HomeMy WebLinkAboutSubsoil Studyl(trt ltumar&Associates, lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO 81601
and Environmentalscientists phone: (970) 945-7gsg
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
An EmployÇc Owncd compony www.kumarusa.com
Office Locations: Denver (HQ), Parkel Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RFCE.,YFD
'-rffii '{rí,*,
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT ¿I,IRONBRIDGE, PHASE III
1796 RIVER BEND \üAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-685
ocroBER 6,2021
PREPARED FOR:
scIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
luke.gosda@sunriseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL.....
FIELD EXPLORATION
SUBSURFACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ........
FOUNDATIONS
FLOOR SLABS
I.INDERDRAIN SYSTEM ........
SURFACE DRAINAGE............
LIMITATIONS....
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 _ LEGEND AND NOTES
FIGURE 4 - SV/ELL-CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
1
....- 3 -
-3-
",-L-
.)
aJ
a
-)
4
5
5
6-
Kumar & Associates, lnc.Project No. 21.7.685
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot2I,Ironbridge, Phase IlI,1796 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 SCIB, LLC dated August 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 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
Design plans for the proposed residence were not available at the time of our study. We assume
the residence will be located between the exploratory borings shown on Figure 1 and be a one or
two-story wood-frame structure over crawlspace or walkout basement with slab-on-grade floors.
Grading for the structure is assumed to be relatively minor with cut depths between about 3 to
8 feet. We assume relatively light foundation loadings, typical of the proposed 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 and appeared to have had minor cut and fill grading during the subdivision
development. The ground surface is relatively flat near the road and gently sloping in the
northern part down to the north-northeast toward the Roaring Fork River. A moderately steep
intervening slope of about 10 to 18 degrees and 10 feet high is present in the middle-south part of
the lot with about 18 feet of elevation difference in the general building envelope. Vegetation
consisted of grass and weeds and scattered brushes with one tree along the western property line
on the moderately steep slope. A small, broad depression near the northeast comer appears to be
from prior grading and the stormwater detention basin to the northeast.
Kumar & Associates, lnc.Project No. 21-7-685
n
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge development.
These rocks are a sequence of gypsiferous shale, fîne-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 work in the area, several sinkholes were obscrvcd
scattered throughout the Ironbridge development. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork River valley. A
sinkhole occurred in the parking lot adjoining the golf cart storage tent in January, 2005 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 were not observed in the immediate area of the subject lot and no evidence of cavities
was encountered in the subsurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. The cause of the small broad depression in the northeast
part of the lot is unknown and appears man-made. However, if building is proposed in that area
we should be contacted for further subsurface exploration and evaluation. Based on our present
knowledge of the subsurface conditions at the site, it cannot be said for certain that sinldroles will
not develop. The risk of future ground subsidence on Lot 21 throughout the service life of the
proposed residence, in our opinion, is low and similar to other lots in the area; 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 August 31,2021. 'l'wo exploratory
borings were drilled at the locations showr on Figure I l.o 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, 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-l586.
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.
Kumar & Associates, lnc.Project No.21.7.685
-3-
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. Below
about 1 foot of topsoil, the subsoils consist of dense to very dense, slightly silty to silty, sandy
gravel with cobbles and probable small boulders in Boring 1 and about 4 feet of stiff sandy to
very sandy clay and silt above dense to very dense, slightly silty to silty, sandy gravel with
cobbles and probable small boulders in Boring 2 down to the explored depths of about 9 and
12 feeL Drilling in the dense, coarse granular soils with auger equipment was difficult due to the
cobbles and boulders and drilling refusal was encountered in the deposit in Borings I and2.
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
a relatively undisturbed drive sample of the silt and clay soil, presented on Figure 4, indicate low
compressibility under light loading and low collapse potential when wetted. Results of gradation
analyses performed on small diameter drive samples of the coarse granular subsoils (minus
IYz-inch fraction) are shown on Figure 5. 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 to moist.
FOUNDATION BEARING CONDITIONS
The upper clay and silt soils encountered in Boring 2 possess a low bearing capacity and
typically a low to moderate settlement potential if wetted. Footings can be used for support of
the building and should be deepened below the upper fine-grained soils and placed on the
underlying dense, sandy gravel with cobble soil to achieve a low movement risk. 'We should
evaluate the exposed bearing conditions at the time of construction for movement potential and
the need to lower the bearing elevation. Design bearing level of sub-excavated areas can be
reestablished with compacted structural fill.
