lnc. o Projec{ No. 21-7-fi5
PURPOSE AND SCOPE OF STITDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 16, Pinyon Mesa, TBD Sage Meadow Road, Garfield County, Colorado. The project site is
shown on Figure l. 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 Eric Aanonsen dated June 28,2021.
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 fîeld e:rploration 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
reçommendations and other geotechnical engineering considerations based on the proposed
construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTTON
Plans for the proposed residence were not available at the time of our study. The proposed
residence is assumed to be a two-story wood frame structure over a crawlspace or basement with
an attached garage. Ground floors will likely be a combination of structural over a crawlspace
and slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths
between about 2 to 5 feet. If a full basement is planned, cut depths could range up to about 10
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 úo re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was v&cant at the time of our field exploration. The ground surface is
moderately to strongly sloping down to the west. Vegetation consists of scattered grasses and
weeds at the front of the lot with generally more sage bnush a¡d a few scattered pines in the
Kumar & Associates, lnc. o Project No.2l-7-ffií
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middle and rear portions of the lot. Mounds of relatively thicker top soil are present near the
middle of the proposed building envelope.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies the Pinyon Mesa
Development. Thcse rocks are a sôquence of gypsiferious shale, {ine-grained sandstone/siltstone
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 properly.
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 r,vas 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
siæ throughout the service life of the proposed structure, in our opinion is low, however the
olilner should be aware of the potential for sinkhole development. If further 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 July 14, 2021. 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-
458 drill rig. The boring was logged by a representative of Kumar & Associates, lnc.
Samples of the subsoils were taken with lTs-inch and 2-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-1586.
The penefiation 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
for review by the project engineer and testing.
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SUBSURFACE COI\DITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. Below
about 1 foot of topsoil, the subsoils consist of about 25 feet of stiff to hard, sandy to very sandy,
silty clay with scattered gravel overlying 2 feet of relatively dense silty, sandy gravel with
cobbles, overlying about 6 feet of interlayered sand and silt with gtavel and clay, underlain by
very dense, silty to clayey sandy gravel with cobbles down to the maximum explored depth of 46
feet,
Laboratory testing performed on samples obøined from the boring included natural moisture
content and density and finer than sand grain-size gradation analyses. Results of a swell-
consolidation testing performed on a relatively undisturbed drive sample of the clay soil,
presented on Figure 3, indicate low to moderate compressibility under natural moisture
conditions and when wetted. The laboratory testing is summmized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist.
FOUNDATION BEARING CONDITIONS
The natural sandy clay soils possess relatively low bearing capacity and settlement potential
mainly 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 gradíng and drainage
contained in this report be followed. The amount of movement, if the bearing soils become \ryet,
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.
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DESIGN RECOMMENDATIONS
FOTINDATIONS
Considering the zubsurface conditions encountered in the exploratory boring and the nature of
the proposed constnrction, \üe recommend the building be founded with spread footings bearing
on a minimum of 6 feet of compacted structural fill below garage and upperJevel crawlspace
footings and a minimum of 3 feet of compacted stmctural fill below the basement level footings,
We should obsen'e the soils foruse 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 required minimum thicknesses of compacted sfuctural fill
should be designed for an allorvable bearing pressure of 1,200 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
movements of about y2 ta I inch could occur if the bearing soils are wetted.
2) The footings should have a minimum width of 24 inches for continuous rvalls 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
afea.
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 should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section ofthis report.
5) The topsoil, sub-excavated 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, silty clay soils)
compacted to at least 98% of standard Proctor density within 2olo of optimum
Kumar & Associates, lnc. o Project No. 21-7-ffii
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moisture content. The structural fill should extend laterally beyond the footing
edges equal to at least % the ñll depth below the footing.
A representative of the geotechnical engineer should evaluate the fill placement
for compaction and observe all footing excavations prior to concrete placement to
evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining sfuctures 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 structurss which are separate from the
residence and can be expected to deflect sufficientþ 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 fine-grained soils.
