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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 24, FOUR MILE RANCH
RED CLIFF CIRCLE
GARFIELD COUNTY, COLORADO
JOB NO. 114 416A
NOVEMBER 14, 2014
PREPARED FOR:
MIKE PICORE
P.O. BOX 519
GLEN\VOOD SPRINGS, COLORADO 81602
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ........................................................................ -1 -
PROPOSED CONSTRUCTION ................................................................................. -1 -
SITE CONDITIONS ................................................................................................... -2 -
SUBSIDENCE POTENTIAL ..................................................................................... -2 -
FIELD EXPLORATION ............................................................................................ -2 -
SUBSURFACE CONDITIONS .................................................................................. -3 -
FOUNDATION BEARING CONDITIONS ............................................................... -4 -
DESIGN RECOMMENDATIONS ............................................................................. -4-
FOUNDATIONS .................................................................................................... -4-
FOUNDATION AND RETAINING WALLS ......................................................... -6 -
FLOOR SLABS ...................................................................................................... -7 -
UNDERDRAIN SYSTEM ...................................................................................... -8 -
SURFACE DRAINAGE ......................................................................................... - 9 -
LIMITATIONS .......................................................................................................... - 9 -
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2 -LOGS OF EXPLORATORY BORINGS
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 and 5 -SWELL-CONSOLIDATION TEST RESULTS
TABLE 1 -SUMMARY OF LABORATORY TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located
on Lot 24, Four Mile Ranch, Red Cliff Circle, Garfield County, Colorado. The project
site is shown on Figure 1. The purpose of the study was to develop recommendations for
foundation design. The study was conducted in accordance with our agreement for
geotechnical engineering services to Mike Picore, dated September 25, 2014.
A field exploration program consisting of exploratory borings was conducted 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,
recommendations and other geotcchnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Design of the proposed residence was not available at the time of our study. For the
purpose of our analysis, we assume the residence will be a one and two story wood frame
structure with a walkout basement level. Ground level floors (such as in the garage)
could be slab-on-grade or structural above crawlspace. 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 assumed type of construction.
When building loadings, location or grading plans are better known, we should be
notified to re-evaluate the recommendations contained in this report.
Job No. 114 416A ~tech
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SITE CONDITIONS
The lot was vacant at the time of our field exploration. The ground surface was relatively
flat with a gentle to strong slope down to the west-southwest and about 6 feet of elevation
difference across the building envelope. Vegetation consisted of sagebrush, grass and
weeds.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Four Mile Ranch
development. These rocks are a sequence of gypsiferous sQale, fine-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 lot. Dissolution of the gypsum under certain conditions can cause
sinkholes to develop and can produce areas oflocalized subsidence.
Sinkholes were not observed on or in the immediate area of the subject lot. No evidence
of cavities was encountered in the subsurface materials; however, the exploratory borings
were 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 24 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 explorat ion for the project was conducted on September 29, 2014. Two
exploratory borings were drilled at the locations shown on Figure 1 to evaluate the
subsurfuce conditions. The borings were advanced with 4 inch diameter continuous flight
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auger powered by a truck-mounted CME-45B drill rig. The borings were Jogged by a
representative of Hepworth-Pawlak Gcotechnica~ Inc.
Samples of the subsoils were taken with l % inch and 2 inch l.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 descnoed
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 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 profiles encountered at the site are shown on Figure 2.
Below about \r'2 foot of organic topsoil, the subsoils consist of very stiff to hard, sandy
clay underlain by dense, basalt gravel, cobbles and probably boulders in a sandy clay
matrix at a depth of about 13~ and 17 feet in the borings. The soils encountered in the
borings are similar to the soils encountered at other nearby lots. The clay soils can
possess an expansion potential when wetted. Drilling in the coarse granular subsoil with
auger equipment was difficult due to the cobbles and boulders and practical drilling
refusal was encountered in the deposit at Boring 1.
Laboratory testing perfonned on samples obtained from the borings during the field
exploration included natural moisture content and density. Swell-consolidation testing
was performed on relatively undisturbed drive samples of the clay soils. The swell-
consolidation test results, presented on Figures 4 and 5, generally indicate low
compressibility wider existing low moisture and light loading conditions and a low to
high expansion potential when wetted under a constant light surcharge with swelling
pressures between about 4,000 psf and 20,000 pst:
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No free water was encountered in the borings at time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The clay soils encountered at the site generally possess an expansion potential when
wetted. The expansion potential can probably be partly mitigated by load concentration
to reduce or prevent swelling in the event of wetting below the fuundation bearing level.
The heave potential can also be reduced by deepening the bearing level such as with a
basement level or by replacing the clay below the bearing level with non expansive
structural fill such as road base. Surface runoff, landscape irrigation, and utility leakage
are possible sources of water which could cause wetting. The expansion potential of the
subgrade should be further evaluated at the time of construction . A low settlement risk
foundation could consist of piles or piers that extend down into the underlying coarse
granular soils. Presented below are recommendations for a shallow foundation. If a deep
foundation is proposed, we should be contacted for additional recommendations.