DESIGN RE COMMENDATIONS
FOLTNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural sandy gravel with cobble soils with a low settlement risk. Structural fill used to
reestablish design bearing level after removing the clay and silt soil should consist of granular
material compacted to at least 98o/o of standard Proctor density at near optimum moisture content
and extend laterally beyond the footing edges a distance equal to at least one-half the depth of fill
below the footing.
Kumar & Associates, lnc.Project No, 21-7-685
-4-
'l'lre tlesign and construction criteria presented below shoulcl bc obscrvccl for a sprcad footing
foundation system.
1) Footings placed on the undisturbed natural sandy gravel with cobble soil or
uurupacl,ed slruclural fill should he clesignecl lrrr an allowahle hearing pressrlre of
2,500 pst'. tsased on experience, we expect settlement of footings designed and
construoted as cliscussed in this section will be about lrto I 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 structurcs should also be designed to resist a
lateral earth pressure conesponding to an equivalent fluid unit weight of at least
55 pcf for the on-site sandy silt and clay soil as backfill or 45 pcf for the on-site or
imported sandy gravel soil as backfill excluding rock larger than 6 inches.
5) The topsoil, clay and silt soils and loose or distrrrbecl soils shoulcl be removed in
footing areas down to the dense sandy gravel with cobble soil. The exposed soils
in footing areas should then be moistened and compacted. Structural fill should
consist of relatively well-graded granular material, such as road base or on-site
granular materials, compacted to at least 98 percent of standard Proctor density at
near optimum moisture content. Structural fill placed in f'ooting areas should
extend laterally beyond the footing edge a distance equal to at least one-half the
depth of fill below the footing.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
'l'he natural granular soils are suitable to support lightly loadcd slab-on-grade construction. Slab
areas underlain by clay and silt soils will have a risk of movement due to compression potential
mainly if the bearing soils are wetted and structural fill at least 2 feet deep should be placed
below the slab in clay and silt areas to limit the movenlent risk. To retluce f.hc effects of some
differential movcment, floor slabs should be separated fi'om all bearing walls and colunurs with
expansion joints which allow unrestrained vertical movement. Floor slab control joints should
be used to reduce damage chre to shrinkage cracking. 'l'he requirements t-or joint spacing and
Kumar & Associates, lnc.Project No.21.7.685
-5-
slab reinforcement should be established by the designer based on experience and the intended
slab use.
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 fill can consist of the on-
site gravel soils devoid of oversized rock, vegetation, and topsoil.
IINDERDRAIN 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
seasonal runoff. Frozen ground during spring runoff can also create a perched condition. V/e
recommend below-grade construction, such as retaining walls and basement areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system. Shallow crawlspace
(about 4 feefhigh) should not need a perimeter underdrain with proper backfill placement and
surface grading.
If required, the drains should consist of drainpipe placed in the bottom of the wall backf,rll
surrounded above the invert level with free-draining granular material. The drain should be
placed at each level of excavation with the drain invert elevation at least 1 foot below lowest
adjacent f,rnish grade and sloped at a minimum Io/o to a suitable gravity outlet or drywell based in
the sandy gravel and cobble soil. Free-draining granular material used in the underdrain system
should contain less than 2o/o passingthe No. 200 sieve, less than 50%o passing the No. 4 sieve and
have a maximum size of 2 inches. The drain gravel backfill should be at least llz feet deep.
SURFACE DRAINAGE
Providing proper surface grading and drainage is very important to the 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.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/o 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 12 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
covered with filter fabric and capped with about 2 feet of the on-site finer grained
soils to reduce surface water infiltration.
Kumar & Associates, lnc,Project No. 21-7-685
-6-
4)Roof downspouts and drains should discharge well beyoncl thc limits of all
backfill,
Landscaping which requires regular heovy irrigation should be located at least
10 feet from foundationwalls. Considcration should bc given to use of xeriscapc
to reduce the potential for wetting of soils below the building caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this arca at this time. V/e 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 concemed 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 fìeld services during construction to review and
monitor the implementation of our recommendations, and to veriff that the recommendations
have been appropriately interpreted. Signifìcant 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, lnc.
