HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.14.14HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 1, FOUR MILE RANCH
RED CLIFF CIRCLE
GARFIELD COUNTY, COLORADO
JOB NO. 114 409A
OCTOBER 21, 2014
PREPARED FOR:
JORDAN ARCHITECTURE
AITN: BRADJORDAN
P.O. BOX 1031
GLENWOOD SPRINGS, COLORADO 81602
(hradjordanarch ilcct rn gnrnil.com)
P.trh•r 103 -~4 l -7119 • Cnlnrndn Spnng~ 719 -613 -5562 • Silvcrthdrne 970-468-1909
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......................................................................... -I -
PROPOSED CONSTRUCTION ................................................................................... -l -
SITE CONDITIONS .................................................................................................... -2 -
SUBSIDENCE POTENTIAL ...................................................................................... :-2 -
FIELD EXPLORATION .............................................................................................. -2 -
SUBSURFACE CONDITIONS ................................................................................... :-3 -
FOUNDATION BEARING CONDITIONS ................................................................. -4 -
DESIGN RECOMMENDATIONS ............................................................................... -4 -
FOUNDATIONS ...................................................................................................... -4 -
FLOOR SLABS ....................................................................................................... -5 -
SURF ACE DRAINAGE .......................................................................................... ~ 6 -
LIMITATIONS ............................................................................................................ -6 -
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2-LOGS OF EXPLORATORY BORINGS
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 THROUGH 7 -SWELL-CONSOLIDATION TEST RESULTS
TABLE l -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 1, 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 the
foundation design . The study was conducted in accordance with our proposal for
geotechnical engineering services to Jordan Architecture, dated September 18, 2014.
A field exploration program consisting of exploratory borings was conducted to obtain
infonnation on the subsurface conditions. Samples of the subsoils obtained during the
field exploration were tested in the laboratory to detennine their 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 conceptual at the time of this report. We
understand the residence will generally be a single story wood frame structure with a 2"i.1
story above the attached garage. Ground floor will be structural over a crawlspace in the
residence and slab-on-grade in the garage. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 4 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.
Joh No. I 14 409A
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SITE CONDITIONS
The site was an undeveloped lot at the time of our field exploration. The ground surface
on the lot is relatively flat with a gentle slope down to the west. A small pond is located
on the western part of the lot. An existing residence occupies the lot to the south (Lot 2).
Vegetation consists of a moderate cover of 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 shale, 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 of localized subsidence. Sinkholes were not
observed 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 l 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 September 24 and 29, 2014. Two
exploratory borings were drilled at the locations shown on Figure I to evaluate the
subsurface conditions. The borings were advanced with 4 inch diameter continuous flight
augers powered by a truck-mounted CME-458 drill rig. The borings were logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Job No. 114 409A
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Samples of the subsoils were taken with 1% 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 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 Logs of Exploratory Borings,
Figure 2. The samples were returned to our laboratory for review by the project engineer
and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils consist of about Yi foot of topsoil overlying stiff sandy and silty clay. Silty
clayey gravel with cobbles and boulders was encountered beneath the clay at depths of 16
and 30Yi feet in Borings l and 2, respectively. Drilling in the dense granular soils was
difficult due to cobbles and boulders and practical refusal to auger drilling was
encountered in Boring l.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and density and unconfined compressive strength. Results of swell-
consolidation testing perfonned on relatively undisturbed drive samples of the clay soils,
presented on Figures 4 -7, generally indicate low compressibility under existing moisture
conditions and light loading and moderate to high compressibility upon increased loading
after wetting. Samples from relatively shallow depth (2 to 5 feet) showed moderate
hydro-compression potential {collapse upon wetting). The sample from Boring l at l 0
feet showed a minor expansion potential when wetted under light loading and a swell
pressure of about 3 ,500 psf.
No free water was encountered in the borings at the time of drilling. Groundwater was
measured in Boring 2 at a depth of 19 feet and no free water was encountered in Boring I
when checked on September 29, 2014. The subsoils were slightly moist to moist above
the groundwater.
