HomeMy WebLinkAboutSubsoil StudyH
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT M46, ROARING FORK MESA
ASPEN GLEN SUBDIVISION
GARFIELD COUNTY, COLORADO
JOB NO. 114 1l3A
MAY 28,2014
PREPARED FOR:
ALISON FOTO
P.O. BOX 7911
ASPEN, COLORADO 8TóT2
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H EPWORTH.PAWLAK GãOTECHN ICAL
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY.....
PROPOSED CONSTRUCTI ON.
SITE CONDITIONS.
SUBSIDENCE POTENTIAL
FIELD EXPLORATION.
SUBSURFACE CONDITIONS..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOIINDATIONS
FOI.INDATION AND RETAINING WALLS.
FLOOR SLABS......
UNDERDRAIN SYSTEM
SITE GRADING
SURFACE DRAINAGE.
LIMITATIONS
REFERENCES
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FTGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - GRADATION TEST RESULTS
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1
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2-
.......... - 3 -
.-3-
-4-
4
4
5
6
7
7
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at
LotM46, Roaríng Fork Mesa, Aspen Glen Subdivision, Garfield Count¡ 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 you dated April 1A,2074.
Chen-Northem, Inc. previously conducted a preliminary geoteclrnical engineering study
for the development (Chen-Northem, 1991) and another geotechnical engineering study
for preliminary plat design (Chen-Northern, 1993).
A field exploration program consisting of exploratory borings was conducted to obtain
infomation 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 ofthe field
exploration and laboratory testing were analyzed to develop recofiÌrnendations for
foundation types, depths and allowable pressures for the proposed building foundation.
This report summarizes the data obtained during this study and presents ow conclusions,
design recommendations and other geotechnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTTON
The proposed residence will be one and two story wood fiame construction above a
walkout basement with an attached garage. Basement and garage floors will be slab-on-
grade. Grading for the structure is assumed to be relatively minor with cut depths
befween about 3 to 9 feet. We assume relatively light foundation loadings, typical ofthe
proposed type of construction.
If building loadings, Iocation or grading plans change significantly from those described
above, we should be notified to re-evaluate the recommendations contained in this report
Job No. 114 I l3A cåEtecn
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SITE CONDITIONS
The lot is vaca¡t and vegetated with sparse grass and weeds. The ground surface is
relatively flat with a gentle slope down to the east and appears to have been graded during
subdivision development with fill placed across the entire lot. The slope becomes very
steep down to the east at the rear of the buildíng envelope. The Roaring Fork River is
located east and several topographic benches below the site. The Eagle Valley Evaporite
Formation is exposed on a hillside just uphill and west of the site along County Road 109.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies most of the lower
Roaring Fork River valley and the Aspen Glen Subdivision. These rocks are a sequ€nce
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 studíes in the alea, several sinkholes were
observed scattered throughout the Aspen Glen development (Chen-Northem, Inc., l99t).
These sinkholes appe¿u similar to others associated with the Eagle Valley Evaporite in
areas of the Roaring Fork Valley.
Sinkholes were not observed in the immediate area ofthe 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 M46 throughout the seruice iife 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.
Job No. I 14 I l3A c,&Etech
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FIELD EXPLORATION
The field exploration for the project was conducted on April 14, 2014. Two exploratory
borings were drilled at the locations shown on Figure I to evaluate the subsurface
conditions. The borings were advânced with 4 inch diameter continuous flight augers
powered by a truck-mounted CME-458 drill rig. The borings were logged by a
repres entative o f Hepworth-P awlak Geotechnical, Inc.
Samples ofthe subsoils were taken with a l3Á nchl.D. spoon sampler. The sampler was
driven into the subsoils at various deptbs 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 ofthe subsoils. Depths at which the samples were taken and the penetration
resistance values are shown on the Logs of Exploratory Borings, Fþre 2. The samples
were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs ofthe subsurface conditions encountered at the site are shown on Figure 2.
