HomeMy WebLinkAboutSoils Report 10.12.2020Kumar & Associates, Inc.®
Geatechnical and Materials Engineers
and Environmental Scienlisls
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 276, IRONBRIDGE
BLUE HERON VISTA
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-517
OCTOBER 12, 2020
PREPARED FOR:
KENDRICK DEVELOPMENT, LLC
ATTN: WES COLE
430 IRONBRIDGE DRIVE
GLENWOOD SPRINGS, CO 81601
(wcale�7u,iron b rid gecl u b.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
BACKGROUND INFORMATION - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
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
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 20.7-517
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 276, Ironbridge, Blue Heron Vista, 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 September 9, 2020. Hepworth-Pawlak
Geotechnical (now Kumar & associates) previously performed a preliminary geotechnical study
for the Ironbridge Villas where Lot 276 is located and presented the findings in a report dated
September 14, 2005, Job No. 105 115-6.
A field exploration program consisting of an exploratory boring 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 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.
BACKGROUND INFORMATION
The proposed residence is located in the existing Ironbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted subsurface exploration and geotechnical
evaluation for development of Villas North and Villas South parcels, Job No. 105 115-6, report
dated September 14, 2005, and performed observation and testing services during the
infrastructure construction, Job No. 106 0367 between April 2006 and April 2007. The
information provided in these previous reports has been considered in the current study of
Lot 276.
PROPOSED CONSTRUCTION
The proposed residence will be a one story, wood frame structure with an attached garage about
1,800 square feet in size and located within the property boundary shown on Figure 1. Ground
floor is proposed to consist of a structural slab -on -grade with no basement or crawlspace.
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Grading for the structure is proposed to be relatively minor with cut or fill depths between about
2 to 3 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.
SITE CONDITIONS
The lot was vacant at the time of the field exploration. The terrain was relatively flat with about
1 to 1'/2 feet of elevation difference down to the southeast. Fill had been placed to elevate the lot
and surrounding area by the previous subdivision grading. Vegetation consisted of sparse grass
and weeds at the time of the field exploration.
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 in the area of the proposed residence on Lot 276 did not encounter voids but the
alluvial fan depth encountered was generally greater than encountered on nearby lots which
could indicate past ground subsidence. In our opinion, the risk of future ground subsidence on
Lot 276 in the Villas North parcel 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.
Kumar & Associates, Inc. ® Project No. 20-7-517
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FIELD EXPLORATION
The field exploration for the current project was conducted on September 21, 2020. 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 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 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, below about 11 feet of relatively dense, mixed sand, silt and gravel fill, consist of
medium dense, silty sand with gravel and stiff to medium stiff, sandy silt with scattered gravel.
Below the sand and silt at about 37 feet deep was dense, slightly silty sandy gravel and cobbles
to the maximum drilled depth of 40 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density, finer than sand size gradation analyses and liquid and plastic limits. Swell -
consolidation testing was not performed on samples of the sand and silt soils but results of
testing on adjacent lots typically indicate low to moderate compressibility under loading and low
collapse potential (settlement under constant load) when wetted. The laboratory testing is
summarized in Table 1.
Free water was not encountered in the boring at the time of drilling and the soils were slightly
moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The upper 11 feet of soils consist of fill placed mainly in 2006 as part of the subdivision
development. The field penetration tests (blow counts) and laboratory tests performed for the
Kumar & Associates, Inc. ® Project No. 20-7-517
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current study, and review of the field density (compaction) tests performed during the fill
construction indicate the structural fill was placed and compacted to the project specified 95% of
standard Proctor density. Debris fan soils which tend to collapse (settle under constant load)
when wetted were encountered below the fill. The amount of settlement will depend on the
thickness of the compressible soils due to potential collapse when wetted, and compression of
the underlying soils after wetting. Relatively deep structural fill will also have some potential for
long term settlement but should be considerably less than the alluvial fan deposit. Sources of
wetting include irrigation, surface water runoff 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 at least 10 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 structural slab
foundation system.
