HomeMy WebLinkAboutSoil's Report.pdfHEPWORTH-PAWLAK GEOTECHNICAL, INC. 5020 Road 154
Glenwood Springs, CO 81601
Fax 970 945-8454
Phone 970 945-7988
PRELIMINARY GEOTECHNICAL STUDY
PROPOSED RESIDENTIAL DEVELOPMENT
PRESHANA FARMS P.U.D.
SOUTHWEST CORNER, INTERSECTION OF HIGHWAY 82
AND COUNTY ROAD 100
GARFIELD COUNTY, COLORADO
JOB NO. 198 501
AUGUST 31, 1998
PREPARED FOR:
JAY WEINBERG
C/O LAND DESIGN PARTNERSHIP
ATTN: RON LISTON
918 COOPER AVENUE
GLENWOOD SPRINGS, COLORADO 81601
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
August 31, 1998
Jay Weinberg
c/o Land Design Partnership
Attn: Ron Liston
918 Cooper Avenue
Glenwood Springs, Colorado 81601
Job No.198 501
Subject: Report Transmittal, Preliminary Geotechnical Study, Proposed Preshana
Farms P.U.D., Southwest Corner, Intersection of Highway 82 and
County Road 100, Garfield County, Colorado.
Gentlemen:
As requested, we have conducted a geotechnical study for the proposed Preshana Farms
P.U.D. The property is suitable for the proposed development based on geologic and
geotechnical considerations.
Subsurface conditions encountered in the exploratory borings drilled in the general
proposed development area consist of about 1 foot of topsoil overlying relatively dense
sandy gravel with cobbles and boulders. At Borings 1 and 3, 3 to 4 feet of sandy clay
was encountered overlying the gravel. Groundwater was encountered between 3 and 7
feet deep in the borings.
Spread footings placed on the natural gravel subsoils and designed for an allowable
bearing pressure of 3,000 psf to 4,000 psf appear suitable at the building sites. We
recommend that foundations be kept shallow to avoid groundwater impacts.
The report which follows describes our investigation, summarizes our findings, and
presents our recommendations suitable for planning and preliminary design. It is
important that we provide consultation during design, and field services during
construction to review and monitor the implementation of the geotechnical
recommendations.
If you have any questions regarding this report, please contact us.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Daniel E. Hardin, P.E.
Rev. by: SLP
DEH/ro
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY 1
PROPOSED DEVELOPMENT 1
SITE CONDITIONS 2
GEOLOGIC SETTING 2
FIELD EXPLORATION 3
SUBSURFACE CONDITIONS 3
GEOLOGIC SITE ASSESSMENT 4
PRELIMINARY DESIGN RECOMMENDATIONS 5
FOUNDATIONS 5
BELOW GRADE CONSTRUCTION 5
FLOOR SLABS 5
SURFACE DRAINAGE 6
PAVEMENT SECTION 6
LIMITATIONS 6
REFERENCES 7
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
H -P GEOTECH
PURPOSE AND SCOPE OF STUDY
This report presents the results of a preliminary geotechnical study for the
proposed Preshana Farms P.U.D. to be located south of State Highway 82 and west of
100 Road, Garfield County, Colorado. The project site is shown on Fig. 1. The
purpose of the study was to evaluate the geologic and subsurface conditions and their
potential impacts on the project. The study was conducted in accordance with our
professional services agreement letter to Jay Weinberg, dated July 15, 1998. A
previous soils report for a parcel of land which included the southern portion of the
subject site and St. Finnbar Farm to the south, was performed by Lincoln DeVore
under Job No. GS -987, dated April 4, 1979. That report was considered in the
preparation of our report.
A field exploration program consisting of a reconnaissance and exploratory
borings was conducted to obtain information on the site and subsurface conditions.
Samples of the subsoils obtained during the field exploration were tested in the
laboratory to determine their classification and other engineering characteristics. The
results of the field exploration and laboratory testing were analyzed to develop
recommendations for project planning and preliminary design. This report summarizes
the data obtained during this study and presents our conclusions and recommendations
based on the proposed development and subsurface conditions encountered.
