HomeMy WebLinkAboutSubsoil Study for Foundation Design 5.15.14G~tech I I I 1 • ' II
HEPWORTH-P/\1NU\f\ f:iEOTECHNICl\l
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 31, PINYON MESA
CLIFFROSE WAY
GARFIELD COUNTY, COLORADO
JOB NO. 114 11 lA
!\'IA Y 15, 2014
PREPARED FOR:
SCOTT DILLARD
,, t
21 COUNTY ROAD 126
GLENWOOD SPRINGS, COLORADO 81601
scotttlill:trd1 ·caf tortit ,!!mail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......................................................................... -1 -
PROPOSED CONSTRUCTION ............................... -.................................................. -1 -
SITE CONDITIONS .................................................................................................... -1 -
SUBSIDENCE POTENTIAL ...................................................................................... :-2 -
FIELD EXPLORATION ............................................................ ." ................................. -2 -
SUBSURFACE CONDITIONS ................................................................................... :-3 -
FOlJNDATION BEARIN"G CONDITIONS ................................................................. -3 -
DESIGN RECOMMENDATIONS .............................................................................. ~ 4-
FOUNDATIONS ...................................................................................................... -4 -
FOUNDATION AND RETArnING WALLS .......................................................... :-5 -
FLOOR SLABS ....................................................................................................... - 7 -
UNDERDRArn SYSTEM ....................................................................................... :-7 -
SURFACE DRArnAGE .......................................................................................... :-7 -
LilVfITATIONS ............................................................................................................ -8 -
FIGURE 1 -LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 -LEGEND AND NOTES
FIGURE 4 -SWELL-CONSOLIDATION TEST RESULTS
TABLE 1-SUMMARY OF LABORATORY TEST RESULTS
·.
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located
on Lot 31, Pinyan Mesa, Cliffrose Way, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the stu~y was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for
geotecbnical engineering services to Scott Dillard, dated April 10, 2014.
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 detennine 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 foWldation. 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 two story wood frame structure over a craw !space with
attached garage. The garage floor will be slab-on-grade. Grading for the structure is
assumed to be relatively minor with cut and fill depths between about 2 to S 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 site was vacant at the time of our field exploration. The site slopes strongly down to
the west at grades of about 8 to 10%. Vegetation consists of sagebrush with an
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understory of sparse grass and weeds. The vegetation had been stripped on the south side
of the lot. There may be minor fill due to overlot grading as part of the original
subdivision development. There is a drainage swale at the north side of the lot.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyan Mesa
development. These rocks are a sequence of gypsiferous shale, fine -grained sandstone
and siltstone with some massive beds of gypsum and limestone. There is a possibility
that massive gypsum deposits associated with the Eagle Valley Evaporite underlie
portions of the lot. 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.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of
cavities was encowitered 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 on Lot 31 throughout the service life of
the proposed residence, in our opinion, is low; however, the owner should be made 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 April 14, 2014. An exploratory
boring was <;frilled at the location shown on Figure 1 to evaluate the subsurface
conditions. The boring was advanced with 4 inch diameter continuous flight augers
powered by a truck-mounted CME-45B driII rig. The boring was logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Job No . 114 ll IA
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Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was
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 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.
SUBSURFACE CONDITIONS
A graphic log of the subsurface condit ions encountered at the site is shown on Figure 2.
The subsoils consist of about ~ foot of topsoil overlyi ng stiff to very stiff with depth,
sandy silty clay with gravelly layers down to the drilled depth of26 feet. Previous
borings in the area indicated that low compressibility granular soils or bedrock are
expected to underlie Lot 31 at depths on the order of 35 feet.
Laboratory testing performed on samples obtained from the boring included natural
moisture content, density and percent finer than sand size gradation analyses. Results of
swell-consolidation testing performed on a relatively shallow undisturbed drive sample,
presented on Figure 4, indicate low compressibility under light loading and moderate
compressibility under increased loading after wetting. The clay sample from 15 feet
deep, presented on Figure 4, showed a moderate swell potential when wetted . The
laboratory testing is summarized in Table l.
No free water was encountered in the boring at the time of drilling and the subsoils were
generally slightly moist.
FOUNDATION BEARING CONDITIONS
The sandy silty clay soils encountered at proposed foundation level tend to settle if they
become wet. The hydro~compressible soi1s have been encountered to about 15 feet deep
in this area. The moderate expansion potential measured on the sample from 15 feet ~s
not typical of the upper bearing soils and will not impact the proposed shallow
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foundation . A shallow foundation placed on these soils will have a risk of settlement if
the soils become wet 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
tenn performance of the structure that the recommendations for surface drainage and
subsurface drainage contained in this report be followed. The amount of settlement, if the
bearing soils become wet, will be related to the depth and extent of subsurface wetting.
We expect that initial settlements will be less than 1 inc~. If wetting occurs, additional
settlements of 2 inches could occur. Settlement in the event of subsurface wetting will
likely cause building distress and mitigation such as deep compaction, a deep foundation,
such as piles or piers extending down below roughly 30 to 35 feet deep or a heavily
reinforced mat foundation, on the order of 2 feet thick, and designed by the .structural
engineer should be used to support the proposed house. If a deep foundation or mat
foundation is desired, we should be contacted to provide further design recommendations.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, the building can be founded with spread footings
bearing on a minimum 5 feet of compacted structural fill with a risk of settlement,
particularly if the bearing soils become wet, accepted by the owner. Control of surface
and subsurface runoff will be critical to the long-term performance of a shallow spread
footing foundation system.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on a minimum 5 feet of compacted structural till should
be designed for an allowable 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 l inch or less.
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Additional settlement of 1 + inches could occur if the bearing soils become
wet. A V3 increase in the allowable bearing pressure can be taken for toe
pressure of eccentrically loaded footings.