Job No. I 14 409A ~h
-4-
FOUNDATION BEARING CONDITIONS
The clay subsoils encountered on the lot possess variable expansion or collapse potential
when wetted. Surface runoff, landscape irrigation, and utility leakage are possible
sources of water which could cause wetting. The settlement/heave potential of the
subgrade should be further evaluated at the time of construction for foundations bearing
on the natural clay site soils.
Placement of foundations on a depth of properly compacted structural fill would serve to
reduce the potential for differential movement of the foundation areas. Structural fill in
foundation areas can consist of imported granular material, such as CDOT Class 6 road
base, placed in 8 inch loose lifts and compacted to at least 98 percent of the standard
Proctor value for the material.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, we recommend the building be founded with spread
footings bearing on the natural subsoils evaluated at the time of construction for bearing
properties or properly compacted structural fill.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural clay subsoils should be
designed for an allowable bearing pressure of 1,500 psf. Footings placed
on a minimum of 3 feet of properly compacted structural fill should be
designed for an allowable bearing pressure of 2,000 psf. Structural fill
should extend a minimum of 2 feet beyond the perimeter of the footings.
Based on experience, we expect initial settlement of footings designed and
constructed as discussed in this section will be about 1 inch or less . There
could be some additional movement {mainly for bearing directly on the
Job No. 114 409A
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clay soils) if the subgrade becomes wetted and precautions should be taken
during design, construction and over the life of the structure to minimize
wetting of the subsoils below the building.
2) The footings should have a minimum width of 18 inches for continuous
walls and 2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided
with adequate soil cover above their bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typically used in this area.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14
feet. Foundation walls acting as retaining structures should also be
designed to resist a lateral earth pressure corresponding to an equivalent
fluid unit weight of at least 55 pcf for the on-site soils as backfill.
5) The topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the firm natural soils. The exposed
soils in footing area should then be moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-
on-grade construction with some risk of movement if the sub-slab soils become wetted.
The settlement/heave potential of the sub-slab area should be further evaluated at the time
of construction. 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 lower
level slabs-on-grade (if constructed) to facilitate drainage and limit capillary moisture
Job No. I 14 409A ~h
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rise. This material should consist of minus 2 inch aggregate with at least 50% retained on
the No. 4 sieve and less than 2% 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 or suitable imported fill devoid of vegetation, topsoil and
oversized rock.
SURF ACE DRAINAGE
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 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 6 inches in the first I 0 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 and foundation areas.
5) Irrigation sprinkler heads and landscaping which requires regular heavy
irrigation, such as sod, should be located at least 5 feet from foundation
walls.
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
Joh No . I 14 409A
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based upon the data obtained from the exploratory borings drilled at the locations
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
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 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 services during
construction to review and monitor the implementation of our recommendations, and to
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications to the recommendations
presented herein. We recommend on-site observation of excavations and foundation
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
James A. Parker, P.E., P.G.
Reviewed by:
Steven L. Pawlak, P .E.
JAP/lj g
Job No. 114 409A
APPROXIMATE SCALE
1· -80
LOT2
COUNTY
ROAD 117
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SPACE
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LOT1 ~ I ~,
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RED CLIFF CIRCLE
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114 409A ~ LOCATION OF EXPLORATORY BORINGS
HEPWORTH·PAWl.AK G E OT'£CHNICAL
FIGURE 1
6080
6075
6070
6065
Qi a> LL
I
8 ·i
iii 6060
6055
6050
6045
114 409A
BORING 1
ELEV.= 6079'
8/12
14/12
WC =159
00 =86
17/12
WC =126
00 =116
18/12
5
-
BORING2
ELEV.= 6078 .5'
9/12
13/12
wc ~12 .8
00•86
14/12
12/12
wc ... 20.1
00-101
131 12
wc-21 .0
00•103
UC r 3200
11/12
4/6,50/1
Note : Explanation of symbols is shown on Figure 3.
~ LOGS OF EXPLORATORY BORINGS
HEPWORTii·PAWLAK GEOTECHNICAL
6080
6075
6070
6065
fil
LL
' c:
.Q
-ro > Q)
6060 iii
6055
6050
6045
FIGURE 2
LEGEND:
8/12
5
[]
T
NOTES:
TOPSOIL; sandy clay, with roots, moist, brown.
