HomeMy WebLinkAboutSoils Report 01.10.2019H-P� KUMAR
Geotechnical Engineering 1 Engineering Geology
Materials Testing 1 Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Parker, Glenwood Springs, and Silverthome, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 6, CERISE RANCH
LARKSPUR DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 18-7-704
JANUARY 10, 2019
PREPARED FOR:
LINDA ORNSTEIN
442 EAST JODY ROAD
BASALT, COLORADO 81621
(easygrits(r,aol.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Project No. 18-7-704
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at Lot 6,
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 Linda Ornstein dated November 20, 2018. Hepworth-Pawlak Geotechnical (now
H-P/Kumar) previously performed a subsoil study on the subject site and presented the findings
in a report dated December 13, 2006, Job No. 106 1001.
A field exploration program consisting of an exploratory boring was conducted for the current
study 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
Development plans for the proposed residence were conceptual at the time of our study. In
general, the residence will be located in the middle of the building envelope shown on Figure 1.
Ground floor will be part structural over crawlspace and slab -on -grade in the garage. Grading
for the structure is assumed to be relatively minor cut depth with fill up to about 5 feet deep
around the house. 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.
Project No. 18-7-704
-2 -
SITE CONDITIONS
The site was vacant and is located on the south (downhill) side of Larkspur drive as shown on
Figure 1. An existing irrigation ditch is located along the southeast (downhill) side of the
building envelope and a drainage ditch constructed as part of the subdivision development is
located on the east side. The ground surface is relatively flat with a gentle slope down to the
south. Elevation difference across the building envelope is about 2 feet. Vegetation consists of
grass and weeds. Eagle Valley Evaporate is visible on the steep hillside on the north side of the
subdivision.
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, sinkholes have been
observed scattered throughout Cerise Ranch and nearby developments. These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the lower 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 6 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 November 28, 2018. One exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The
Project No. 18-7-704
-3 -
boring was advanced with 4 inch diameter continuous flight augers powered by a truck -mounted
CME -45B drill rig. The boring was logged by a representative of H-P/Kumar.
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 Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about 11/2 feet of topsoil overlying soft, sandy silty clay with scattered
organics. Relatively dense, silty sandy gravel with cobbles was encountered beneath the clay at
a depth of 171/2 feet. Drilling in the dense granular soils with auger equipment was difficult due
to the cobbles and drilling refusal was encountered in the deposit at a depth of 24 feet in
Boring 1.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density finer than sand grain size gradation analyses, liquid and plastic limits and
unconfined compressive strength determination. Results of swell -consolidation testing
performed on a relatively undisturbed drive sample of the clay, presented on Figure 3, indicate
moderate compressibility under conditions of loading and wetting. The unconfined compressive
strength test showed the sample to have soft consistency. The laboratory testing is summarized
in Table 1.
Free water was encountered in the boring at a depth of 6 feet at the time of drilling and also at
6 feet when checked 2 days later and the subsoils were very moist to wet.
The subsurface profile encountered by the current study is consistent with that encountered by
the previous study which encountered between 12 and 15 feet of soft clay above dense gravel
and groundwater at a depth of 51/2 to 6 feet.
Project No. 18-7-704
-4 -
FOUNDATION BEARING CONDITIONS
The sandy clay soils within about the upper 171/2 feet are very moist to wet and moderately to
highly compressible under loading. The underlying silty gravel soils possess moderate bearing
capacity and typically low settlement potential. At assumed excavation depths we expect the
subgrade to expose sandy clay soils. Lightly loaded spread footings placed on these soils should
be suitable for support of the proposed residence provided that the risk of settlement is
acceptable to the owner. As an alternative, a deep foundation system, such as driven piles, screw
piles or helical piers, could be used to extend the bearing level down to the relatively dense
granular soils encountered in the boring at a depth of 171/2 feet below the ground surface. A deep
foundation system would provide moderate to high load capacity with a relatively low settlement
risk. If a deep foundation system is proposed, we should be contacted to provide additional
recommendations.
Groundwater was measured in the boring at a depth of 6 feet below the existing ground surface.
It has been our experience in the area that the groundwater level can fluctuate and rise during
spring runoff and irrigation season. Based on the anticipated excavation depths, groundwater
may not be encountered at footing grade; however, the excavation should be kept relatively
shallow to avoid encountering soft/wet soils. The crawlspace area should be well ventilated and
footing grade kept within about 2 to 3 feet below existing round surface. As an alternative, slab -
on -grade construction could be used throughout the entire structure to eliminate the risk of
wetting or excessive humidity in the crawlspace.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, the building can be founded with spread footings bearing on the upper
natural clay soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
Project No. 18-7-704
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 800 psf. Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch. Additional long-term differential settlement on the order of 1 to
2 inches could occur across the structure and will depend on the depth of clay
soils below the footings and actual loading conditions.
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 heavily 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 clay soils as backfill. Wet on-site soils may need to be dried
out prior to use as backfill.
5) Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the undisturbed natural soils. Excavations should be
performed with track -mounted equipment from outside the building footprint to
reduce disturbance of the bearing soils. It may be necessary to stabilize the
subgrade soils with geogrid and a shallow depth of granular soils in order to
provide a working platform for footing construction. Fill depths above existing
ground surface should not exceed about 2 feet to limit loading on the clay soils
from the weight of the fill. The exposed soils in footing area should then be
moisture adjusted to near optimum and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
Project No. 18-7-704
6
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade
construction. The clay soils are compressible and slab settlement and distress could occur. Slab
grade should be kept within about 2 feet above existing ground surface to limit additional
loading of the clay soils. 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 relatively
free draining gravel (such as %-inch screened rock) should be placed beneath slabs to break
capillary rise and provide slab support. 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 should consist of
imported granular soils (such as road base) devoid of vegetation, topsoil and oversized rock.
We recommend vapor retarders conform to at least the minimum requirements of ASTM E1745
Class C material. Certain floor types are more sensitive to water vapor transmission than others.
For floor slabs bearing on angular gravel or where flooring system sensitive to water vapor
transmission are utilized, we recommend a vapor barrier be utilized conforming to the minimum
requirements of ASTM E 1745 Class A material. The vapor retarder should be installed in
accordance with the manufacturers' recommendations and ASTM E1643.
UNDERDRAIN SYSTEM
Free water was encountered during our exploration, and 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 crawlspace areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system.
Project No. 18-7-704
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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. 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.
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
covered with filter fabric and 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
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
Project No. 18-7-704
-8 -
from the exploratory boring drilled at the location 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 boring 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,
HTP- KUMAR
James H. arsons, E.I.
Reviewed by:
Steven L. Pawlak,
JHP/kac
Project No. 18-7-704
LOT 5
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18-7-704
H-PtiKUMAR
LOCATION OF EXPLORATORY BORINGS
Fig. 1
w —
La
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a
w
0
5
10
15
20
25
BORING 1
EL. 6352'
2/12
WC=27.9
DD=91
3/12
WC=28.0
DD=94
—200=86
LL=30
PI=11
UC=700
LEGEND
/
TOPSOIL; ORGANIC SILTY CLAY, DARK BROWN.
CLAY (CL); SILTY, SANDY, SOFT, VERY MOIST TO WET WITH
DEPTH, BROWN, LOW PLASTICITY.
GRAVEL AND COBBLES (GM); SILTY, SANDY, PROBABLE BOULDERS,
DENSE, WET, BROWN. ROUNDED ROCK.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD
3/12I PENETRATION TEST.
2/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 2 BLOWS OF
A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
4/12
42/12
0.2 DEPTH TO WATER LEVEL AND NUMBER OF DAYS AFTER
= DRILLING MEASUREMENT WAS MADE.
DEPTH AT WHICH BORING CAVED WHEN CHECKED ON 11/30/18.
t PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORING WAS DRILLED ON NOVEMBER 28, 2018
WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE
PLAN PROVIDED.
3. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED (HP GEOTECH JOB No. 106 1001).
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD BE
CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE
METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN
MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER LEVEL SHOWN ON THE LOG WAS MEASURED AT
THE TIME AND UNDER CONDITIONS INDICATED. FLUCTUATIONS IN
THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216);
—200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140);
LL = LIQUID LIMIT (ASTM D 4318);
PI = PLASTICITY INDEX (ASTM D 4318);
UC = UNCONFINED COMPRESSIVE STRENGTH (psf) (ASTM D 2166).
18-7-704
H-PwKUMAR
LOG OF EXPLORATORY BORING
Fig. 2
Cerlee icnch\D
CONSOLIDATION - SWELL
0
—1
—2
— 3
— 4
— 5
— 6
—7
10 APPUED PRESSURE — KSF
10
100
18-7-704
H-P-'-bKUMAR
SWELL—CONSOLIDATION TEST RESULTS
Fig. 3
SAMPLE OF: Sandy Silty Clay
FROM: Boring 1 ® 2.5'
WC = 27.9 %, DD = 91 pcf
NO
MOVEMENT
WETTING
UPON
—
These test restate apply only to the
samples tested. The testing report
shall not he reproduced. except in
full. without the written approval of
Kumar and Associates, Inc. Swell
Consolidation testing performed in
accordance with ASTM D-4546.
10 APPUED PRESSURE — KSF
10
100
18-7-704
H-P-'-bKUMAR
SWELL—CONSOLIDATION TEST RESULTS
Fig. 3
Project No. 18-7-704
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SOIL TYPE
Sandy Silty Clay
Sandy Silty Clay
i
UNCONFINED
COMPRESSIVE
STRENGTH
(psf)
700
RG LIMITS
PLASTIC
INDEX
(%)
ii
ATTERBEI
LIMIT
(%)
0
M
PERCENT
PASSING
NO. 200
SIEVE
00
GRADATION
O
a
co
GRAVEL
(%)
NATURAL
DRY
DENSITY
(pcf)
1.4
94
NATURAL
MOISTURE
CONTENT
(%)
ON
N
N
28.0
LOCATION
DEPTH
(ft)
N
1 SAMPLE
BORING