1) A heavily reinforced structural slab placed on around 10 feet of structural fill
should be designed for an allowable bearing pressure of 1,500 psf or subgrade
modulus of 125 tcf. 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
Kumar & Associates, Inc. ® Project No. 20-7-517
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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.
The foundation walls, where provided, 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 (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.
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 50 pcf for backfill consisting
of the on -site soils. Cantilevered retaining structures which are separate from the building 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 40 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.
Kumar & Associates, Inc. ® Project No. 20-7-517
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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
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.
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.
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. 20-7-517
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
Precautions to prevent wetting of the bearing soils, such as proper backfill construction, positive
backfill slopes, restricting landscape irrigation and use of roof gutters need to be taken to limit
settlement and 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
2'/2 inches in the first 10 feet in paved areas. Graded swales should have a
minimum slope of 3%.
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. 20-7-517
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LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at the time of this study. 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.
Steven L. Pawlak, P.E.
Reviewed by:
Daniel E. Hardin, P.
SLP/kac
Kumar & Associates, Inc.
Project No. 20-7-517
5959
LOT 277
BORING 1
•
10 0 10 20
APPROXIMATE SCALE -FEET
LOT 275
BLUE HERON VISTA
20-7-517
Kumar & Associates
LOCATION OF EXPLORATORY BORING
Fig. 1
13
1
1
1
0
5
10
15
r-
w
w
20
w
25
BORING 1
EL. 5958'
x
35/12
WC=5.2
DD=122
52/12
WC=12.7
DD=123
36/12
WC=9.7
DD=122
— 200=42
22/12
WC=10.1
— 200=51
LL=19
PI=2
LEGEND
b;
TOPSOIL; CLAYEY SILT AND SAND, ROOT ZONE.
FILL; SILTY SAND TO SANDY SILT WITH GRAVEL, SLIGHTLY CLAYEY,
MEDIUM DENSE, SLIGHTLY MOIST TO MOIST, MIXED BROWN,
SUBANGULAR ROCK.
SAND AND SILT (SM—ML); SCATTERED GRAVEL TO GRAVELLY,
SLIGHTLY CLAYEY, MEDIUM DENSE/STIFF TO MEDIUM STIFF, MOIST,
BROWN.
GRAVEL AND COBBLES (GM —GP), SLIGHTLY SILTY, DENSE, MOIST,
BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D SPLIT SPOON STANDARD
PENETRATION TEST.
35/12 SAMPLE BL.
AD
DRIVE 140—POUND HAMMEROWCOUNTFALLING 30
INDICATES INCHESTHAT WERE35BLOWS REQUIREOFD
TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORING WAS DRILLED ON SEPTEMBER 21, 2020
WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
8/12 2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE
PLAN PROVIDED.
31/12
: — WC=5.8
T.'. DD=108
• —200=28
r=:
30
35
11/12
•
- 40
20-7-517
Kumar & Associates
3. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.
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 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);
LL = LIQUID LIMIT (ASTM D 4318);
PI = PLASTICITY INDEX (ASTM D 4318).
LOG OF EXPLORATORY BORING
Fig. 2
It -FA
Kumar & Associates, Inc.®
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-517
SAMPLE LOCATION
Boring
1
DEPTH
NATURAL
MOISTURE
CONTENT
(ft) (%)
2 1/2
5
5.2
12.7
NATURAL
DRY
DENSITY
(pcf)
122
123
GRADATION
GRAVEL SAND
(%) (%)
PERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
UNCONFINED
COMPRESSIVE
STRENGTH
jpsf]
10
15
25
9.7
10.1
5.8
122
108
42
51
28
19
2
SOIL TYPE
Silty Clayey Sand with
Gravel Clay (Fill)
Silty Clayey Sand with
Gravel (Fill)
Very Silty Sand with
Gravel (Fill)
Very Sandy Silt with
Gravel
Silty Gravelly Sand