HomeMy WebLinkAboutSoils Report 11.26.2007HI P
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HEPWORTH - PAWLAK GEOTECHNICAL
Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email: hpgeo@hpgeotech.com
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED DUPLEX
LOT D-23, ASPEN GLEN SUBDIVISION
ELK TRACK LANE
GARFIELD COUNTY, COLORADO
JOB NO. 107 0818
NOVEMBER 26, 2007
PREPARED FOR:
JORDAN ARCHITECTURE
ATTN: BRAD JORDAN
P.O. BOX 1031
GLENWOOD SPRINGS, COLORADO 81602
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION _ 1
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 8 -
REFERENCES - 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
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed duplex residence to be
located on Lot D-23, Aspen Glen Subdivision, Elk Track Lane, 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 October 26, 2007. Chen -Northern, Inc. (1991 and 1993) previously
conducted preliminary geotechnical engineering studies for the development and
preliminary plat design.
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,
design recommendations and other geotechnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed duplex will be a one and two wood frame structure over a partial basement
level and partial crawlspace located in the area shown on Figure 1. The attached garage
and basement floors will be slab -on -grade. Grading for the structure is assumed to be
relatively minor with cut depths between about 3 to 8 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.
Job No. 107 0818
GPis�YeCh
-2 -
SITE CONDITIONS
Lot D-23 is located in the eastern part of the development and was vacant at the time of
our field exploration. The lot is off of Elk Track Lane cul-de-sac between the road and the
12th Fairway to the east. The ground surface in the building area is relatively flat with a
gentle slope down to the southwest and about 2 to 3 feet of elevation difference. The lot
has been disturbed by past grading for the golf course, including a landscape berm and
pond at the northeast, rear side of the lot. An open ditch flows along the southern
property line. The pond appears to be lined. Vegetation consists of grass and weeds.
Scattered cobbles were exposed on the ground surface.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
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
studies for the subdivision (Chen -Northern, 1991 and 1993), several sinkholes were
observed scattered throughout the Aspen Glen development, mainly east of the Roaring
Fork River. These sinkholes appear similar to others associated with the Eagle Valley
Evaporite in areas of the Roaring Fork River Valley. The closest mapped sinkhole to Lot
D-23 is located about 730 feet to the west and the perimeter of a broad subsidence area
that contains the sinkhole is about 240 feet from the lot.
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 D-23 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.
Job No. 107 0818
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FIELD EXPLORATION
The field exploration for the project was conducted on November 1, 2007. Two
exploratory borings were drilled at the locations shown on Figure 1 to evaluate the
subsurface conditions. The borings were advanced with 4 inch diameter continuous flight
augers powered by a truck -mounted CME -45B drill rig. The borings were logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with 1% 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 sirriilar 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 1 foot of topsoil overlying stiff to very stiff, sandy silty clay
to depths of 141/2 and 91/2 feet in Borings 1 and 2, respectively. Relatively dense, slightly
silty sandy gravel, cobbles and boulders was encountered below the clay soils to the
drilled depths of 12 and 151A feet. Drilling in the dense granular soils with auger
equipment was difficult due to the cobbles and boulders and drilling refusal was
encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and density. Results of swell -consolidation testing performed on a
relatively undisturbed drive sample of the clay soils, presented on Figures 4 and 5,
generally indicate low to moderate compressibility under conditions of loading and
wetting. The sample from Boring 2 at 5 feet showed a low to moderate expansion
potential when wetted under a constant light surcharge.
Job No. 107 0818
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No free water was encountered in the borings at the time of drilling or when checked 13
days later and the subsoils were slightly moist.
FOUNDATION BEARING CONDITIONS
The upper clay soils have low bearing capacity with variable settlement/heave potential.
These soils are typically known to be compressible under loading when wetted and the
expansion potential indicated by the laboratory testing can be neglected in the design but
the settlement/heave potential of the clay soils should be further evaluated at the time of
construction. The underlying gravel and cobble soils have moderate bearing capacity and
minor settlement potential. The groundwater level in the area is relatively deep and a
basement level should be feasible with little risk of groundwater impacts. We assume that
the pond to the northeast and the ditch to the south of the building site are lined and will
have no impact on the development. The lined condition and water management plan
should be confirmed with the developer.
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 soils.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf. Footings or piers that bear
entirely on the dense gravel and cobble soils can be designed for an
allowable bearing pressure of 4,000 psf. 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
movements on the order oft to 1 inch if the bearing soils are wetted.
