HomeMy WebLinkAboutSubsoils Report for Foundation Designrcn l(umar & Asslslats, lnc. 6
Geotechnical and Materials Engineers
and Envirsnmental $cieniists
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970)945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
Office tocations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 116,IRONBRrDGE
263 SILVER MOUNTAIN DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 23-7-329
JULY 19,2023
PREPARED FOR:
SCOTT FENSKE
P.O. BOX 1323
CARBONDALE' COLORADO 81623
Fenske970@smail.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
FOUNDATION AND RETAINING WALLS
FLOOR SLABS......
I-]NDERDRAIN SYSTEM .......
SITE GRADING
SURFACE DRAINAGE....
LIMITATIONS
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - GRADATION TEST RESULTS
TABLE 1 -SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No.23-7-329
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot ll6,Ironbridge, 263 Silver Mountain 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 Scott Fenske dated June2,2023. Hepworth-Pawlak Geotechnical (now
Kumar & Associates) previously conducted a preliminary subsoil study for Lots 108 to 118 and
presented the findings in a report dated December 6,2002, 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 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 reconrmendations and other geotechnical engineering considerations based
on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Building plans for the residence had not been developed at the time of our study. In general, the
proposed building witl be in the upper part of the lot and be a 1 or 2 story structure, possibly
above a walkout lower level. Ground floor could be slab-on-grade or structural above
crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between
about 2 to 6 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 and the ground surface appeared mostly
natural. The ground surface slopes moderately steep down to the southeast with about 5 feet of
elevation difference across the upper part of the building envelope. A fence separated the upper
part of the building envelope from the lower portion. Vegetation consisted of sagebrush, grass
and weeds in the building area.
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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 sinkholes to develop and can produce areas of
localized subsidence. During previous studies for Ironbridge and other developments, broad
subsidence areas and sinkholes have been observed including sinkholes in the central to northern
parts of Ironbridge. These sinkholes appeared similar to others associated with the Eagle Valley
Evaporite in areas of the lower Roaring Fork River valley.
Sinkholes were not observed in the immediate area of the subject lot or in the southern part of
Ironbridge. 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 116 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 June 30, 2023. 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 wers logged by a representative of Kumar &
Associates.
Samples of the subsoils were taken with a I3Ainchl.D. spoon sampler. The sampler was 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 consist of about I foot of topsoil overlying very dense, slightly silty sandy gravel and
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cobbles with boulders. Drilling in the coarse granular soils with auger equipment was diffrcult
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
content and gradation analyses. Results of gradation analyses performed on small diameter drive
samples (minus I%-inch fraction) of the coarse granular subsoils are shown on Figure 3.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The natural gravel and cobble soils encountered below the topsoil are suitable for support of
spread footing foundations with moderate bearing capacity and relatively low settlement
potential. All topsoil and any clay soils should be removed from beneath the proposed building
area. At typical foundation depths for the general proposed type of construction, we expect the
excavation will be into the gravel and cobble soils.
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.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils should be designed for an
allowable bearing pressure of 3,000 psf. Based on experience, we expect settlement of
footings designed and constru"t"Tfu"ussed in this section will be about 1 inch or less.
2) The footings should have a minimum width of 16 inches for continuous walls and2 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 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.
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5) The topsoil, clay and any 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.
6) A representative of the geotechnical engineer should 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 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 soils. Backfill should not contain organics
or rock larger than about 5 inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffrc, 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 90o/o 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 95o/o 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 0.50. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 pcf. The
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
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the sides of the footings to resist lateral loads should be a granular material compacted to at least
95%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 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 free-
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
material should consist of minus 2-inch aggregate with at least 50%o retained on the No. 4 sieve
and less than2o/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 fill can consist of the
on-site gravel soils devoid of vegetation, topsoil and oversized rock.
I.]NDERDRAIN 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 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 system.
The drains should consist of PVC 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 lzVo to a suitable gravity outlet. Free-draining granular material used in the underdrain
system should contain less than 2Yo 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 l%feet
deep and covered with filter fabric such a Mirafi 140N.
SITE GRADING
The risk of construction-induced slope instability atthe site appears low provided cut and filI
depths are limited. We assume the cut depths for the basement level will not exceed about
10 feet. Fills should be limited to about 8 feet deep. Embankment fills should be compacted to
at least 95Yo of the maximum standard Proctor density near optimum moisture content. Prior to
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fil1placement, the subgrade should be carefully prepared by removing all vegetation and topsoil
and compacting to at least 95o/o of the maximum standard Proctor density. The fill should be
benched into the portions of the hillside exceeding 20Vo grade. Permanent unretained cut and fill
slopes should be graded at2horizontal to 1 vertical or flatter and protected against erosion by
revegetation or other means.