2) The footings should have a minimum width of 20 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 heavily reinforced top and bottom
to span local anomalies such as by assuming an unsupported length of at
least 14 feet. The foundation should be configured in a "box like" shape to
help resist differential movements. 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 and any loose or disturbed soils should be removed below the
building area. The exposed soils in footing area should then be moistened
and compacted. Structural fill should consist oflow penneable soil
compacted to at least 98% standar~ Proctor density within 2% of optimum
moisture content.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RET AfNING 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 residence and can be expected to deflect sufficiently to
mobilize the full active earth pressure condition should be designed for a lateral earth
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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 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 that retain more than 6 feet of soil.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill 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 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 0.30. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 300 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 95% of the maximum standard Proctor density at a
moisture content near optimum.
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FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-
on-grade construction with settlement risk similar to that described above in the event of
wetting of the sub grade soils. 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 ofrelatively well graded sand and gravel such as road
base should be placed beneath the garage slab to limit capillary moisture rise. This
material should consist of minus 2 inch aggregate with at least 50% retained on the No. 4
sieve and less than 12% 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 on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
An underdrain should not be placed around shallow footing depth structures such as the
garage and shallow crawlspace areas .
SURF ACE 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 optirnwn moisture and
compacted to at least 95% of the maximum standard Proctor density in
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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 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 at least 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 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 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 r ecommendations submitted in this report are
based upon the data obtained from the ex"}>loratory boring drilled at the location 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 inteipolation 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 dwing
construction appear differ~nt from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made .
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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 evolv es, we should provide continued consultation and field services du1ing
construction to review and monitor the implementation of our recommendations, and lo
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 foundation
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
Steven L. Pawlak, P.E.
DEH/ksw
cc : Patrick Stuckey -(stuciirch1ii 'r.evmcn~Llict )
Glenwood Structural and Civil, Inc. -Adolfo Gorra (QSl.'.la•soiiris.net)
Job l\u. 114 1111\ c~ech
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Hoowcrth-Powlak G.otec:hnl~al
114111A
CLIFFROSE WAY
BENCH MARK: GROUND AT PROPERTY
CORNER; El.EV. • 100.0', ASSUMED.
LOCATION OF EXPLORATORY BORING Figure 1
95
90
85
Qi
Q) u.
I 80 a
~
ii w
75
70
65
114111A
BORING 1
ELEV.= 94.9'
9/12
25/12
WC•S.6
00 ... 99
25112
WC•6.1
00•100
·200•87
30/12
WC•7.0
00-111
30/12
WC=8.0
00•108
·200-=90
34/12
NOTE: Explanation of symbols is shown on Figure 3.
~ H worth-Powlak Geotechnlccl
LOG OF EXPLORATORY BORING
95
90
85
Q)
Q)
lL
80 I
c
0 = ~ ~
75
70
65
Figure 2
LEGEND:
~ TOPSOIL; organic, sandy sill and clay, firm. moist, brown.
D Cl.A Y; silty, sandy, slighlfy mo ist, stiff to very stiff with depth, light brown.
p Re lative ly undisturbed drive sample; 2-lnch 1.0. Californ ia liner sample.
39,12 Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were
1 requ:red to drive the Cal ifomla or SPT sampler 12 inches.
NOTES:
1. The exploratory bori ng was drilled on Apr il 14, 2014 with a 4-inch diameter continuous flight power auger.
2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan
provided.
3. The exploratory boring elevatron was measured by instrument level and refers to the Bench Mark shown on Figure 1.
4. The exploratory boring location and elevation should be considered accurate only to the degree Implied by the
method used.
5. The lines between materi als shown on the exploratoiy bori ng log represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered In the boring at the time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results :
WC == Water Content (%)
DD • Ory Density (pcO
-200 = Percent passing No. 200 sieve
114111A ~
HEPWORTH-PAWLAK GCOTECHNICAI.
LEGEND AND NOTES Figure 3
Moisture Content = 5.6 percent
Ory Density • 99 pcl
Sample of: Sandy Silty Clay
From: Boring 1 at 5 Feet
0 l..
"#. I~~ c 1 0
·~ \ ~ \ a.
in 2
I Expansion ' c \ 0 upon ·u;
U) 3 wetting I) Q) a.
E 0
(.)
4
0 .1 1.0
APPLIED PRESSURE -ksf
10 100
Moisture Content == 7.0 percent
Dry Density = 111 pct
4 Sample of: Sandy Clay
1>-......._
From : Boring 1 at 15 Feet
3
"""" "#. ~ 8 p..._
Ci5 2 c Expansion ~~ <O
~ upon \ I 1 wetting
c: I\ 0
'(ii i'\ (/)
Q) ....
Q. 0 ~
E 0
(.) I>
1
2
0 .1 1.0
APPLIED PRESSURE -ksf
10 100
114111A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4
H£PWORTH•PA.WLAK GEOTl:CHNICAL.
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No . 114 lllA
SUMMARY OF LABORATORY TEST RESULTS
SAMPl.f LOCATION NATURAL GRADATION ATTERBfRG LIMITS UNCONFINED
MOISTURE NATURAL GRAVEL SANO PERCENT COMPRESSIVE SOILOft PlASTIC • BORING DEPTH CONTI NT ORV DENSITY PASSING NO. LIQUIOUMIT STRENGTH (%) (%) 200SIEVE INDEX BEDROCK lYPE
(ft) '"' (pcf) '"' '"' (PSFJ
1 5 5.6 99 Sandy Silty CJay
10 6.1 100 87 Sandy Silty Clay
15 7.0 111 Sandy Clay
20 8.0 108 90 Slightly Sandy Clay