PROPOSED DEVELOPMENT
The proposed development will consist of 47 residential lots and about 3,000
feet of roadway. The existing equestrian facilities will remain. We assume the
residences will be typical of the area and be two to three stories with slab -on -grade or
shallow crawlspace. The development will be serviced with sewer tie-in to Ranch at the
Roaring Fork and on-site water wells.
If development plans change significantly from those described, we should be
notified to re-evaluate the recommendations presented in this report.
H -P GEOTECH
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SITE CONDITIONS
The Preshana Farms P.U.D. is located in the Roaring Fork valley about three
miles upstream of Carbondale. The property covers part of the SW Vs of Sec. 31, T. 7
S., R. 87 W. Blue Creek borders the property on the south. The proposed building
envelopes are on a gently sloping terrace that averages about 15 feet above the modern
Roaring Fork River channel. The valley floor has an average longitudinal slope of
about 1% down to the west. At the time of this study the property was developed with
equestrian facilities including an indoor riding arena, stables, storage barn and an old
concrete foundation. To the south of the equestrian facilities, there are two 1 -story
wood frame houses and a small garage. Most of the borings were drilled in the existing
pasture and polo field. The surrounding native vegetation consists of cottonwood trees,
grass and brush. The lower lying parts of the terrace on the south side of the property
are wetlands.
GEOLOGIC SETTING
Regional geologic mapping shows that formation rock in the project area is the
Pennsylvania -age Eagle Valley Evaporite (Kirkham and Widmann, 1997). Outcrops are
not present on the valley floor in the project area, but outcrops are present on the
Roaring Fork valley sides to the north and south. The Eagle Valley Evaporite is made
up of gray and tan gypsum, anhydrite and halite with interbedded siltstone, claystone,
shale, and dolomite. The gypsum, anhydrite and halite are soluble in fresh water. The
bedding structure in most places is convoluted because of flow deformation in the
plastic gypsum, anhydrite, and halite. Subsurface voids and related sinkholes are
sometimes present in areas underlain by the Eagle Valley Evaporite because of the
solubility of the gypsum, anhydrite, and halite. Evidence of sinkholes was not observed
on the property.
Late Pleistocene -age Roaring Fork alluvium is present below the terrace where
the proposed building sites are located. The exploratory borings show that the river
alluvium on the terrace is greater that 12 feet deep. It consists of a stratified deposit of
rounded gravel, cobbles and boulder in a silty sand matrix.
H -P GEOTECH
-3 -
FIELD EXPLORATION
The field exploration for the project was conducted on August 7, 1998. Six
exploratory borings were drilled at the locations shown on Fig. 1 to evaluate the
subsurface conditions. The borings were advanced with 4 inch diameter continuous
flight auger powered by a truck -mounted CME -55 drill rig. The borings were logged
by a representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with 13/e inch I.D. 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 testis 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, Fig. 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
Fig. 2. The subsoils consist of about 1 foot of topsoil overlying relatively dense,
slightly silty sandy gravel containing cobbles and boulders. Drilling in the dense gravel
with auger equipment was difficult due to the cobbles and boulders and drilling refusal
was encountered in the deposit. In Borings 1 and 4, sandy silty clay was encountered
below the topsoil between depths of 1 to 5 feet.
Laboratory testing performed on samples obtained from the borings included
natural moisture content and density and gradation analyses. Results of a
swell -consolidation test performed on a sample of sandy silty clay are shown on Fig. 4
and indicate the clay is moderately to highly compressible under conditions of loading
and wetting. Results of gradation analyses performed on small diameter drive samples
(minus 11/2 inch fraction) of the natural coarse granular soils are shown on Fig. 5. The
laboratory testing is summarized in Table I.
H -P GEOTECH
-4 -
Free water was encountered in the borings at the time of drilling and when
checked 4 days later at depths of 3 to 7 feet. The subsoils were slightly moist to wet.
GEOLOGIC SITE ASSESSMENT
Geologic factors that should be considered in project planning and development
are the potential for sinkholes and earthquake related ground shaking.