CLAY (CL); silty, sandy at inteNals, stiff, slightly moist to moist with depth, brown, fine calcareous veins at
various depths.
GRAVEL (GC-GM); silty, clayey, with cobbles, very dense, moist to wet, brown and black subangular to
rounded rocks.
Relatively undisturbed drive sample; 2-inch l.D. California liner sample .
Dnve sample; standard penetration test (SPD. 1 3/8 inch l.D. spht spoon sample, ASTM D-1586 .
Drive sample blow count; indicates that B blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Free water level in Boring 2 and number of days following drilling measurement was taken .
Indicates slotted PVC pipe installed in boring to depth shown.
Practical drilling refusal.
1. Exploratory borings were drilled on September 24 and 29, 2014 with 4-inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approx imately 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. Water level readings shown on the logs were made at the t ime and under the conditions indicated. No free water
was encountered at the time of drilling or in Bor ing 1 when checked 5 days later. Fluctuations in water leve l may
occur with time .
7. Laboratory Testing Results :
WC = Water Content (%)
DD ,., Dry Dens ity (pcf)
UC = Unconfined Compressive Strength (psf)
114 409A ~
HEPWORTli·PAWLAK GitOTECHNICAL
LEGEND AND NOTES FIGURE 3
Moisture Content -15 .9 percent
Dry Density = 86 pcf
Sample of : Silty Clay
From: Boring 1 at 5 Feet
0
ri ,
1
i-v __ Compression -~ ._. --upon
?fl. 2 ( wetting
c: \ 0 'iii en 3 Q) ..... a.
E
0 u
4
5 \
6 \
7 \
I I
8 \
\
9 \
10 \
11
0 .1 1.0 10 100
APPLIED PRESSURE • ksf
114 409A ~ SWELL-CONSOLIDATION TEST RESULTS FIGURE 4
HEPWORTH•PAWLAK GEOTECHNICA1.
Moisture Content ... 12.6 percent
Dry Density = 115 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 1 O Feet
Cft 1
c .Q
(/) c co 0 I\. a. x '~ LU r--~ I -i"'ooi"o )......_ c r--... 0 1 """<)
"(ii \ (/)
Q) ...... a.
E 2 0 u Expansion
upon
wetting
01 1.0 10 100
APPLIED PRESSURE-ksf
114 409A ~ SWELL-CONSOLIDATION TEST RESULTS FIGURE 5
HEPWORTH•PAWV.K GEOTEC:HNICAL
.
Moisture Content = 12.8 percent
Dry Density = 86 pcf
Sample of: Silty Clay
From: Boring 2 at 2 Feet
0
~ -r--~ i-~ '1)
1
Compression
~ --D upon
2 _,,.,, ~"" "" wetting '(fl.
c ~ (_ v ........ ~
0 ·u;
(/)
3 Q) ....
0.
E c 0
(.) \ 4
5 \
6
7 \
8 \
(
9 \
\
10
11 \
' 12
0.1 1.0 10 100
APPLIED PRESSURE -ksf
114 409A ~ SWELL-CONSOLIDATION TEST RESULTS FIGURE 6
HEPWORTH-PAWLAK GEOTECHNICAL
.
Moisture Content = 20 .1 percent
Dry Density = 101 pcf
Sample of: Sil ty Sandy Clay
From: Boring 2 at 1 O Feet
No Movement
r--upon wett ing
0 ---r---r-r-t.'l~ 1
!'--...._
-~ ~ 0 -z 2 .........
0 \ Ci5 ~ (J) w 3 a:
11. \ ~
0 u 4
5
0.1 1.0 10 100
APPLIED PRESSURE ( ksf )
114 409A ~tech SWELL-CONSO LI DATI ON TEST RESULTS FIGURE 7
HEPW O R TH-PAWLAK GEOTEC HNICAL
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No.114409A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT ATTERBERG LIMITS UNCONFINED
MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE
BORING DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH SOIL TYPE
(%) (%) SIEVE
(ft\ l%\ (ccfl f%} f%l tPSF\
1 5 15.9 86 Silty Clay
10 12.6 116 Sandy Silty Clay
2 2 12.8 86 Silty Clay
10 20.1 101 Silty Clay
15 2 1.0 103 3200 Silty Clay