HomeMy WebLinkAboutSubsoils Report for Foundation Designrcn lfumar & Associates, lnc.'
Geotechnical and Materials Engineers
and Environmental Scientists
An Emdoycc Orncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970)945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
rvu.w.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 41, PHASE 3o IRONBRTDGE
BLUE HERON DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-436
JULY 19,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUrTE 52018
ASPEN, COLORADO 81611
luke. gosda@sunriseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ...
PROPOSED CONSTRUCTION
SITE CONDITIONS....
GEOLOGY.
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS .
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGIIRES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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.......- 3 -
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..-3-DESIGN RECOMMENDATIONS ......
LIMITATIONS .........- 6 -
Kumar & Associates, lnc. o Project No.21-7-436
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 41, Phase 3, Ironbridge, Blue Heron Drive, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to SCIB, LLC dated May 10, 202I.
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 analyzedto 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
At the time of our study,lans for the residence had not been The building is
proposed within the upper part of the building envelope shown on Figure 1 and will generally be
a wood frame structure over a
Grading for the structure will
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 subject site was vacant at the time of our field exploration. The site is split in two relatively
flat benches separated by a relatively steep 5-foot tall slope. The borings were drilled on the
upper bench as shown on Figure 1. Vegetation consists of grass and weeds. The downhill side
of Blue Heron Drive appears to be a fill bench for residence construction placed during the
subdivision development. The Roaring Fork River is located downhill about %mlle to the north.
Kumar & Associates, lnc. @ Project No, 21-7-436
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GEOLOGY
The geologic conditions were described in a previons repoft conducted for planning and
preliminary design of the overall subdivision development by Hepworth-Pawlak Geotechnical
(now Kumar & Associates) dated October 29,1997, Job No. 197 327. The natural soils on the
lot mainly consist of sandy silt and clay alluvial fan deposits overlying gravel terrace alluvium of
the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of rounded
gravel, cobbles, and boulders typically up to about 2 to 3 feet in size in a silty sand matrix and
overlies siltstone/claystone bedrock.
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge subdivision.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum and limestone. Dissolution of the gypsum under certain conditions can
cause sinkholes to develop and can produce areas of localized subsidence. A sinkhole occurred
in the parking lot adjoining the golf cart storage tent in January, 2005 located several hundred
feet south of Lot 41 which was backfilled and compaction grouted. To our knowledge, that
sinkhole has not shown signs of reactivation such as ground subsidence since the remediation.
Sinkholes possibly related to the Evaporite were not observed in the immediate area of the
subject lot. Based on our present knowledge of the subsurface conditions at the site, it cannot be
said for certain that sinkholes related to the underlying Evaporite will not develop. The risk of
future ground subsidence on Lot 41 throughout the service life of the proposed building, 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 June 15, 202I. 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-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with l%-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 fhe
Kumar & Associates, lnc. @ Project No.21-7.436
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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 consist of a shallow topsoil/root zone overlying sandy clay with gravel fill to between 8
and 9 feet deep where between about 4 to 6 feet of very stiff, sandy silty clay soils was
encountered. Dense, silty sandy gravel with cobbles was encountered below the clay soils to the
maximum explored depth of 21 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell-consolidation testing performed on
relatively undisturbed drive samples of the natural clay soils, presented on Figures 4 and 5,
indicate low compressibility under existing low moisture conditions and light loading and a low
to moderate expansion potential when wetted under constant light surcharge. Results of a
gradation analysis performed on small diameter drive samples (minus L%-inch fraction) of the
granular soils 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 and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
Spread footing foundations placed on the relatively dense fill soil above the natural clay soils or
underlying dense gravel soils should be adequate for support of the proposed residence with
relatively low settlement potential. Although the tested clay soil samples showed an expansion
potential when wetted, our experience in this area is that the existing fill and clay soils are
typically not expansive. Footings bearing entirely on the underlying dense gravel soils should
have a low settlement risk. The bearing condition of the soils exposed in the excavation should
be further evaluated at the time of construction for potential sub-excavation and replacement
with compacted structural fill in the event of expansive conditions.
DESIGN RECOMMENDATIONS
FOTINDATIONS
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 relatively dense fill soils or the underlying natural soils if encountered.
Kumar & Associates, lnc. o Project No. 21-7-436
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The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the relatively dense fill soils or the underlying natural soils
should be designed for an allowable bearing pressure of 1,500 psf. Based on
experience, we expect initial settlement of footings a.rffiiiAronstructed as
discussed in this section will be about 1 inch or less. Post-construction settlement
could he aronnd I inch mainly if the bearing soils are we,tted.
2) The footings should have a minimum width of 18 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
atea.
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 12 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure coresponding to an equivalent fluid unit weight of at least
55 pcf for the onsite soils as backfill. A sliding coefficient of 0.35 and passive
earth pressure of 350 pcf equivalent fluid unit weight can be used to resist lateral
loading on foundation walls.
5) Any topsoil and loose disturbed soils should be removed and the footing bearing
level extended down to the relatively dense fill soils or natural soil beneath the
fill. The exposed soils in footing area should then be moistened and
Additional structural fill can consist of the onsite soils compacted to atleast 98Yo
of standard Proctor density at near optimum moisture content. New structural fill
should extend laterally beyond the footing edges a distance equal to at least one-
half the fill depth below the footing.
A representative of the geotechnical engineer should observe all footing
excavafions prior to concrete placement to evaluate hearing conditions.
FLOOR SLABS
The on-site fill soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. The natural clay soils, if encountered, should be evaluated for expansion potential
and the need for sub-excavation and placement of structural fill. To reduce the effects of some
differential movernent, floor slabs should be separated fi'om all bearing walls and colurms with
6)
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expansion joints which allow unrestrained vertical movement. Floor slab control joints should
be used to reduce darnage 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 road base should be placed beneath garage level slabs.