The probability of encountering sinkhole related problems at the proposed
building sites is considered to be low, but the developer and prospective home owners
should be aware that the building sites cannot be considered totally sinkhole risk free
since the Eagle Valley Evaporite is present in the subsurface. The potential presence of
subsurface voids should be considered when planning site-specific foundation studies.
If indications of voids or sinkhole related problems are identified by these studies, the
problem area should be avoided or the feasibility of engineered mitigation evaluated.
Engineered mitigation that can sometimes be used to mitigate sinkhole related problems
include:
• Void stabilization by grouting or excavation and backfilling
• Deep foundation systems
• Structural bridging
• Mat foundations or other rigid foundation systems
The project area could experience moderately strong earthquake related ground
shaking. Modified Mercalli Intensity VI ground shaking should be expected during a
reasonable service life for the residences, but the probability for stronger ground shaking
is low. Intensity VI ground shaking is felt by most people and causes general alarm, but
results in negligible damage to structures of good design and construction. All occupied
structures should be designed to withstand moderately strong ground shaking with little
or no damage and not to collapse under stronger ground shaking. The region is in the
Uniform Building Code, Seismic Risk Zone 1. Based on our current understanding of
the earthquake hazard in this part of Colorado, we see no reason to increase the
commonly accepted seismic risk zone for the area.
H -P GEOTECH
-5 -
PRELIMINARY DESIGN RECOMMENDATIONS
The conclusions and recommendations presented below are based on the
proposed development, the site reconnaissance, subsurface conditions encountered in the
exploratory borings, and our experience in the area. The recommendations are suitable
for planning and preliminary design but site specific studies should be conducted for
individual lot development.
FOUNDATIONS
Bearing conditions will vary depending on the specific location of the building on
the property. Based on the nature of the proposed construction, spread footings bearing
on the natural granular subsoils should be suitable at the building sites. We expect the
footings bearing on the relatively dense gravel soils can be sized for an allowable
bearing pressure in the range of 3,000 psf to 4,000 psf. The upper clay soils appear soft
and may need to be removed and replaced with compacted fill or the bearing level
deepened to dense gravel. Foundation walls should be designed to span local anomalies
and to resist lateral earth loadings when acting as retaining structures. The footings
should have a minimum depth of 36 inches for frost protection.
BELOW GRADE CONSTRUCTION
Ground water level is shallow throughout the project area. Due to the shallow
water level, it will probably not be practical to protect below grade areas from wetting
and hydrostatic pressure buildup by use of an underdrain system. We recommend that
slab -on -grade floors be placed near to above existing grade and crawlspaces be shallow
and at least 2 feet above the ground water level. Basements and deep crawlspaces
should be avoided.
FLOOR SLABS
Slab -on -grade construction should be feasible for bearing on the natural soils.
There could be some post construction slab settlement in clay subgrade areas. To reduce
the effects of some differential movement, floor slabs should be separated from all
bearing walls and columns with expansion joints. Floor slab control joints should be
H -P GEOTECH
-6 -
used to reduce damage due to shrinkage cracking. A minimum 4 inch thick layer of
free -draining gravel should underlie slabs to facilitate drainage.
SURFACE DRAINAGE
The grading plan for the subdivision should consider runoff through the project
and at individual sites. Water should not be allowed to pond next to buildings. Exterior
backfill should be well compacted and have a positive slope away from the building for a
distance of 10 feet. Roof downspouts and drains should discharge well beyond the Limits
of all backfill.
PAVEMENT SECTION
The near surface soils encountered in the borings below the topsoil consists
mainly of silty sand and gravel which is a fair to excellent material for support of
pavement materials. We recommend the pavement section for the site roads consist of 3
inches of asphalt pavement on 6 inches of Class 6 aggregate base course. The subgrade
should be evaluated for pavement support at the time of construction. Clay subgrade
soils should be subexcavated and replaced with coarse granular subbase material.
LIMITATIONS
This study has been conducted according to generally accepted geotechnical
engineering principles and practices in this area at this time. We make no warranty
either expressed or implied. The conclusions and recommendations submitted in this
report are based upon the data obtained from the field reconnaissance, review of
published geologic reports, the exploratory borings located as shown on Fig. 1, the
proposed type of construction and our experience in the area. 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.
