HomeMy WebLinkAboutSoils Report 01.27.2020IC+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 294, IRONBRIDGE DEVELOPMENT
RIVER BEND WAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 18-7-255.01
JANUARY 27, 2020
PREPARED FOR:
KENDRICK DEVELOPMENT, LLC
ATTN: WES COLE
430 IRONBRIDGE DRIVE
GLENWOOD SPRINGS, COLORADO 81601
wcole(2 ironbrid2ectub.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 3 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5
NONSTRUCTURAL FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 8 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 18-7-255.01
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 294, Ironbridge Development, River Bend Way, 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 Kendrick Development, LLC dated January 22, 2020. Hepworth-Pawlak
Geotechnical (now Kumar & Associates) previously performed a preliminary geotechnical study
for the Ironbridge Villas and presented the findings in a report dated September 14, 2005, Job
No. 105 115-6.
On April 11, 2018, H-P/Kumar (aka Kumar & Associates) drilled an exploratory boring 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
Design level plans were not available for this subsoil study. We understand the proposed
structure on Lot 294 will be a single story, 1,800 ft2 residence. Cut depths are expected to range
between about 2 to 3 feet. Foundation loadings for this type of construction are assumed to be
relatively light and typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The site was vacant and vegetated with grass and weeds at the time of our field exploration. The
site was covered with about 'A to 1 foot of snow at the time of this report. The terrain is
Kumar & Associates, Inc. ® Project No. 18-7-255.01
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relatively flat with an estimated 1 foot of elevation difference across the building envelope. A
two tiered-small-.b1ock-retaini.ag_waItand-single=family-residences_are-to-the-west,-single family
residences are to the north, south, and east, and River Bend Way and the Roaring Fork River are
to the east.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge 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. A sinkhole opened in the cart storage parking lot located east of the Pro
Shop and west of the Villas North parcel in January 2005. Other irregular bedrock conditions
have been identified in the affordable housing site located to the northwest of the Villas North
parcel. Irregular surface features that could indicate an unusual risk of future ground subsidence
were not observed in the Villas North parcel, but localized variable depths of debris fan soils and
bedrock quality encountered by the previous September 14, 2005 geotechnical study in the Villas
North development area could be the result of past subsidence. The subsurface exploration
performed west of the proposed residence on Lot 294 did not encounter voids but the alluvial fan
depth encountered was considerably greater than encountered on nearby lots which could
indicate past ground subsidence. In our opinion, the risk of future ground subsidence on Lot 294
throughout the service life of the proposed residence is low and similar to other areas of the
Roaring Fork River valley where there have not been indications of ground subsidence, but 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 April 11, 2018. One exploratory boring
was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The 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 HP Kumar (aka Kumar &
Associates).
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Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers
wereddriven-into the-subsoils-aocariaus-depths-with-blows_from-a_L4a-pound-hamcner fa.iiing3f
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 4 feet of medium dense, silty clayey sand and gravel fill overlying
about 15 feet of stiff, sandy silt and clay, underlain by dense, slightly silty sandy gravel and
cobbles with probable boulders. Drilling in the dense granular soils with auger equipment was
difficult due to the cobbles and probable boulders and drilling refusal was encountered in the
deposit.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density, finer than sand size gradation analyses and unconfined compressive
strength. Results of swell -consolidation testing performed on a relatively undisturbed drive
sample of the silt and clay soil, presented on Figure 3, indicate low to moderate compressibility
under conditions of loading and wetting. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist with depth.
FOUNDATION BEARING CONDITIONS
Approximately 4 feet of fill from overlot grading was encountered in our boring. The residence
to the south, Lot 293, was constructed with an 8 to 9 -foot deep basement level. Backfill used for
the basement may be encountered during foundation excavations for the proposed residence on
Lot 294. The natural silt and clay soils encountered below the fill possess a relatively low
bearing capacity and a low to moderate risk of settlement. The amount of settlement will depend
on the thickness of the compressible soils due to potential collapse when wetted, and potential
compression of the underlying soils after wetting. Properly compacted structural fill will also
Kumar & Associates, Inc. ® Project No. 18-7-255.01
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have some potential for long term settlement but should be considerably less than the underlying
natural-silt_and_claxsail.. Sources..otwetting_includeeirrigatian,--surface water runo£1=and-utility
line leaks. A heavily reinforced structural slab or post -tensioned slab foundation designed for
significant differential settlements is recommended for the building support.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with a heavily reinforced
structural slab foundation bearing on about 3 feet of compacted structural fill. A post -tensioned
slab foundation could also be used.
The design and construction criteria presented below should be observed for a shallow
foundation system.
1) A heavily reinforced structural slab placed on about 3 feet of structural fill should
be designed for an allowable bearing pressure of 1,500 psf. A post -tensioned slab
if used should be designed for a wetted distance of 10 feet but at least half of the
slab width, whichever is more. Based on experience, we expect initial settlement
of the slab foundation designed and constructed as discussed in this section will
be about 1 inch or less. Additional settlement could occur if the bearing soils
were to become wetted. The magnitude of the additional settlement would
depend on the depth and extent of wetting but may be on the order of 1 to
1'/2 inches.
2) The thickened sections of the slab for support of concentrated loads should have a
minimum width of 20 inches.
3) The perimeter turn -down section of the slab should be provided with adequate soil
cover above the bearing elevation for frost protection. Placement of foundations
at least 36 inches below exterior grade is typically used in this area. If a frost
protected foundation is used, the perimeter turn -down section should have at least
18 inches of soil cover.
