HomeMy WebLinkAboutSoils Report 03.16.2020K+A
Kumar & Associates, Inc. ®
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Company
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT C-3, ASPEN GLEN
THUNDERSTORM
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-170
MARCH 16, 2020
PREPARED FOR:
GERRY AND KIRSTEN McDANIEL
87 DIAMOND A RANCH ROAD WEST
CARBONDALE, COLORADO 81623
ki rsten(7you then tity.org
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 - 3 -
DESIGN RECOMMENDATIONS _ 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 7 -
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
Kumar & Associates, Inc. ® Project No. 20-7-170
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot C3, Aspen Glen, Thunderstorm, 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 Gerry and Kirsten McDaniel dated March 3, 2020. Chen -Northern, Inc. previously
conducted a preliminary geotechnical study for preliminary plat design under their Job
No. 4 112 92, dated December 20, 1991 and May 28, 1993.
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
Development plans for the lot were not available at the time of our study and we understand the
findings will be considered in the purchase of the lot. In general, we assume the proposed
residence will be a 1 and 2 -story structure possibly above a basement level with a garage at the
main level. Crawlspace could also underlie part of the residence area. Basement and garage
floors will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut
depths between about 4 to 10 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.
Kumar & Associates, Inc. ® Project No. 20-7-170
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SITE CONDITIONS
The lot is located off the east side of Thunderstorm cul-de-sac as shown on Figure 1. The
ground surface is relatively flat with around 2 feet of elevation difference down to the west
across the building envelope. Vegetation consisted of grass and weeds.
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. Several sinkholes were observed by Chen -Northern
scattered throughout the Aspen Glen property during the subdivision development. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the
Roaring Fork River valley.
The lot is not located within a broad subsidence area and sinkholes were not observed in the
immediate area of the subject lot. The closest mapped sinkhole within a broad subsidence area is
located about 1/3 mile west 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
he said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot
C3 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 March 5, 2020. 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 -
Kumar & Associates, Inc. ® Project No. 20-7-170
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mounted CME -45B drill rig. The borings were logged by a representative of Kumar &
Associates.
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 encountered, below about 1 foot of topsoil, consist of about 14'/2 to 21'/2 feet of stiff,
silty sandy clay overlying dense, slightly silty sandy gravel and cobbles with probable boulders
down to depths of 18 to 26 feet. Drilling in the coarse 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 and finer than sand size gradation analyses. Results of swell -consolidation
testing performed on relatively undisturbed drive samples of the clay soils, presented on Figures
4 and 5, indicate low compressibility under light loading and natural low moisture condition and
minor to low expansion potential when wetted. The samples showed moderate compressibility
under additional loading after wetting. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the soils were typically
slightly moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The soils encountered at assumed excavation depths consist of low bearing capacity clay with
variable expansion and compressibility potential mainly when wetted. It has been our experience
in the area that the soils can possess collapse potential (settlement under constant load) when
Kumar & Associates, Inc. ® Project No. 20-7-170
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wetted. Tightly loaded spread footings can be used for building support with mitigation to limit
the movement potential and risk of building distress mainly if the natural clay soils become
wetted. Compacted structural fill at least 3 feet deep is recommended below shallow footings
such as for the garage and crawlspace areas. Structural fill should consist of low permeable soils
such as the onsite clay to limit penetration of water to the lower natural soils. If the risk of
settlement and distress is not acceptable, a deep foundation extending down to the dense gravel
and cobble soils should be used.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils or compacted structural fill with a risk of settlement as described below.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils at basement level or on at least 3
feet of compacted structural fill at shallow depth should be designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. Additional settlement on the order of 1/2 to 1 inch is
possible if the underlying soils are wetted and would likely be differential
between shallow and deeper bearing levels.
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.
Kumar & Associates, Inc. ® Project No. 20-7-170
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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) The topsoil, minimum depth of clay soil below footing level and loose or
disturbed soils should be removed down to the stiff natural clay soils. The
exposed soils in footing area should then be moistened and compacted. Structural
fill placed below footing areas should extend horizontally out from the edge of the
footing to a distance equal to at least 1/2 the depth of fill below the footing and be
compacted to at least 98% of standard Proctor density at near optimum moisture
content.
6) A representative of the geotechnical engineer should evaluate structural fill for
compaction and observe all footing excavations for bearing conditions prior to
concrete placement.
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 (if any) 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.
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.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
Kumar & Associates, Inc. ® Project No. 20-7-170
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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. Backfill consisting of a granular soil such as road base and
compaction to at least 98% standard Proctor density could be used to reduce the settlement risk.
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, can be used to support lightly loaded slab -on -grade
construction with a risk of settlement mainly if the bearing soils are wetted. Compacted
structural fill feet deep could be used to reduce the movement potential of shallow slabs. 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.
