HomeMy WebLinkAboutSubsoil Studylcn l(¡¡mar & Assoclates, lnc.6
Geotechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
An Emdoycc olfncd compqny www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RFCEIVED
ocï 2 I 202r
"#fr,i'l,îl3',t?#il,17SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT SS,IRONBRTDGE
RIVER BANK LANE
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-168
APRIL 14,202I
PR.EPARED FOR:
RED DEER REALTY
ATTN: LEO CARMICHAEL
0766 RIVER BEND \ryAY
GLENWOOD SPRINGS, COLORADO 81601
@
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
BACKGROUND INFORMATION ..
PROPOSED CONSTRUCTION
SITE CONDITIONS
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOTINDATION BEARING CONDITIONS .......
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM...
SURFACE DRAINAGE.......
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 . LOGS OF EXPLORATORY BORINGS
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 4 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No. 21.7.168
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 88, Ironbridge, River Bank Lane, 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
selices to Red f)eer Realty dated February 2,2021.
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.
BACKGROUND INFORMATION
Hepworth-Pawlak Geotechnical (now Kumar & Associates) previously performed a preliminary
geotechnical sfudy at the subject lot, report dated May 29,2007, Job No. l0l 107 311.
Information from that report has been reviewed and considered in the preparation of this report.
PROPOSED CONSTRUCTION
The proposed residence will be a one and two-story structure over crawlspace with a slab-on-
grade garage located as shown on Figure 1. Cut depths are assurred to be between 2 to 4 feel
and foundations loadings are assumed to be relatively light, typical of the proposed type of
construction
If building location, grading or loading information changes, we should be notified to re-evaluate
the recommendations presented in this report.
SITE CONDITIONS
The lot was vacant and the ground surface appeared mostly natural at the time of our field
exploration. The terrain is gently sloping down to the northeast at a grade of about 3 to 4o/rin
the building area then drops at a steep grade down about 12 feet to the gentle slope in the
Kumar & Associates, lnc, @ Project No. 21.7-168
a
northeast part of the lot. Elevation difference across the building areas is about 2 feet.
Vegetation consists of scaftered sage brush, grass and weeds. The Roaring Fork River is located
around 100 feet beyond the northeast property line.
SUBSIDENCE POTENTIAL
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. 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 areas of localized subsidence. During previous work
in the area, several sinkholes were observed scattered throughout the Ironbridge Subdivision.
These sinkholes appear similar to others associated with the Eagle Valley Evaporite in other
areas of the lower 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; howevcr, the exploratory boring was 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 88, throughout the service life of the proposed strucflre, in our
opinion, is low and similar to other platted lots in the area; however, the owner should be made
awaÍe 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 24,2021. Three exploratory
borings were drilled at the approximate locations shown on Figure I to evaluate the subsurface
conditions. The borings were advanced with 4-inch diameter continuous flight augers powered
by a truck-mounted CME-458 drill rig. The borings were logged by a representative of Kumar
& Associates.
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 the
subsoils. Depths at which the samples were taken and the penetration resistance values are
Kumar & Associates, lnc, o Project No. 2l-7-168
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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 the topsoil, consist of stiff sandy silty clay to depths of I to 3% overlying dense,
slightly silty, sandy gravel and cobbles with probable boulders. 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
oontent and density and gradation analyses. Results of swell-consolidation testing performed on
a relatively undisturbed drive sample of the clay, presented on Figure 4, indicate low to moderate
compressibility under conditions of loading and wetting. Results of gradation analyses
performed on small diameter drive samples (minus lYzinch fraction) of the coarse granular
subsoils are shown on Figure 5. 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.
FOUNDATION BEARING CONDITIONS
The fîne-grained clay soils possess low bearing capacity and low to moderate settlement
potential. The underlying coarse granular, sandy gravel and cobble soils possess moderate
bearing capacity and relatively low settlement potential. At assumed excavation depths, the
subgrade could transition the clay and coarse granular soils. Spread footings bearing entirely on
the coarse granular soils are recommended for foundation support of the residence to limit
settlement potentiai.
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 granular soils or compacted structural fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
Kumar & Associates, lnc. o Project No. 2'l-7-168
l)
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Footings placed on the undisturbed natural granular soils or compacted structural
fill should be designed for an allowable bearing pressure of 3,000 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be about I inch or less.
The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above theil bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
aÍea..
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 Walls"
section of this report.
The topsoil, clay and loose or disturbed soils should be removed and the footing
bearing level extended down to the relatively dense natural granular soils. The
exposed soils in footing area should then be moistened and compacted. Structural
fill placed to reestablish design bearing level should consist of a relatively well
graded granular soil compacted to at least 98% of standard Proctor density at near
optimum moisture content.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
2)
3)
4)
FOUNDATION AND RETAINTNG 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 mainly granular 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 alateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 40 pcf for backfill consisting of the on-site rnainly granular soils.
Backfîll should not contain organics or rock larger than about 6 inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffrc, constn¡ction materials and equipment. The
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Kumar & Associates, lnc. @ Project No. 21-7.168
5
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 maxlmum
standard Proctor density at a moisture content at or slightly above 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 pressute on the wall. Some settlernent 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 0.50. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 pcf. The
coeffîcient 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 granular soil compacted to atleast95o/o
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 tnovetnent, 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 relatively
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 507o retained on the No. 4 sieve
and less than l2o/o passing the No. 200 sieve.