Mark Gayeski, E.I.T
Reviewod by:
Steven L. Pawlak,
SLPlkac
s)
(r,5222I
Õ
Kumar & Associates, lnc.Project No.21-7.685
Cì!b"rJ.[lJ,\ê9Yw4I<of"eryo//B¿oT\IIl11À\\p)iI2v,!@2åt8qqh3+6qiIq\PrÞ\iS¡ÈÞ5a:aËl\)(¡-t-IlÐox--lrU)c)r-rrlI-ftrnrrt--{sr-o{!¡Ë¡'-l(oo^.¡ı 'è¿'ñ>Õ;-meñÏ;rü;iecoð9;ìlfly az@qo8 Iz<o^{êIIÈ-r| \-III*&c,6eè)?-/ "!ti\ı\wrt<W<,N)I!Io,cocnxc3o)eoaU,oo.0)+oØt-Oc)-.{Oz.O-lr1lXTt-On-loÐEonz.C)U)-1(o
E
9
I
\
I
BORING 1
EL. 5954'
BORING 2
EL. 5923'
0 0
72/12
WC=0.4
+4=63
-200=8
1 4/ 1?-
WC=6.4
DD=101
-2ô0=78
5 5
t-
t¡J
t¡JtL
IIFfLt!Õ
so/s 3s/12 F
L¡J
LdtL
I-F-fL
L¡Jo
10 1071/12
15 15
21 -7 -685 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
E
I
N
TOPSOIL; SILT AND SAND WITH ORGANICS, GRAVELLY AT BORING 1 , FIRM, SLIGHTLY MOIST,
LIGHT BROWN.
CLAY AND SILT (CL-ML); SANDY TO VERY SANDY, STIFF, SLIGHTLY MOIST, TAN' SLIGHTLY
POROUS.
GRAVEL (CV); SanoY TO VERY SANDY, COBBLES, SLIGHTLY SILTY TO SILTY, PROBABLE
SMALL BOULDERS, DENSE TO VERY DENSE, SLIGHTLY MOIST TO MOIST, MIXED LIGHT BROWN
& LIGHT GRAY.
!
i
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE.
DRTVE SAMPLE, 1 5/8-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST.
72/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 72 BLOWS OF A 14o_POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
I PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 31, 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 (PCi) (ASTU D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM 06915);
-2OO= PERCENTAGE PASSING NO. 200 SIEVE (ASTM 01140).
21 -7 -685 Kumar & Associates LEGTND AND NOTES Fig.3
¡
I
I
¡
I
SAMPLE OF: Sondy Silt ond Cloy
FROM:Boring2e.2.5'
W0 = 6.4 %, ÐD = 101 pcf
-2OO = 78 %
1
0
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
o\
JJ
L¡J
=tJ1
I
zotr
ô
Io
UIzo(J
-,|
I
I
I
l
I
I
I
I
I
I
l
-2
-5
-4
1 1.0 APPLIED t0 100
orly to th6
¡n
21 -7 -685 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
SIEVE ANALYSISHYDROMETER ANALYSIS
fIME READINGS
¿4 HRS 7 HRS
U.S. SIANDARD SÊRIÈS
!4A 4ti 4Aı 4iâ 41ô 4A 5"6' I
CLEAR SQUARÊ OPÊNINGS
t/aâ 3/¿" 1 1/r"
/
/
/
/
l
I
I
l
l
I
I
I
1
l
l I I
I
I
f
=
100
90
ao
70
60
50
,{o
30
20
fo
o
0
10
20
50
40
50
60
70
80
90
100
=b
DIAMETER OF IN MI
CLAY TO SILT COBBLES
GRAVEL 63 % SAND 29
LIQUID LIMIT
SAMPLE OF: Slightly Silty Sondy Grovel
PLASTICITY INDEX
SILT AND CLAY A %
FRoM: Boring 1 @ 2.5' & 5' (comblned)
lhsss losl ¡ssulls opply only to lh6
sqmples wh¡ch wer6 loslod. Th6
tssllng r€porl sholl nol be roproduc€d,
excepl ln full, wllhoul lhe wr¡tten
opprovql of Kumqr & Associolos, lnc.
Slsvo onolysls lssllng ls perlormed ln
occordoncê wlth ASTM 06913, ASTM D7928,
ASTM Cl36 ond/or ASÍM Dllilo.
GRAVELSAND
FINE MEDIUM lcornse FIN E COARSE
21 -7 -685 Kumar & Associates GRADATION TEST RESULTS Fig. 5
IG'Tlfu¡nar & Associates, lnc.'Geotechnical and Materials Engineersand Environmental ScientistsTABLE ISUMMARY OF LABORATORY TEST RESULTSNo.21-7-685SOIL TYPESlightly Siþ Sandy GravelSandy Silt and ClaylosfìUNCONFINEDCOMPRESSIVESTRENGTHATTERBERG LIMITSLIQUID LIMIT(ololt%tPLASÏICINDEXPERCENTPASSING NO,200 stEvE878SANDti/"|296RADATION(:/"')GRAVEL63101NATURALMOISTURECONTE}¡TNATURALDRYDENSITYlocfllo/ol0.46.4tftìDEPTHzrt&scombined2v,SAMPLE LOCATIONBORING12