HomeMy WebLinkAboutSubsoils Report for Foundation DesignHEPWORTH-PAWLAK GEOTECHNICAL
Hepworth-Pawlak Geutechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone:970-945.7988
Fax 5�i0-9+5 �454
eutail: hpgca*hpgevrech.cun7
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED POWERS ART
LEARNING CENTER
POWERS RANCH
13114 STATE HIGHWAY 82
CARBONDALE,COLORADO
JOB NO. 108193A
MAY 30, 2008
PREPARED FOR:
KIMIKO POWERS
c/o BLACK SHACK ARCHITECTS
ATTN: GLEN RAPPAPORT
P.O. BOX 1847
BASALT, COLORADO 81621
Parker 303-841-7119 -1 Colorado Springs 719-633-5562 • Silverthorrie 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY........................................................................ - 1 -
PROPOSED CONSTRUCTION................................................................................. - 1 -
SITECONDITIONS................................................................................................... - 2-
FIELD EXPLORATION .................................. - 3-
SUBSURFACE CONDITIONS.................................................................................. - 3-
DESIGN RECOMMENDATIONS............................................................................. 4-
FOUNDATIONS ................................................ -
FLOORSLABS...................................................................................................... - 5 -
UNDERDRAINSYSTEM...................................................................................... - 6-
SURFACEDRAINAGE......................................................................................... - 6-
LIMITATIONS................................................................................................... - 7-
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 TO 8 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 9 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for the proposed Powers Art Learning
Center to be located on Powers Ranch, 13114 State Highway 82, 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 Kimiko Powers
dated April 28, 2008.
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 art learning center will be a 38 foot tall concrete structure with a 9,000
square foot footprint. Ground floor will be slab -on -grade. 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.
Job No. 108 193A G9 tech
-2-
SITE CONDITIONS
The proposed art learning center will be located on the south facing side of the Roaring
Fork River valley. The ranch house is located about 800 feet northwest, an existing earth
house is 500 feet northeast and a gravel pit is located to the southeast. The building area is
currently used as irrigated pasture. The ground surface slopes down to the south at grades
of 6 to 8 percent in the building area.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Powers Ranch.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and sihstone 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 site.
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. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of
the Eagle Valley.
Sinkholes were not observed in the immediate area of the subject property. 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 this site throughout the service life of
the proposed building, 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. 108 193A GgF-)teCh
-3-
FIELD EXPLORATION
The field exploration for the project was conducted on May 1 and 2, 2008. Six
exploratory borings staked by others and 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 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 6 to 12 inches of topsoil, consists of 10 to 27 feet of medium
stiff to very stiff sandy silty clay overlying silty sandy gravel with cobbles and small
boulders at depths of 11 to 28 feet. A slightly silty gravelly sand layer was encountered
in Boring 5 overlying the dense granular soils. Drilling in the dense granular soils with
auger equipment was difficult due to the cobbles and boulders.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and gradation analyses. Results of swell -consolidation testing
performed on relatively undisturbed drive samples, presented on Figures 4 to 8, indicate
low to moderate compressibility under conditions of loading and wetting. A sample tested
from Boring 2 at 2 Feet showed a minor swell potential when wetted. Results of
Job No. 108 193A
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gradation analyses performed on small diameter drive samples (minus 1 %Z inch fraction)
of the coarse granular subsoils are shown on Figure 9. The laboratory testing is
summarized in Table 1.
No free water was encountered in the borings at the time of drilling or when checked 14
days later and the subsoils were generally moist to very moist.
BEARING CONDITIONS
The fine-grained soils are generally suitable for low -bearing pressure spread footing
foundations provided some potential settlement can be tolerated particularly if the bearing
soils are wetted. If differential settlements can not be tolerated or if high column loads
are anticipated, then a deep foundation such as drilled or driven piles bearing on the
relatively incompressible underlying gravel may be more suitable. We can provide
recommendations for a deep foundation if desired.
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 provided some differential settlement can be
tolerated.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural granular soils should be
designed for an allowable bearing pressure of2,000 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be about 1 inch or less. Additional settlement
Job No. 108 193A
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-5-
on the order of 1 inch is possible if the bearing soils become wet. The
settlement could be differential across the building.
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 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 (if any) should also be
designed to resist a lateral earth pressure corresponding to an equivalent
fluid unit weight of at least 50 pcf.
5) All existing topsoil and any loose or disturbed soils should be removed and
the footing bearing level extended down to the natural soils. The exposed
soils in footing areas 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, are suitable to support lightly loaded slab -
on -grade 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
basement level 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.
