HomeMy WebLinkAboutSoils Report 03.13.2019I(+A
Kowa llaeneis, C.
Geotechnim3 and IlLtitrials Errgireers
and ffvirt mentaiSatrn1sts
5020 County Road 154
Glenwood Springs, CO 89801
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Company www.kurnarusa.corn
Office Locations: Denver (HO), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
AUG 1 5 2O1
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 94, IRONBRIDGE
322 RIVER BEND WAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 19-7-158
MARCH 13, 2019
PREPARED FOR:
TIM & NATALIE McDONALD
8964 WINDHAVEN DRIVE
PARKER, COLORADO 80134
(c u buffbuff(iicio u d.com)
(riatoliemcdoneald(a. mac.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 - 3 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS -4-
FOUNDATION AND RETAINING WALLS - 5 -
FLOORSLABS -6-
UNDERDRAIN SYSTEM - 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EYP 1 ORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 & 5 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. Project No. 19-7-158
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
94, Ironbridge, 322 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 proposal for geotechnical engineering services
to Tim and Natalie McDonald dated February 26, 2019. Hepworth-Pawlak Geotechnical
Engineering (now Kumar and Associates, Inc.) previously conducted a preliminary subsoil study
for Lots 90 through 100 of Rose Ranch (now Ironbridge) and presented the findings of in a
report dated December 31, 2002, Job No. 101 196-1.
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
The proposed residence will be a one-story structure with a two-story attached garage. Ground
floor will be structural over crawlspace in the living areas and slab -on -grade in the garage.
Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5
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. 19-7-158
-2 -
SITE CONDITIONS
The lot was vacant and covered with about three inches of snow at the time of field exploration.
The ground surface appeared natural and was gently sloping down to the east with about 1V2 feet
of elevation change across the proposed building area. The Roaring Fork River is located about
100 yards east of the building area and about 15 to 20 feet lower than the lot. Vegetation
consists of grass and weeds.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge development.
These rocks arc 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 studies for the subdivision development, several
sinkholes were observed scattered throughout the Ironbridge area. These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the 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 94 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 8, 2019. Three 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. 19-7-158
-3 -
mounted CME -45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with 13/5 inch and 2 inch ID. 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, below about 4 foot of topsoil, consist of 2%a to 6 feet of soft to very stiff sandy to very
sandy silty clay overlying dense, silty, sandy gravel and cobbles. Drilling in the dense granular
soils with auger equipment was difficult due to the cobbles and, possibly, boulders and drilling
refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and gradation analyses. Results of swell -consolidation testing performed on relatively
undisturbed drive samples, presented on Figures 4 and 5, indicate moderate to high
compressibility under conditions of loading and wetting in the clay soils from Boring 1 at 21/2
feet and low compressibility under light loading at natural moisture content and a low expansion
potential when wetted in the clay sample from Boring 3 at 2'A feet. Results of gradation analyses
performed on small diameter drive samples (minus 11/2 inch fraction) of the coarse granular
subsoils are shown on Figure 6. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling. The subsoils were very
moist in Boring 1 and slightly moist to moist in the other borings.
FOUNDATION BEARING CONDITIONS
The sandy silty clay soils have variable compressibility and low bearing capacity and are
generally unsuitable for support of building loads. The underlying silty sandy gravel and cobble
Kumar & Associates, Inc. Project No. 19-7-158
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soils have low compressibility and moderate bearing capacity and are suitable for support of
shallow spread footings with low settlement potential_ Where sandy silty clay soils are
encountered, sub -excavation and replacement with structural fill appears feasible to reestablish
design bearing level.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural coarse granular soils or compacted structural fill after removal of the compressible
sandy silty clay soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural gravel soils (below clay soils) or
compacted structural fill should be designed for an allowable bearing pressure of
2,500 psf. Based on experience, we expect settlement of footings designed and
constructed as discussed in this section will be about 1 inch or less.
2) The footings should have a minimum width of 16 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 reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 12 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Wails'
section of this report.