DESICfN-UCOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, the residence can be fuunded with spread footings
placed on the undisturbed natural soils or on compacted structural fill with a risk of
movement and distress .
The design and construction criteria presented below should be observed for a spread
fuoting foundation system.
1) Footings placed on the undisturbed natural soils can be designed for an
allowable bearing pressure of3,000 psf The footings should also be
designed for a minimum dead load pressure of 1,000 psf. In order to
satisfy the minimum dead load pressure under lightly loaded areas, it may
be necessary to concentrate loads by using a grade beam and pad system.
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Wall-on-grade construction is not recommended at this site to achieve the
minimum dead load. The minimum dead load pressure is not needed
where structural fill is used below the footing bearing level.
2) Based on experience, we expect initial settlement of footings designed and
constructed as discussed in this section will be up to about 1 inch. There
could be additional differential movement of about 1 inch if the bearing
soils were to become wetted to a limited depth of about 10 feet. A larger
movement may occur under deeper wetting. Structural fill used to limit
the differential foundation movement potential should be at least 3 feet
deep below bearing level and consist of a well graded granular material
such as CDOT Class 6 base course and be compacted to at least 98% of its
standard Proctor value. The structural fill should extend beyond the
footing edges a distance equal to at least 'fl the fill depth below the footing
but at least 2 feet.
3) The footings should have a minimum width of 16 inches for continuous
footings and 24 inches for isolated pads.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomalies and limit the risk of differential movement. One method
of analysis is to design the foundation wall to span an unsupported length
of at least 12 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) 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 the exterior grade is
typically used in this area .
6) Prior to the footing construction, the topsoil and loose or disturbed soils
should be removed to expose the undisturbed natural soils.
7) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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FOUNDATION AND RETAINING WALLS
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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 60 pcf
for backfill consisting of the on-site soi1s. 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 50 pcffor backfill consisting of
the on-site 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
walJs 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
buiJdup behind waUs.
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. Use
of a granular soil such as road base could be used to limit the backfill settlement potential.
The lateral resistance of foundation or retaining wall fuotings 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
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backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of350 pct: 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 oftbe footings to resist lateral loads
should be compacted to at least 95% of the maximum standard Proctor density at a
moisture content near optimum.
FLOOR SLABS
The clay soils possess an expansion potential and slab heave could occur if the subgrade
soils were to become wet. Slab-on-grade construction may be used where the potential
movement can be tolerated, such as the garage, provided precautions are taken to limit
potential movement and the risk of distress to the building is accepted by the owner. A
posit~ve way to reduce the risk of slab movement, which is commonly used in the area, ~
to construct structurally supported floors over crawlspace. The expansion potential of the
subgrade should be further evaluated at the time of construction.
Slab-on-grade construction may be used in the garage area provided the risk of distress is
understood by the owner. We recommend placing at least 3 feet of nonexpansive, well
graded granular materiaL such as road base, below floor slabs in order to help mitigate
slab movement due to expansive soils.
To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from cil.I bearing walls and columns with expansion joints which allow
unrestrained vertical movement. Interior non-bearing partitions resting on floor slabs
should be provided with a slip joint at the bottom of the wall so that, if the slab moves,
the movement cannot be transmitted to the upper structure. This detail is also important
for wallboards, stairways and door frames. Slip joints which will a11ow at least 1 Yi inches
of vertical movement are recommended. Floor slab control }lints should be used to
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reduce damage due to shrinkage cracking. Slab reinforcement and control joints should
be established by the designer based on experience and the intended slab use.
Required fill beneath slabs should consist of imported granular material, such as road
base. The fill should be spread in thin horizontal lifts, adjusted to at or above optimum
moisture content, and compacted to at least 95% of the maximum standard Proctor
density. All vegetation, topsoil and loose or disturbed soil should be removed prior to fill
placement.
The above recommendations will not prevent slab heave if the expansive soils underlying
slabs-on-grade become wet. However, the recommendations will reduce the effects if
slab heave occurs. All plumbing lines should be pressure tested before backfilling to help
reduce the potential for wetting.
UNDERDRAINSYSTEM
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
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 level of excavation and at least 1 foot below lowest adjacent finish
grade and sloped at a minimum 1 % to a suitable gravity outlet. Free-draining granular
material used in the underdrain system should contain less than 2% passing the No. 200
sieve, less than 50% passing the No. 4 sieve and have a maximum size of2 inches. The
drain gravel backfill should be at least I !h feet deep. Where void form is exposed below
the foundation wal~ an impervious membrane such as a 20 mil PVC liner should be
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placed beneath the drain gravel in a trough shape and attached to the foundation wall with
mastic to prevent water flow to beneath the building.