All foundation and retaining structures 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
backfrll 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 retainirg structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in unifonn lif* and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfïll placed in pavement and
walkway areas should be compacted to at least 95% 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
backfrll should be expected, even if the material is placed correctly, and could result in dishess to
facilities constructed on the back{ill. Backfill 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 footing. Resistance to sliding at the bottoms of the footings can be calculated
6)
Kumar & As¡ociates, lnc. e Projec{ I'lo. 21-7.ffi5
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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 pressr¡re values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the süain which will
occur at thc ultimate strength, particularly in the case of passive resistance.
FLOOR SLABS
The natural on-site soils, exclusive oftopsoil" 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 contol 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-nch 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 less than}%
passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% 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.
I.INDERDRAIN SYSTEM
Although f¡ee water was not encountered during our exploration, it has been our experience in
the area and where clay soils are present that local perched gtoundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a
perched condition. Wo recommend below-grade construction, such as retaining walls, deep
crawlspace and basement areas, be proüected 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
sourcss.
Kumar &Associates, lnc. ê Projec't No.2l-7-565
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The drains should consist of drainpipe placed in the botûom of the wall backfill surrounded above
the invert level with free-draining gtanular 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 IYoto
a suitable gravity outlet or sump and pump. Free-draining granular material used in the
underdrain system strould contain less than 2% passing the 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 lYz 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 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 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 10 feet in paved areas. Free-draining wall backfill should be
covered with filær fabric and capped with about 2 feet of the on-site soils to
reduce surface w ater infiltration.
4) Roof downspouts and drains should discharge well beyond the limis of all
backfill.
5) Landscaping which requires regular heavy inigation should be located at least
l0 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 accepûed geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
Kumar & Associates, lnc. o Project No. 21-7-fi5
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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 l, 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 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 senices 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
ofexcavations and foundation bearing skata and testing ofstructural fill by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associates, Inc.
Mark Gayeski, E.I.T
lîU¿t" t
Reviewed by:
.'''-\''',':.i -.
Daniel E. Hardin, P.E.
MG: DEH/ljf
Kumar & Associates, lnc, ¡.Project No. 21-7.565
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BENCHMARK:
MANHOLE, ELEVATION 100,,
ASSUMED
1
APPROXIMÂTE SCALE_FEET
21 -7-565 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
o
5
20
25
10
f5
30
35
40
45
F-f¡¡
l¡.¡l!
Itt-o-
t¡J(f
BORING I
EL. 98.9'LEOEND
TOPSOIK SILT ANO CIAY, SANDY W]TH GRÀVEL, ROOTS ù
ORGANICS, SOFT TO FIRII, SUGHTIY ¡TOFT, U6HT BROM{.
CLAY (CL); SILTY, SANDY T0 VERY SANDY, UTH GRAVEL,
OCCASIONAL ROOTS AT SHALTOW DÊPTHS, ÌRACE TO SIIGHTLY
CALCAREOUS, OCCASIONAL TRACE POROSIW, STIFF TO HARD,
SLIOHTLY I¡OISÍ TO I¡OIST, UGHT TO TIEDIUM BROWI{, TAt{ AND
CRAYISH-TAN.
GRAVEL (Gr¡Ì SANDY, StLfi T0 CLÂYEY WTH C0BBI."ES, VERY
DEilSE, SLIGHTLY l¡0lST T0 l¡0lST, TAN, GRAY & BR0Wl,t.
rNTER|¡YERED SAND AND SrLT (SM-r¡Lh WITH SCATTERTD GRÀVEL
AilD Cl¡Y, SUGHfIY CALCAREoUS, ltEDlUt¡ 0ENSE 0R VERY
STIFF, SUGHTLY [O1ST, LIGHT BNOWil.
DRIVE SAIIPLE, 2-INCH I.D. CAUFORNIA UNER SAI¡PU.
