HomeMy WebLinkAboutSoils Report 07.10.2006Gtech
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 RESIDENCE
LOT 15, CERISE RANCH
GARFIELD COUNTY, COLORADO
JOB NO. 106 0474
JULY 10, 2006
PREPARED FOR:
WALKER CONSTRUCTION
ATTN: IAN WALKER
600 E. HOPKINS AVENUE, SUITE 203
ASPEN, COLORADO 81611
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 -
FLOORSLABS -6-
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 8 -
LIMITATIONS - 8 -
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
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at
Lot15, Cerise Ranch, Larkspur Drive, 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 Walker Construction dated May 16, 2006.
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 residence will be a one story, wood frame structure over a walkout
basement level with an attached garage located roughly in the area of the exploratory
borings shown on Figure 1. Basement and garage floors will be slab -on -grade. We
understand that cut depths for the walkout basement level are planned to be up to about
10 to 12 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.
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2
SITE CONDITIONS
Lot 15 was vacant at the time of our field exploration and is located on the south side of
Larkspur Drive. The ground surface in the proposed building area is relatively flat with a
gentle slope down to the south. An abandoned irrigation ditch crosses the lot at the north
(uphill) side of the building envelope and an active ditch is located just below the
building envelope. The downhill ditch was flowing at the time of our field exploration.
Vegetation consists of grass and weeds. Eagle Valley Evaporite is visible on the valley
hillside to the north.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian. age Eagle Valley Evaporite underlies the Cerise Ranch
Subdivision. 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,
several sinkholes were observed scattered throughout the Cerise Ranch Subdivision.
These sinkholes appear similar to others associated with the Eagle Valley Evaporite in
areas of the Roaring Fork River valley.
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_ 15 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.
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FIELD EXPLORATION
The field exploration for the project was conducted on June 15 and 16, 2006. 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 LD. 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, below about one foot of topsoil, consist of mainly sandy silty clay with
scattered gravel at Boring 1 overlying silty sandy gravel at a depth of 32 feet in Boring 1
and 22 feet in Boring 2. The clay soils are stiff to soft with depth.
Laboratory testing performed on samples obtained from the borings included natural
moisture content, density and percent finer than No. 200 sieve size (silt and clay fraction)
gradation analysis. Results of swell -consolidation testing performed on relatively
undisturbed drive samples 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 3 feet showed a minor expansion potential after wetting. The
laboratory testing is summarized in Table 1.
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Free water was encountered in the borings at depths of 19 feet in Boring 1 and 10 feet in
Boring 2 when checked on June 28, 2006. The upper sandy silty clay soils were typically
moist.
FOUNDATION BEARING CONDITIONS
Based on the subsoil conditions encountered in the borings, a spread footing foundation
bearing on the upper sandy silty clay soils appears feasible with some risk of settlement.
A deep foundation (such as driven piles) which extends down to the relatively dense
gravel subsoils could be used to provide a moderate load capacity and a low settlement
risk. We should be contacted if a deep foundation is proposed.
4
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 upper 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 sandy silty clay soils should be
designed for an allowable bearing pressure of 1,200 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be up to about 1 to 11/2 inches.
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.
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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 a lateral earth pressure as discussed below in
"Foundation and Retaining Walls".
5) All topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the natural undisturbed soils. The
exposed soils in footing area should then be lightly compacted. If water
seepage is encountered, we should be contacted for evaluation and
additional recommendations.
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 fine-grained soils. Cantilevered retaining structures
which are separate from the building 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 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
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retaining structure. An underdrain should be provided to prevent hydrostatic pressure
buildup behind walls.
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 bacicfill 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. 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 with some risk of differential settlement. 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
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designer based on experience and the intended slab use. A minimum 4 inch layer of free -
draining gravel should be placed beneath basement slabs to facilitate drainage. This
material should consist of minus 2 inch aggregate with at least 50% retained on the No. 4
sieve and less than 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. Hi lily moist
soils could require drying before reuse as structural fill.
UNDERDRAIN SYSTEM
Although free water was encountered in the borings below proposed excavation depths, it
has been our experience in the area that the groundwater level can rise and local perched
groundwater can develop during times of heavy precipitation, irrigation season 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 or sump and pump. 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 1' feet deep. An
impervious membrane, such as 20 or 30 mil PVC liner, should be placed below the drain
gravel and attached to the foundation wall with mastic.
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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.
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 soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) 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
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
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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,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Trevor L. Knell, P.E.