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
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 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 covered with filter fabric and capped
with about 2 feetof the on-site finer grained soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all backfill.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area atthe 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 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 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
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should provi*l* e*ntinued *onsultation and field serviees during canstructio* t* review and
monitor the irnptrementati*n of our recommeildations, and t* verifu that the rce*rnrtl*ndations
have been appropriately interpreted. Signiticant design ehanges may require additional analysis
or m*difieations to the re*ommendations pr*s*nted herein. Ws recommend on*sitc *bseryation
of exeavaticns and fbundatian bearilrg strata and testing of structural fill by a r*pres*ntative of
the geoteehnicai engi*eer.
Respectfi.illy Submitte*l,
Kumar &
Robert L.
Reviewed by:
$
Daniel H. F{ardin, P.E.
RLD,kac
lJ3v
affie
K[mar & As*ociates* l:tc. e Prejes't F,le. 33"?"*?$
IMPROVEMENT S(IRVEY PLAT
LOT 116
IRONBRIDGE PLANNED UNIT DEVELOPMENT, PI{ASE 1
GAEtr'IELD COUNTY, COLOITADO
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APPROXIMATE SCALE-FEET
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23-7 -329 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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BORING 1
EL. 100'
BORING 2
EL. 1 03'
0 0
86/ 12
WC=1 .1
+4=57
-2OO=12
50/4.s
WC=1.4
+4=54
-2OO=1 1
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LEGEND
TOPSOIL. ORGANIC SANDY SILT WITH SCATTERED COBBLES, LOOSE, SLIGHTLY MOIST, TAN
GRAVEL
VERY DE
(cM);
NSE,
SANDY, SLIGHTLY SILTY TO SILTY, WITH COBBLES AND PROBABLE BOULDERS,
SLIGHTLY MOIST, TAN.
I DRTVE SAMPLE, 1 3/9-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST
6^ IA^ DRIVE SAMPLE BLOW COUNT' INDICATES THAT 86 BLOWS OF A 14o-POUND HAMMERoo/ tz FALLTNG so TNcHES WERE REQUIRED To DRtvE THE SAMPLER t2 tNcHES.
f nnacrrcaL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JUNE 30,2023 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 HAND LEVEL. THE
GROUND SURFACE AT BORING 1 WAS ASSUMED TO BE 1OO' FEET.
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 DRITLING
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (NSTV OSSIS);
-2OO = PERCENTAGE PASSING No. 200 SIEVE (ASTM Dl 140).
23-7 -329 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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90
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70
80
50
40
30
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HYDROMETER ANALYSIS SIEVE ANALYSIS
TIUE READINOS
2/a HRS 7 HRS
U.S. STANDARO SERIES
rso r& 130 lra llo aaalnn /
CLEAR SOUARE OPENINOS
t tt t/^. I I t..
.......:.:::::::.:::::t:.:::::::::::::::::::::
......................t.......................
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SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
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50
80
76
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DIAMETER OF IN MI
2'O
LLIMETERS
CLAY TO SILT COBBLES
GRAVEL 57 % SAND 31 %
LIQUID LIMIT - PLASTICITY INDEX
SAMPLE 0F: Silly Sondy Grovel
SILT AND CI.AY 12 %
FROM:Boring1O2.5'
2
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80
70
60
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DIAMETER OF IN MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 54 % SAND
LIQUID LIMIT
SAMPLE OF: Sllghtly Silty Sondy Grovel
35%
PLASTICITY INDEX
SILT AND CLAY 11 %
FROM: Borlng 202.5'
Thclc lcrl rr8ulls opply only lo lh6
3qmpllE whlch w6rr bslsd. Th.
lcsllng ropori shqll nol bc r.produccd,
cxccpl ln full. wlthoul lht vrlthn
opprcYol of Kumqr & AEsoclolct, lnc.
Slcvr qnolyslr l.sllng ls parfomrd ln
occordonc. wlth ASTM 06913, ASTM D7928,
ASTM C136 ond/or ASTM Dll40.
SIEVE ANALYSISHYDROMETER ANALYSIS
U.S- ST NDARD SERIES
2/a HRS 7 HRSG VtN r3 ltN t trtN It
TII'E READINGS
60vt{ 19utN 4ulN
GRAVELSAND
COARSE FINE COARSEFINEMEDIUM
Fig. 3GRADATION TEST RESULTS23-7 -329 Kumar & Associates
lGrtH,ffil,ffiffiruHi'Yrd**
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Slightly Silty Sandy Gravel1135541.4
Silty Sandy Gravel
SOIL TYPE
1231571.12%
(%)
SAND
(%)
GRAVEL
DEPTH
I
BORING
ATTERBERG LIMITS
LIQUID LIMIT
UNCONFINED
COMPRESSIVE
STRENGTH
PERCENT
PASSING NO.
2()() SIEVE
NATURAL
DRY
DENSTTY
NATURAL
MOISTURE
CONTENT
PLASTIC
INDEX
2%2
No. 23-7-329