I
1¿¡l2DRlYE SAIIPLE BLOW COUllT. INDICATES THAT 14 B!0WS 0F,', ..A
Í4O-POUND HAI¡TIER FAI.UHG 30 INCHES WERE REQUIRED
TO DRIVE THE SAI¡PITR 12 INCHES.
NOTES
I. THE ÐOLORATORY BORING WAS DRIII"ED ON JULY 14, 2O2I WTH
A ¿.INCH DIAYTTEN CONÎIIIUOUS FLIGI{T POSER AUCER.
2. THE LOCATION OF THT EXPLORAIORY BORING WÀS I¡EASURED
APPROXII¡ATELY BY PACING FROM FEATURES SHOWN ON THE SITE
PI"AN PROVIDEÐ.
5. THE TIfVATION OF THE TXPTORAÏORY BORING WÀS I¡EASURED BY
INSTRUTINT tfVEL ÀND RTFËR TO THE BENCHMARK ON FIG. I.
1. ÎIE EXPLORATORY BORING LOCÂTION AND TLEVATION SHOUI.D BE
CONSIDERED ACCURAÎE ONLY TO THE DfGRET II¡PUED BY THE
I/ETHOD USED.
5. THE TINES BETTVEEN MATERIAIS SHOWN ON THI EXPLORATORY
BORING LOG REPRESENÍ THE AFPROXIUATT BOUNDARITS BEÍTIEEN
MÀTTRIAL TYPES AND THE TRANSITIONS UAY BE GRADUAL
6. GROUNDIYÂIER ilÀS NOT ENCOI,NTERED IN THT BOR]NC AT THE
TIIIE OF DRI]I¡NG.
7. I.AEORAÌORY TEST RESULTS:
rlc = wATrR GoNTENT (X) (ASnr D 2216);
DD = DRy DENSIÌY (pct) (lSrU D 2216)¡
-200 = PERCTNTAGE PASSING N0. 200 SIEVE (ÀSTII D tl10).
14/12
15/12
WC=9.0
DD=1 06
-200=80
10/12
12/12
WC=10.6
DD=1 05
1e/12
WC=l2,2
DD=1 1 6
-200=68
to/12
Y{C=7.7
DD=l 1 6
-2OO=79
32/12
27 /12
50/4.5
96/1O
WC=6.4
-200=29
DRTVE SAMPLE, I 5/S-|NCH r.D. SPLTT SP00N STANDARD
PENEfRÂTON TEST.
21-7-565 Kumar & Associates LOGS OF EXPLORATORY BORING Fig. 2
^0)s
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J
¡¡J.
aft
t-2
zg
t-
$-ro
a
=o<)-4
SAMPLE OF: Sondy Sllty Cloy
FROI{:BorlnglCl0'
l{C = lO.6 Í, DD = 105 pcf
ADDITIONAL COMPRESSION
UNOER CONSÏANT PRESSURE
DUE TO WTNNG
I
¡
¡
I
I
1
l
.1
l
I
21-7-565 Kumar & Associates SWELL-CONSOLIDATION TEST RESULT Fig. 5
lGrtiffi,ffi:ffü**
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ÏABLE I
SUMMARY OF LABORATORY TEST RESULTS
No.-7-565
I
BORING
45
20
I 5
I 0
5
tftì
DEPIH
SAMPLE LOCATION
6.4
7.7
t2.2
10.6
9.0
t%ì
NATURAL
MOISTURE
CONTENT
116
116
105
106
NATURAL
DRY
DËNSITY
focll
GRA\GL
(%)
SAND
(%)
29
79
68
80
PERCENT
PASSING NO.
200 slEvE
lo/ol
LIQUID LIMIT
ftt
PLASTIC
INDEX
ATTÊRBERG LIMITS
lDsfl
UNCONFINED
COMPRESSIVE
STRE}IGTH
Clayey Sattdy Gravel
Sandy Silty Clay with
Gravel
Sandy Clay
Sandy Silty Clay
Sandy Siþ Clay
SOIL WPE