Kumar & Associates, Inc. ® Project No. 20-7-170
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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 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, non -perforated sump or drywell based in the underlying gravel and
cobble deposit. 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. 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
Proper grading and drainage will be very important to keeping the bearing soils dry and limiting
the building settlement and potential distress. 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.
Kumar & Associates, Inc. ® Project No. 20-7-170
-8-
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
3 inches in the first 10 feet in paved areas. Free -draining wall backfill should be
covered with filter fabric and capped with at least 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.
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 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
Kumar & Associates, Inc. ® Project No. 20-7-170
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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,
Kumar & Associates, Inc.
Steven L. Pay
Reviewed by:
)0,
Daniel E. Hardin, P.E.
SLP/kac
Kumar & Associates, Inc. ® Project No. 20.7-170
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20-7-170
Kumar & Associates
LOCATION OF EXPLORATORY BORINGS
Fig. 1
{
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1-
w
w
1-
0
w
0
0
5
10
BORING 1 BORING 2
//
15
10/12
20
13/12
WC=7.3
DD=105
12/12
WC=8.6
DD=107
-200=80
10/12
WC=13.9
DD=112
25
' 1 13/6, 50/2
0
/ 14/12
/ WC=6.9
/ DD=98
//_1 -200=83
/
5
12/12
WC=8.4
DD=106
10
15
6/6, 17/6
WC=1 6.8
DD=105 -
f-200=90 -
20
25
30 30
1-
w
0_
a
w
0
20-7-170
Kumar & Associates
LOGS OF EXPLORATORY BORINGS
Fig. 2
2i
LEGEND
7
r
r
TOPSOIL; ORGANIC SANDY SILT AND CLAY, BROWN.
SILT AND CLAY (ML—CL); SANDY, STIFF, SLIGHTLY MOIST TO MOIST WITH DEPTH, RED.
SLIGHTLY POROUS AND CALCAREOUS TRACES.
GRAVEL AND COBBLES (GM—GP); SLIGHTLY SILTY, SANDY, PROBABLE BOULDERS, DENSE,
SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
IDRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
13/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 13 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
t PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 5, 2020 WITH A 4—INCH—DIAMETER
CONTINUOUS—FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF THE
EXPLORATORY BORINGS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM 02216);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
20-7-170
Kumar & Associates
LEGEND AND NOTES
Fig. 3
CONSOLIDATION - SWELL
CONSOLIDATION - SWELL
2
1
0
—1
— 2
— 3
1
0
— 1
— 2
—3
10 APPLIED PRESSURE — KSF
10
100
SAMPLE OF: Sandy Silt and Clay
FROM: Boring 1 ® 2.5'
WC = 7.3 %, DD = 105 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
---mwi_________.______________.___:L><---'
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
I
IL---ilt
1M
1
These teat results apply only to the
*ample. Seated. The leehng report
.hop net he reproduced. except a
full, wilh.ot the written oplaraeol M
Kumar and Associates, Inc. Swell
Consolidation lasting performed In
accordance with ASTM D-4546.
10 APPLIED PRESSURE — KSF
10
100
10 APPLIED PRESSURE — KSF
10
100
20-7-170
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
SAMPLE OF: Sandy Silt and Clay
FROM: Boring 1 ® 10'
WC = 13.9 %, DD = 112 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
---mwi_________.______________.___:L><---'
I
IL---ilt
These teat results apply only to the
*ample. Seated. The leehng report
.hop net he reproduced. except a
full, wilh.ot the written oplaraeol M
Kumar and Associates, Inc. Swell
Consolidation lasting performed In
accordance with ASTM D-4546.
10 APPLIED PRESSURE — KSF
10
100
20-7-170
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
CONSOLIDATION - SWELL
2
1
0
1
2
3
4
10 APPLIED PRESSURE — KSF
10
100
20-7-170
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 5
SAMPLE OF: Sandy Silf and Clay
FROM: Boring 2 ® 5'
WC = 8.4 %, DD = 106 pcf
/
�
— EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
10><
--..--.11
............NAN
These test molts apply only to the
ample. tested. The testing report
Moll not be reproduced, except in
full, without the written approval of
Kumar and A.eaciatee, Inc. Swell
Coneoildotion testing performed in
accordance with ASTM D-454&
10 APPLIED PRESSURE — KSF
10
100
20-7-170
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 5
Kumar & Associates, Inc.®
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-170
SAMPLE LOCATION l
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITY
(pcf)
GRADATION
ERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(psf)
SOIL TYPE
BORING
DEPTH
(ft)
GRAVEL
(%)
SAND
(%)
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
r
1
21/2
7.3
105
Sandy Silt and Clay
5
8.6
107
80
Sandy Clayey Silt
10
13.9
112
Sandy Silt and Clay
2
21/2
6.9
98
83
Sandy Silt and Clay
5
8.4
106
Sandy Silt and Clay
15
16.8
105
90
Slightly Sandy Clayey Silt