4) The foundation should be constructed in a "box -like" configuration rather than
with irregular extensions which can settle differentially to the main building area.
Kumar & Associates, Inc. ® Project No. 18-7-255.01
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The foundation walls, where provided, should be heavily reinforced top and
hottom-to.span_local-anomalies..such- hy_assuming.an. unsupported.length of .t
least 14 feet. Foundation walls acting as retaining structures (if any) should also
be designed to resist lateral earth pressures as discussed in the "Foundation and
Retaining Walls" section of this report.
5) The organic root zone and any loose or disturbed soils should be removed.
Additional structural fill placed below the slab bearing level should be compacted
to at least 98% of the maximum standard Proctor density at a moisture content
near optimum and can consist of the onsite or similar soils.
6) A representative of the geotechnical engineer should evaluate the compaction of
fill materials and 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
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 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 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 placed 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
Kumar & Associates, Inc. ® Project No. 18-7-255.01
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this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backf i 1 l. -should -be expected, even -i -the-mateeialas-placed.correctly,_and_could.-result,in..distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger
than about 6 inches.
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 350 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.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support lightly loaded slab -on -grade construction
separate from the building foundation. The fill soils can be compressible when wetted and result
in some post -construction settlement. To reduce the effects of some differential movement,
slabs -on -grade should be separated from the building to 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 well -graded sand and
gravel, such as road base, should be placed beneath slabs for support. This material should
consist of minus 2 -inch aggregate with at least 50% retained on the No. 4 sieve and less than
12% 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.
Kumar & Associates, Inc. ® Project No. 18-7-255.01
7
UNDERDRAIN SYSTEM
It is our understanding that the finished floor elevation at the lowest level of the proposed
residence will be at or above the surrounding grade. Therefore, a foundation drain system is not
required. 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, be protected from
wetting and hydrostatic pressure buildup by an underdrain system.
If finished floor elevation of the proposed residence has a floor level below the surrounding
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
SURFACE DRAINAGE
Providing proper surface grading and drainage is important to limiting wetting of bearing soils
and potential building 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.
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 5 feet in unpaved areas and a minimum slope of
21/2 inches in the first 10 feet in paved areas.
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.
Kumar & Associates, Inc. ® Project No. 18-7-255.01
8
LIMITATIONS
1.
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 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,
Kumar & Associates, inc.
Shane J. Robat, P.E.
Reviewed by:
Steven L. Paw ak,
SJR/kac
Kumar
Project No. 1
LOT 295
(VACANT)
BORING 1
■
LOT 294
Fl
7r-r'"
LOT/293 - --
(EXISTING RESIDENCE
WITH BASEMENT)
10 0 is 20
APPROXIMATE SCALE—FEET
18-7-255.01
Kumar & Associates
LOCATION OF EXPLORATORY BORING
Fig. 1
BORING 1
LEGEND
0
5
— 10
— 15
20
25
0
01
1
33/12
10/12
WC=9.4
DD=102
— 200=78
9/12
WC=7.7
DD=112
10/12
WC=13.5
DD=111
— 200=81
UC=2,600
56/12
7
/
FILL: SILTY CLAYEY SAND AND GRAVEL, MEDIUM DENSE,
SLIGHTLY MOIST, BROWN.
SILT AND CLAY (ML—CL); SANDY, STIFF, SLIGHTLY MOIST TO
MOIST WITH DEPTH, BROWN.
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.
33/12 DRIVE SAMPLE SLOW COUNT. INDICATES THAT 33 BLOWS OF
A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
I PRACTICAL AUGER DRILLING REFUSAL.
NOTES
1. THE EXPLORATORY BORING WAS DRILLED ON APRIL 11, 2018
WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING.
3. THE ELEVATION OF THE EXPLORATORY BORING WAS NOT
MEASURED AND THE LOG OF THE EXPLORATORY BORING IS
PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATION 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 WAS NOT ENCOUNTERED IN THE BORING AT THE
TIME OF DRILLING.
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);
UC = UNCONFINED COMPRESSIVE STRENGTH (ASTM D 2166).
18-7-255.01
Kumar & Associates
LOG OF EXPLORATORY BORING
Fig. 2
1
4
�r.
CONSOLIDATION - SWELL
1
0
1
— 2
— 3
—4
10 APPLIED PRESSURE - KSF
10
1 DO
18-7-255.01
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 3
SAMPLE OF: Sandy Silt and Cfay
`FRO : ar ng OTO'
WC = 7.7 %, DD = 112 pcf
NO MOVEMENT UPON
WETTING
Mode lest melte an only t the
pro Iniad. Th. 1e117nq npoct
YwC not be ropmduced, i ce I In
lull, without the written approval of
Kumon end NeonI tw, Inc. Swell
ccordantlon WuUc performed In
accordance with ASiY 0-4M&
10 APPLIED PRESSURE - KSF
10
1 DO
18-7-255.01
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 3
Kumar & Associates, Inc."'
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION
NATURAL
MOISTURE
I CONTENT
(%)
NATURAL
DRY
DENSITY
(pcf)
GRADATION
PERCENT
PASSING NO.
SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(Dsfl
+
SOIL
TYPE
BORING
DEPTH
(ft)
GRAVEL
(%)
SAND
(%)200
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
1
5
9.4
102
78
Sandy Silt and Clay
10
7.7
112
Sandy Silt anti Clay
15
13.5
111
81
2,600
Sandy Silt and
Clay