SURF ACE DRAINAGE
Keeping the bearing soils dry below the structure will be critical to the satisfactory
performance of the residence. The following drainage precautions should be observed
during construction and maintained at all times after the residence has been completed:
1) Excessive wetting or drying of the foundation excavations and underslab
areas should be avoided during construction. Drying could increase the
expansion potential of the clay soils.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at least 95% of the maximum standard Proctor density in
pavement 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill. Natural vegetation lined drainage swale~ should have a minimum
slope of3%.
5) Irrigation sprinkler heads and landscaping which requires regular heavy
irrigation, such as sod, should be located at least 10 feet from fuundation
waUs. Consideration should be given to use ofxeriscape to limit wetting
below the building from landscape irrigation water.
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
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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 extrapolation of the subsurface
conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is perfonned. If conditions
encountered during construction appear to be different from those described in this report,
we should be notified at once so 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 infonnation. 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
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications of 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,
HEPWORTH -PAWLAK GEOTECHNICAL, INC.
Reviewed by:
SLP/ljg
Job No. 11 4 416A ~tech
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RED CLIFF CIRCLE
APPROXIMATE SCALE
1" = 60' ----r-~----
;002 - -
LOT23
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H worth-Pawlak Geotechnlcal
114416A
----
LOT24
--------
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OPEN SPACE
-1000
LOT25
LOCATION OF EXPLORATORY BORINGS Figure 1
1010
1005
1000
Q)
if
I 995 c:
0
~
Q) w
990
985
980
114416A
BORING 1
ELEV.= 1008'
24/12
wc .. a.2
00 .. 102
32/12
50/12
WC=9.5
DO:a112
BORING2
ELEV.= 1002.5'
26/12
72/12
WC=7.8
00=126
50/9
76/12
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Note : Explanation of symbols is shown on Figure 3.
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H worth-Pawlak Geolechnlcal
LOGS OF EXPLORATORY BORINGS
1010
1005
1000
995
Q)
Cl> u.
' c
0 ~ w
990
985
980
Figure 2
.f"
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LEG&'ID:
TOPSOIL; organic sandy slit and clay, firm, moist, dark brown.
CLAY (CL); sandy, scattered gravel, very stiff to hard with depth, slightly moist. red-brown, medium plasticity,
slightly calcareous and porous to calcareous streaks with depth.
GRAVEL (GC); clayey, sandy, cobbles, probable boulders, dense, slightly moist, mixed brown, with basalt rock.
Relatively undisturbed drive sample: 2~inch l.D. Galifornia liner sample.
Drive sample: standard penetration test (SPl), 1 3/8 inch l.D. split spoon sample, ASTM 0-1586.
24112 Drive sample blow count; indicates that 24 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
T Practical drilling refusal.
NOTES:
1. Exploratory borings were drilled on September 29, 2014 with 4-lnch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory borings were obtained by Interpolation between contours shown 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 transitions may be gradual.
6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density {pc~
114 416A ~ Heoworth-Pawfak Geob1c:hnk:al
LEGEND AND NOTES Figure 3
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Moisture Content = 8.2 percent
Dry Density = 102 pct 0 Sample of: Sandy Clay, Calcareous --.... From: Boring 1 at 2 Feet .... ""' ~ )
1 I l""--.
~ r--r---I""-........ ~
*' 2 ...
' ""' Compression c:; ~ f'-.... 0 r-.. t--.._ upon "iii
tll 3 wetting Q) ....
0. ' E j I
0 \ ()
4
'
5 \
' iD
6
0.1 1.0 10 100
APPLIED PRESSURE -ksf
Moisture Content = 9.5 percent
Dry Density = 112 pcf
Sample of: Sandy Clay
*' From: Boring 1 at 1 O Feet
c: 0 1
'Ci) c l4i>----.., ro
0.
............. ~ 0
• '~ '~ c: -0 "') ·u;
tll 1 Q) .... Expansion 0.
E upon 0
(..) 2 wetting
0.1 1.0 10 100
APPLIED PRESSURE -ksf
114416A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4
He11worth-Powhlk Geo\oc:hnlc:al
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Moisture Content = 7.8 percent
Dry Density = 126 pct
Sample of: Sandy Clay
From: Boring 2 at 8 Feet
5
4 1P---_
~ ~
3 !'r,
Expansion '\
i'\ upon \
2 wettina "'
<fl. '~ c 0
'iii 1 a \, a.
~
I 0 c
0
Ci)
UJ
~ a. 1 E
0 (.)
0.1 1.0 10 100
APPLIED PRESSURE -ksf
114416A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 5
Hepwarth-P4111ak Geal11chnlcal
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 114 416A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL GRADATION AmRBERG LIMITS UNCONFINED
MOISTURE NATURAL PERCENT COMPRESSIVE GRAVEl SANO PlASTIC SOIL OR BORING DEPTH CONTENT DRVOENSITV PASSING NO . UQUIOLIMIT STRENGTH
l"l (%) 200SIEVE INDEX BEDROCK TYPE
(h) (") (pd) (%) (%1 (PSF)
1 2 8.2 102 Sandy Clay. Calcareous
10 9.5 112 Sandy Clay
2 8 7.8 126 Sandy Clay