Job No. 108 193A
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 and topsoil.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our
experience in the area 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, basement
and crawlspace areas (if any), be protected from wetting and hydrostatic pressure buildup
by an underdrain system. The proposed slab -at -grade construction should not require an
underdrain.
If installed, 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 of 2 inches. The
drain gravel backfill should be at least 1'/z feet deep. An impervious membrane such as
20 mil PVC 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 arts learning center building has been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
Job No. 108 193A GVgtech
-7-
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 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 2% 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) 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.
6) We assume the proposed "pond" adjacent to the building will be lined to
prevent seepage from adversely impacting the foundation bearing soils.
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
Job No. 108 193A
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.
Louis E. Eller
Reviewed by:
Daniel E. Hardin, P.E.
LEE/vad
Job No. 108 193A - GeC�tE'1
APPROXIMATE SCALE
1 " = 100'
PROPOSED 6270
PARKING / 6280
LOT /
6260 I /
EXISTING - BORING 1 ' 1 6290
DRIVE � - -' � / i ! � I
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f I I I
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6250 BORING 2 1 I I
PROPOSED
6260 r PROPPOSED ! BUILDING /
\ DRIVE 1 I I
BORING 3 / BORING 6/' /
\ 1 'BORING 5 •' ,' / /�
BORING 4
6270
6280
I EXISTIN(
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108 193A
LOCATION OF EXPLORATORY BORINGS I Figure 1
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LOGS OF EXPLORATORY BORINGS Figure 2
LEGEND:
2 TOPSOIL; organic sandy silt and clay, firm, slightly moist, brown.
CLAY (CL); sandy, silty, low plasticity, medium stiff to soft, moist to very moist, brown.
SAND (SP-SM); slightly silty, medium dense, moist, brown. (Boring 5 only)
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GRAVEL (GP -GM); with cobbles and small boulders, sandy, slightly silty to silty, dense, slightly moist, brown,
rounded rock.
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.
I
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.
NOTES:
1. Exploratory borings were drilled on May 1 and 2, 2008 with 4-inch diameter continuous flight power auger.
2. Locations of exploratory borings were staked by others and located 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
and checked by instrument level.
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 14 days later. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DID = Dry Density (pcf)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
LL = Liquid Limit (%)
PI = Plasticity Index (%)
UC = Unconfined Compressive Strength (psf)
108 193A
LEGEND AND NOTES I Figure 3
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Moisture Content = 7.6 percent
Dry Density = 115 pcf
Sample of: Sandy Clay
From: Boring 2 at 2 Feet
upon
wetting
0.1 1.0 10 100
APPLIED PRESSURE - ksf
Moisture Content = 14.9 percent
Dry Density = 108 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 10 Feet
Compression
upon
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0.1
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APPLIED PRESSURE - ksf
100
108 193A ' SWELL -CONSOLIDATION TEST RESULTS f Figure 4
Moisture Content = 17.8 percent
Dry Density = 108 pcf
Sample of: Sandy Silty Clay
From: Boring 3 at 5 Feet
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Moisture Content = 15.9 percent
Dry Density = 106 pcf
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108 193A
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Dry Density = 96
Sample of: Sandy Silty Clay
From: Boring 4 at 20 Feet
No Movement
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percent
pcf
0.1 1.0 10 100
APPLIED PRESSURE - ksf
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Moisture Content = 16.4
Dry Density = 113
Sample of: Sandy Silty Clay
From: Boring 5 at 2 Feet
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APPLIED PRESSURE - ksf
percent
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100
108 193A ( SWELL -CONSOLIDATION TEST RESULTS I Figure 6
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Dry Density = 104
Sample of: Very Silty Clay
From: Boring 5 at 10 Feet
No Movement
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percent
pcf
0.1 1.0 10 100
APPLIED PRESSURE - ksf
Moisture Content = 18.2
Dry Density = 109
Sample of: Sandy Silty Clay
From: Boring 5 at 15 Feet
Compression
upon
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percent
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0.1 1.0 10
APPLIED PRESSURE - ksf
108 193A ~ SWELL -CONSOLIDATION TEST RESULTS
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Dry Density = 112
Sample of: Sandy Silty Clay
From: Boring 6 at 4 Feet
No Movement
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0.1 1.0 10 100
APPLIED PRESSURE - ksf
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108 193A
Moisture Content = 10.3
Dry Density = 113
Sample of: Sandy Silty Clay
From: Boring 6 at 19 Feet
Compression
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APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
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