Reviewed by:
Steven L. Pawlak, P.E.
TLK/ksw
Job No. 106 0474
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ELEVATION - FEET
- 6375
- 6370
- 6365
6360
6355
6350
- 6345
- 6340
6335
BORING 1
ELEV. =6374'
11/12
18/12
6/12
13/12
WC=10.7
DD=119
1/12
7/12
BORING 2
ELEV. =6363'
10/12
WC=11.0
DD=110
-200=88
8/12
WC=14.0
DD=110
6355
6375
6370
6365
6360
1/12
4/12
Note: Explanation of symbols is shown on Figure 3.
6350
6345
6340
6335
ELEVATION - FEET
106 0474
I I EPWOR F I• PAW LAK GEOTECI I N ICAL
LOGS OF EXPLORATORY BORINGS
FIGURE 2
LEGEND:
® TOPSOIL; silty clay, soft, slightly moist to moist, dark brown, organic.
i
1
CLAY (CL); silty, slightly sandy to sandy, with scattered gravel at Boring 1, medium stiff to stiff and slightly moist
to medium stiff to soft and very moist to wet with depth, brown to light brown. Low plasticity.
GRAVEL (GM); silty, sandy, with cobbles, dense, wet, brown.
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.
11/12 Drive sample blow count; indicates that 11 blows of 140 pound hammer falling 30 inches were required to drive
the California or SPT sampler 12 inches.
0,13 Depth to free water and number of days following drilling that measrurement was taken.
—3. Depth at which boring had caved when measured on June 28, 2006.
NOTES:
1. Exploratory borings were drilled on June 15 and 16, 2006 with 4 -inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from building envelope corners shown on
Figure 1.
3. Elevations of exploratory borings were obtained by interpolation between contours 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 Togs represent the approximate boundaries between
material types and transitions may be gradual.
6. Water level readings shown on the logs were made at the time and under the conditions indicated. Fluctuations in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (% )
DD = Dry Density (pcf )
-200 = Percent passing No. 200 sieve
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1IEPWORTI I-PAWLAK GEOTECHNICAL_
LEGEND AND NOTES
FIGURE 3
COMPRESSION (% )
COMPRESSION - EXPANSION (% )
0
1
2
3
1
0
1
01
1.0 10
APPLIED PRESSURE ( ksf )
100
Moisture Content = 10.7 percent
Dry Density = 119 pcf
Sample of: Sandy Silty Clay with Gravel
From: Boring 1 at 15 Feet
Moisture Content = 11.0 percent
Dry Density = 110 pcf
Sample of: Slightly Sandy Silty Clay
From: Boring 2 at 3 Feet
I I
1
_..
_________T---...0....„,
��
•
No movement
upon wetting
Expansion
upo
wetting
1
01
1.0 10
APPLIED PRESSURE ( ksf )
100
01
1 0 10
APPLIED PRESSURE ( ksf )
100
106 0474
HEPWORTH-PAWLAK GEOTECHNICAL
SWELL -CONSOLIDATION TEST RESULTS
FIGURE 4
Moisture Content = 11.0 percent
Dry Density = 110 pcf
Sample of: Slightly Sandy Silty Clay
From: Boring 2 at 3 Feet
_________T---...0....„,
Expansion
upo
wetting
1
01
1 0 10
APPLIED PRESSURE ( ksf )
100
106 0474
HEPWORTH-PAWLAK GEOTECHNICAL
SWELL -CONSOLIDATION TEST RESULTS
FIGURE 4
COMPRESSION (% )
CO N 1 0
Moisture Content = 14.0 percent
Dry Density = 110 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 5 Feet
--------________No
movement
upon wetting
1
Y
0 1
1 0 10 100
APPLIED PRESSURE (ksf )
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HEPWORTH-PAWLAK GEOTECHNICAL
SWELL -CONSOLIDATION TEST RESULTS
FIGURE 5
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 106 0474
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRYGRAVEL
DENSITY
(pcf)
GRADATION
PERCENT
PASSING
NO. 200
SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(PSF)
SOIL
TYPE
BORING
NO.
DEPTH
(ft)
(%)
SAND
(%)
LIQUID
LIMIT
(%)
PLASTIC
1 INDEX
i%°)
1
15
10.7
119
Sandy silty clay with
gravel
2
3
11.0
110
88
Slightly sandy silty clay
5
14.0
110
Sandy silty clay
L _