5) All topsoil, sandy silty clay soils and loose or disturbed soils should be removed
and the footing bearing level extended down to the relatively dense natural coarse
Kumar & Associates, Inc. Project No. 19-7-158
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granular soils. The exposed soils in footing area should then be moistened and
compacted. As an alternative, design footing grade can be re-established with
compacted structural fill. Structural fill should consist of a relatively well graded
sand and gravel such as CDOT Class 6 road base compacted to at least 100% of
standard Proctor density and extended laterally beyond the footing edges a
distance equal to at least half the depth of fill below the footing.
6) A representative of the geotechnical engineer should evaluate the fill placement
for compaction 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 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 40 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 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
Kumar & Associates, Inc. Project No. 19-7-158
-6 -
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 0.50. 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 a 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, are suitable to support lightly loaded slab -on -grade
construction. 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 sand and
gravel should be placed beneath slabs for structural support and 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 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 granular soils or imported road base 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 that local perched groundwater can develop during times of heavy precipitation or
Kumar & Associates, Inc. Project No. 19-7-158
-7 -
seasonal runoff. Frozen ground during spring runoffcan also 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 t foot below lowest adjacent finish grade and sloped at a minimum I% to
a suitable gravity outlet sump and pump or drywell. 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 1'Vz 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 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.
Kumar&Associates, Inc. Project No. 19-7-158
-8 -
The conclusions and recommendations submitted m this report are based upon the data obtained
from the exploratory borings drilled at the locations indicated on Figure 1, the proposexi 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
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.
/0,, fa -
Shane M. Mello, Staff Engineer
Reviewed by:
Daniel E. Hardin, P " R 4 4 3 .
' s/rrrtq
cc: Patrick W. Stuckey (stli Q 0.cast.net)
Kumar & Associatt , h}�.
Project No. 19-7-158
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DD=95
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DD=117
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19-7-158
Kumar & Associates
LOGS OF EXPLORATORY BORINGS
1
DEPTH-FEET
Fig. 2
LEGEND
dsz
TOPSOIL; ORGANIC CLAY AND SILT. FIRM. MOIST. DARK BROWN.
CLAY (CL); SILTY. SANDY TO VERY SANDY, MEDIUM STIFF 70 VERY STIFF. SLIGHTLY MOIST
TO VERY MOIST, GRAYISH BROWN TO BROWN.
GRAVEL AND COBBLES (GM); SANDY TO VERY SANDY. SILTY TO SLIGHTLY SILTY. DENSE,
SUGHTLY MOIST TO MOIST. MIXED BROWN.
I DRIVE SAMPLE, 2—INCH I.D. CAUFORNIA UNER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
412 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 4 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
DEPTH AT WHICH BORING CAVED.
f PRACTICAL AUGER REFUSAL.
NATE$_
1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 8. 2019 WITH A 4—INCH DIAMETER
CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY INSTRUMENT LEVEL AND
REFER TO THE BENCHMARK ON FIG. 1.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
• GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
19-7-158
Kumar & Associates
LEGEND AND NOTES
Fig. 3
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CONSOLIDATION - SWELL
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19-7-158
SAMPLE OF: Sandy Silty Clay
FROM: Boring 1 0 2.5'
WC = 18.7 %. DD = 106 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1.0 APPLIED PRESSURE - KSi
Kumar & Associates
10 100
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
CONSOLIDATION - SWELL
2
SAMPLE OF: Sandy Silty Clay
FROM: Boring 3 0 2.5'
WC = 8.1 X, DD = 117 pcf
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19-7-158
10 APPIJED PRESSURE — KSF 10 100
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
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Geotechnical and Materials Engineers
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kumarusa.com
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No, 19-7.158
SAMPLE LOCATION
NATURAL NATURAL
MOISTURE DRY
CONTENT DENSITY I
(%) Oct)
GRADATION
PERCENT
NO.
Papp SIEVE
ATFERSERG LIMITS
PLASTIC
LIQUID LI
LIQUMB INDEX
(%) PA)
UNCONFINED
COMPRESSIVE
STRENGTH
(Pe)
SOIL TYPE
BORING
DEPTH
(t)
GRAVEL SAND
l%) (y,)
1
21/2
18.7
106
Sandy Silty Clay
5
11.9
95
60
Very Sandy Silt and Clay
3
21/2
8.1
117
Sandy Silty Clay
5
5.8
99
59
Very Sandy Silty Clay
10
3.9
55
34
11
Slightly Silty Sandy Gravel
r