2) The footings should have a minimum width of 18 inches for continuous
walls and 2 feet for isolated pads.
Job No. 107 0818
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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 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) Any existing fill, 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.
FOUNDATION 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 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 pressure computed
on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of
the on-site soils. Backfill should not contain vegetation, topsoil or oversized rock.
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.
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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 0.35 for the clay soils and 0.5 for the
gravel and cobble soils. 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.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -
on -grade construction. The clay soils have variable settlement/heave potential when
wetted which could result in some slab movement and distress if the bearing soils become
wet. 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
Job No. 107 0818
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facilitate drainage. 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 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.
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, sump and pump or
drywell that outlets in the underlying gravel soils. 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 of 2 inches. The drain gravel
backfill should be at least 11/2 feet deep. Where footings bear on the upper clay soils and
an underdrain is needed, an impervious membrane such as a 20 mil PVC liner 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
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.
Job No. 107 0818
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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 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 about 2 feet of the on-
site silt and clay 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 and sprinkler heads
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
based upon the data obtained from the exploratory borings drilled at the locations
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 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
arc not responsible for technical interpretations by others of our information. As the
project evolves, we should provide continued consultation and field services during
Job No. 107 0818
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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,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Jordy Z. Adamson, Jr., P.E.
Reviewed by:
Steven L. Pawlak, P.E.
JZA/vad
REFERENCES
Chen -Northern, 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen
Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated December 20, 1991, Job No. 4 112 92.
Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design,
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated May 28, 1993, Job No. 4 112 92.
Job No. 107 0818
EXISTING POND
--"*":416
APPROXIMATE SCALE
1'-40'
GOLF COURSE
LOT D26
ELK
TRACK
LANE
107 0818
LOT D23
1
1
1
I
1 I
1
1
I
1
1
1
1
1
BORING 2
■
BENCH MARK: ELECTRICAL BOX CONCRETE
SLAB; ELEV. = 100.0', ASSUMED.
LOT D24
He. worth -Pail
Goat e nIcaI
LOT D22
LOT D21
LOCATION OF EXPLORATORY BORINGS Figure 1
Elevation - Feet
f-- 105
100
95
90
85
BORING 1
ELEV.= 102.5'
8/12
8/12
WC=13.7
DD=100
8/12
WC=16.1
DD=108
26,6,28/2
BORING 2
ELEV.= 99.7'
7/12
WC=7.2
DD=86
16/12
WC=8.3
DD=99
38/6,8/0
Note: Explanation of symbols is shown on Figure 3.
105
100
95 —
90
85
Elevation - Feet
107 0818
Hepworth-Pawlak C °technIcal
LOGS OF EXPLORATORY BORINGS
Figure 2
LEGEND:
2 Topsoil; sandy silty clay, organics, roots, firm, slightly moist, brown.
Clay (CL); silty, sandy, stiff to very stiff, slightly moist, reddish brown.
Gravel (GM); silty, sandy, with cobbles and boulders,derise, slightly moist, brown.
L
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586.
8/12 Drive sample blow count; indicates that 8 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
T
Practical drilling refusal.
Depth at which boring had caved when checked on November 14, 2007.
NOTES:
1. Exploratory borings were drilled on November 1, 2007 with 4 -inch 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 measured by instrument level and refer to the Bench Mark shown on Figure 1.
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 or when checked 13 days later. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
107 0818
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Hepworth -Pawls v ;vol
LEGEND AND NOTES
Figure 3
Compression %
Compression %
0
1
2
3
4
0
1
2
3
4
Moisture Content = 13.7 percent
Dry Density = 100 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 5 Feet
Compression
-upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 16.1 percent
Dry Density = 108 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 10 Feet
/ Compression
upon
'''e wetting
\\(\....:."'e
i
0.1
1.0 10
APPLIED PRESSURE - ksf
100
107 0818
H
Hepworth—Pawlak Geotachnlao1
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
0
X
w
0
c
a
w
0
1
2
4
5
6
7
2
1
0
2
.41
0.1
Moisture Content = 7.2 percent
Dry Density — 86 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 2 Feet
Expansion
upon
wetting
1.0 10
APPLIED PRESSURE - ksf
1
100
Moisture Content = 8.3 percent
Dry Density = 99 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 5 Feet
Expansion
upon
wetting
0.1
107 0818
1.0 10
APPLIED PRESSURE - ksf
H
Hepworth—Pawl c tectrnlcof
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 5