This material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4
sieve and less than l2o/o passing the No. 200 sieve. The gravel layer below the basement floor
slab should be relatively free draining with less than2%o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required filI can consist of the on-
site fill soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
The proposed slab-on-grade garage floor and shallow crawlspace (less than 4 feet), if used,
should not require a perimeter underdrain system provided that the site grading recommendations
contained in this report are followed. We recommend that below-grade construction, such as
retaining walls, deep crawlspace and basement areas, be protected from wetting and hydrostatic
pressure buildup by an underdrain system.
Where installed, 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 loh to a suitable gravity outlet or drywell based in the gravel soils. Free-
draining granular material used in the underdrain system should contain less than 2o/o passingthe
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 lYz feet deep. An impervious membrane such as 30 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 surface grading and drainage will be critical to prevent wetting of the bearing soils and
satisfactory performance of the foundation. 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.
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2)Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95%o 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.
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 l0 feet in paved areas. Free-draining wall backfill, where
used, should be covered with filter fabric and capped with about 2 feet of the on-
site soils to reduce surface water inhltration. Graded swales should have a
minimum slope of 3%.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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.
3)
4)
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
l"hal 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 veriff that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
s)
Kumar & Associates, lnc. @ Project No. 21-7-436
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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.
James H. Parsons, P
Reviewed by:
ffi,/.
Steven L. Pawlak, P.E.
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Kumar & Associates, lnc. @ Project No. 21-7-436
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21 -7 - 436 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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BORING 1
EL. 106.5'
BORING 2
EL. 106.5'
0 0
18/ 12
WC=5.8
DD=1 1 1
-2OO=72
1e/ 12
5
34/ 12
WC=7.1
DD=116
-200=82
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24/ 12
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DD=112
-200=83
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25 25
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+4=36
-20Q=17
21 -7 - 436 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
FILL: SLIGHTLY GRAVELLY SANDY SILTY CLAY, VERY STIFF, SLIGHTLY MOIST, LIGHT TO
MEDIUM BROWN & REDDISH BROWN.
CLAY (CL); SILTY, SANDY, VERY STIFF, SLIGHTLY MOIST, TAN AND BROWN TO REDDISH
BROWN.
GRAVEL (OC); SINOY, CLAYEY, SLIGHTLY SILTY, DENSE, SLIGHTLY MOIST TO MOIST, BROWN
GRAVEL (GP-GM); SANDY, SILTY, WITH COBBLES, VERY DENSE, SLIGHTLY MOIST, LIGHT GRAY
AND TAN.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
i DRIVE SAMPLE, 1 3/8-tNeH t.D. SpLtT SPOON STANDARD pENETRATION TEST
1e/i, DRIVE SAMPLE BLOW COUNT. INDICATES THAT 18 BLOWS OF A 14O-POUND HAMMER'"/ '' FALLTNG 30 tNcHES wERE REQUIRED To DRtvE THE sAMpLER 12 tNcHES.
f eucrrcAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JUNE 1 5, 2021 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 OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
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.
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 (pct) (lSrV D2216);
+4 = PERCENTAGE RETAINED oN No. 4 SIEVE (ASTM D6913);
_2OQ= PERCT.NIAGI PASSING NO. 2OO SITVL (ASIM IJ1140).
21 -7 - 436 Kumar & Associates LEGEND AND NOTES Fig.3
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SAMPLE OF: Silty Sondy Cloy
FROM: Boring 1 @ 10'
WC = 8.5 %, DD = 110 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21 -7 -436 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULTS Fig. 4
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HYDROMETER ANALYSIS
TIME READINGS
24 HRS 7 HRS 4t
u.s,CLEAR SQUARE OPENINGS
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20
50
40
50
60
70
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IN MILLIMETERS
152
DIAMETER OF PA
CLAY TO SILT COBBLES
GRAVEL 36 % SAND
LIQUID LIMIT
SAMPLE OF: Silty V€ry Grovelly Sond
47%
PLASTICITY INDEX
SILT AND CLAY 17 %
FROM: Boring 2 @ 15' & 20' (Combined)
Thos6 l€sl rssulls qpply only lo lh€
sqmplgs which w6r6 lsslgd. Th6
losllng r€porl sholl nol b6 rsproduced,
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Sl6ve qnolysls lssllng ls performsd ln
qccordonce wlth ASTM D6913, ASTM D7928,
ASTM C136 ond/or ASTM Dl1,+0.
GRAVELSAND
COARSEFINEMEDTUM ICOARSE FINE
21 -7 -436 Kumar & Associates GRADATION TEST RESULTS Fig.6
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Sandy Gravelly Clay (Fill)
Sandy Gravelly Clay (Fill)
Silty Sandy Clay
Sandy Gravelly Clay (Fill)
Silty Sandy Clay
Silty Sand and Gravel
(osfl
UNCONFINED
COMPRESSIVE
STRENGTH
72
(o/"1
PLASTIC
INDEX
ATTERBERG LIMITS
(%l
LIQUID LIMIT
PERCENT
PASSING NO.
200 s|EVE
83
82
71
SAND
$t
47
GRADATION
(:/"1
GRAVEL
36
116
t04
SAMPL= LOCATION
DEPTHBORfiIG
NATURAL
DRY
DENSITY
NATURAL
MOISTURE
CONTENT
5.8 111
7.3 112
8.5 i 110
11
9.7
0.3
2%
7V,
01
5
01
ts &20
combined
1
2
No.21-7-436