Kumar & Associates, lnc. @ Project No. 2'l-7-168
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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 mainly granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
It is our understanding the proposed finished floor elevation at the lowest level is at or above the
sunounding grade and the crawlspace will be relatively shallow, around 3 feet deep. Therefore,
a foundation drain system is not required. 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 and basement areas, be protected from wetting and
hydrostatic pressure buildup by an underdrain and wall drain system.
lf the finished floor elevation of the proposed structure is revised to have a floor level below the
sumounding grade, we should be contacted to provide recommendations for an underdrain
system. All earth retaining structures should be properly drained.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) lnundation ofthe 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 95Vo 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irigation should be located at least
5 feet from foundation walls.
Kumar & Associates, Inc. @ Project No. 21-7-168
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LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. lVe 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 possibilify of mold or other biological contaminants (MOBC) deveioping
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. trVe 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 verifu 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 sfuctural fill by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associates, Inc.
Steven L. Pawlak P.E.
Reviewed by:
t
Daniel E. Hardin,
SLPlkac
Kumar & Associates, lnc. o Project No,21-7168
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BOilNO 5
o
2
8ffi
EORING B-to
oo.,""
(o$$LOT 87
{o
BORING
%r,B-l
9*o
Çro
LOT 89
BENCH MARK:
REBAR WITH CAP, ELEV.= 100', ASSUMED
LEGEND:
.) BORING FOR CURRENT STUDY
O aoR¡I.Io FOR PERVIOUS STUDY
H-P GEOTECH, DATE MAY 29, 2OO7
20 0
APPROXIMATE SCALE-FEET
21-7 -168 Kumar & Associates LOCATION OF TXPLORATORY BORINGS 1Fig.
BORING 1
EL.=97.8'
BORING 2
EL.=96.8'
BORING 2
EL.=95.7'
0
5
10
LEGEND
18/ t2
WC=15.4
DD=1 16
so/ 4
1o/12
43/6,
50/3.5
71 /12
31 /6,
50/s
5a/3
0
5
10
t-t¡lt¡lL-
I-F--fLt¡tô
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LrJ
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N
IOPSOIL: ORGANICS SANDY SILT AND CLAY, FIRM, BROWN'
CLAY (CL); SILTY, SANDY, VERY STIFF, MOIST' BRoWN.
GRAVEL AND COBBLES
SLIGHTLY MOIST, GREY
(GM-GP); SLIGHTLY SILTY' SANDY, PROBABLE BOULDERS' DENSE,
BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE
DR|VE SAMPLE, 1 3/9-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST.
Aı ]A^ DRIVE SAMPLE BLOW COUNT. INDICATES ÎHAT 18 BLOWS OF A 14o-POUND HAMMER
'o/ '' F^LLTNG 30 TNCHES WERE REQU|RED To DRtvE THE SAMPLER 12 lNcHES.
PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 24, 2021 WITH A 4-INCH_DIAMEÏER
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 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 ÏHE
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 D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTU OOSIS);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1 1 4O);
W
!
i
I
WC=1.4
+4=60
-200=1 0
Fig. 2Kumar & Associates LOGS OF EXPLORATORY BORINGS21 -7 -1 68
3
2
1
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=ltt
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z.o
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o
Jotnz.o()
0
-1
*2
-5
-4
t.0 - KSF t0 tü,
SAMPLE OF: Scndy Cloy
FROM: Boring 1 @ 1'
WC = 15.4 %, DD = 116 pcf
EXPANSION UNDER CONSTANÏ
PRESSURE UPON WETTING
I )
Fig. 3SWTLL-CONSOLIDATION ÏTST RESULTS21 -7 -168 Kumar & Associates
2ı
2
,
100
90
80
70
ôo
50
40
50
20
to
o
o
ro
2A
30
,ao
50
60
70
ao
00
ã
E
Ë
too
.oot
IN
CLAY TO SILT COBBLES
GRAVEL 60 % SAND
LIQUID LIMIÏ
SAMPLE OF: Sllghtly Sllly Sondy Grovel
30%
PLASIICITY INDEX
SILT AND CLAY 10 %
FROM: Borlng 2 O 4' ond 7' Comblned
Ther. lesl ¡e8ulls opply only lo lhe
3omplôs whlch roÞ lesl€d. Tho
hsllng ruporl rholl not b. ruproduc.d,
oxc€pl lñ full, v¡thoul th6 w¡ltlon
qpprovql ol Kumqr & A¡¡oclolas, lnc.
Sbv. dnolysb l.sllng ls p.rfom.d ¡n
oocordoncà wllh ASIM 06915. ASIM D792E,
ASTM C156 qnd/ôr ASTM Dlt,lo.
SIEVE ANALYSISIJYDROMEIER ANALYSIS
TIYÊ READINGS
2' HRS 7 HRS
u,s.
-' rl tl
rL
IL
r--:i tu; l l
I
I.,i--ij_
I_--J- --- f --i -1;=
--t ,-l-it
SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
21 -7 -1 68 Kumar & Associates GRADATION TIST RESULTS Fis. 4
lc,I*mimffü-*
-
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
No.2l.7-168
AT]LtutTsSAMPTLOCATIONGRADAIION
PLASTIC
INDEX
¡0Át lbçfl
UNCONFINED
COMPRESSIVE
SIRENGTH SOIL TYPEBORING
¡ffì
DEPTH
IOAI
NATURAL
MOISTURE
CONTENT
fftfl
NATURAL
DRY
DENS]fY
GRAVEL
(%)
SAND
{%}
PERCENT
PASSING NO.
200 slEvE
P/"1
LIQUID LIMIT
Sandy Clay15.4 116
Slightly Silty Sandy Gravel24 &,'7
combined 1.4 60 30 r0