H -P GEOTECH
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This report has been prepared for the exclusive use by our client for planning and
preliminary design purposes. We are not responsible for technical interpretations by
others of our information. As the project evolves, we should provide continued
consultation, conduct additional evaluations and review and monitor the implementation
of our recommendations. 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.
Daniel E. Hardin, P.E.
Reviewed By:
Steven L. Pawlak, P.E.
DEH/ro
cc: High Country Engineering - Attn: Tim Beck
REFERENCES
Kirkham, R.M. and Widmann, B.L., 1997, Geology Map of the Carbondale Quadrangle,
Garfield County, Colorado: Colorado Geological Survey Open File Report 97-3.
H -P GEOTECH
APPROXIMATE SCALE
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198 501
HEPWORTH --- PAWLAK
GEOTECHNICAL, INC.
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198 501
HEPWORTH — PAWLAK
GEOTECHNICAL, INC.
LOGS OF EXPLORATORY BORINGS
Fig. 2
LEGEND:
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TOPSOIL; sandy silty clay, organic. medium stiff. slightly moist, dark brown.
CLAY (CL); silty. sandy, medium stiff to soft with depth, moist to very moist with depth, brown.
SAND (SM); silty, gravelly with scattered cobbles, medium dense, slightly moist, brown.
GRAVEL (GM—GP); sandy, slightly silty, with cobbles and boulders, dense, slightly moist to wet
with depth. 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.
Drive sample blow count; indicates that 37 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 and number of days after drilling measurement was made.
-' Depth at which boring had caved when checked 4 days after drilling.
1
NOTES:
Practical rig refusal.
1. Exploratory borings were drilled on August 7, 1998 with a 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 based an contours 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 time and under the conditions indicated.
Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content ( % )
DO = Dry Density ( pcf )
+4 = Percent retained on No. 4 sieve.
—200 = Percent passing No. 200 sieve.
198 501
HEPWORTH - PAWLAK
GEOTECHNICAL, INC.
LEGEND AND NOTES
Fig. 3
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Moisture Content = 23.5 percent
Dry Density Weight = 102 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 4 Feet
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APPLIED PRESSURE — ksf
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HEPWORTH — PAWLAK
GEOTECHNICAL, INC.
SWELL—CONSOLIDATION TEST RESULTS
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I Fig. 4
24 HR. 7 101
45 ION. 15 MN.
100
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70
60
50
40
30
20
10
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HYDROMETER AHALYSIS
IDE READINGS
60 104.19 10N. 4 1111t 1 YN.
.001 .002 .005 .009 .010 .037
CLAY TO SLT
GRAVEL 51 %
LIQUID LIMIT
U.S.1STANDARD 'SERIES
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.074 .150 .8W1.111 2.36 4.75 9.5123 140 37.5 76.2 12152 203
DIAMETER OF PARTICLES IN MIWMETERS
SAND tl16Va SEs
MENUS ICOMSE 1 FINE I COARSE
FINE
SAND 40 %
SILT AND CLAY 9 %
PLASTICITY INDEX %
SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 2 at 4 Feet
HYDRCYEIER MIAOW
TIYE READINGS
24 HR. 7 HR
45 MN. 16 MN. 60 YN.19 IAN . 4 UHL 1 UN.
100
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11.5. STANDARD SERIES
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DIAMETER OF
CLAY TO SLT
GRAVEL 58 %
UQUID LIMIT
SAMPLE OF: Slightly Silty Sandy Gravel
FINE
SEW ANALYSIS
I/4 3/61/2334' 11 3' 56' YO
CLEAR SQUARE OPENINGS
{
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20
30
40
50
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80
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.600 1.18 2.36 4.75 95125140 37.5 742 1 27 52 203
PAR11CLES IN MILLIMETERS
SMD
1 MEDIUM (COARSE
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COBBLES
SAND 36 % SILT AND CLAY 6 %
PLASTICITY INDEX %
FROM: Boring 6 at 2 Feet
198 501 ( HEPWORTH - PAWLAK i
GEOTECHNICAL, INC. L
GRADATION TEST RESULTS
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