The subsoils consist of about I ro lVz feet of man-placed fiIl overlyíng slightly silty sandy
gravel with cobbles and small boulders. Drilling in the dense 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 content and gradation analyses. Results of gradation analyses performed on
small diameter drive samples (minus 1% inch fraction) of the coarse granular subsoils are
shown on Figure 4. The laboratory testing is surnmarized tnTable 1.
Job No. I 14 I l3A cåEteclr
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No free water was encountered in the borings at the time of drilling or when checked 43
days later and the subsoils were slightly moist.
FOUNDATION BEARING CONDITIONS
The natural gnnular soils are adequate for support of spread footing foundations and
slab-on-grade floors. The man-placed fill encountered at the site was relatively shallow
and should be removed from beneath footings and slabs.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, r¡/e recoiltmend 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
fo oting foundation system.
1) Footings placed on the undisturbed natural glanular soils 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 shouid have a minirnumwidthof 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
tlpically used in this area.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomalies such as by assumíng an unsupported length of at least 10
Job No. I 14 1134 c,äBtecrr
5
s)
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.
All existing fill, topsoil 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.
A representative of the geotechnical engineer should obsele all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOI-INDATION 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 45 pcf
for backfill consisting of the on-site granuiar 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 ofthe on-site granular soils.
All foundation and retaining sttuctures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and
equipment. The pressures recontmended 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.
Backfïll should be placed in uniform lifts and compacted to at least 90%o of the maximum
standard Proctor density at a moisture content near optimurn. Backfill in pavernent and
6)
Job No. 114 I l3A c,äEtecn
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walkway ateas should be compacted to at Teast 95To of the maxirnum 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 footíng. 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
backfrll against the sides of the footings can be calculated using an equivalent fluid unit
weight of 400 pcf The coefñcient of friction and passive pressure values recommended
above assume ultimate soil strength. Suitable factors of safety should be included in thc
design to lirnit the strain which will occur at the ultimate strength, particularly in thc case
ofpassive 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 man-placed fill and topsoil, are suitable to support
lightly loaded slab-on-grade construction. To reduce the effects of some difflerential
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 nch aggregate with at least 50% retained on the No. 4 sieve and less than 2%
passing the No. 200 sieve.
Job No. I 14 I l3A eåEtecrr
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All fill materìals for support of floor slabs should be compacted to at least 95%o of
maximum standard Proctor density at a rnoisture content near optimum. Required fill can
consist ofthe on-site glanular soils devoid ofvegetation, topsoil and oversized rock.
LINDERDRAIN 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 runoffl Frozen ground during spring runoffcan also 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 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 I foot below lowest adjacent finish
grade and sloped at a minimum l%, to a suitable gravity outlet. Free-draining granular
material used in the underdrain systern 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 l% feet deep.
SITË GRADING
The risk of construction-induced slope instability at the site appears low provided the
building is located above the steep slope as planned and cut and fill depths are limited.
We assume the cut depths for the basement level will not exceed one level, about I to l0
feet. Fills should be limited at the downhill side of the residence where the slope
steepens. Embankment fills should be cornpacted to at least 95% of the maximum
standard Proctor density near optimum moisture content. Prior to fill placement, the
subgrade should be carefuily prepared by rernoving all vegetation and topsoil and
Job No. 114 I l3A cåEtecn
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compacting to at least 95Yo of the maximum standard Proctor density. The fill should be
benched into the portions of the hillside exceeding 20io grade.
Permanent unretained cut and fill slopes should be graded at2horizontal to 1 vertical or
flatter and protected against ercsion by revegetation or other means. The risk of slope
instability will be increased if seepage is encountered in cuts and flatter slopes may be
neeessary. If seepage is encountered in permanent cuts, an investigation should be
conducted to determine if the seepage will adversely afflect the cut stability. This offrce
should review site grading plans for the project prior to construction.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and
maintained at all times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be
avo ided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at least 954/o of the maxirnum 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 l2 inches in the first 10 feet in unpaved
areas and a rninimum 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.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area aI this time. We make no wan'anty either
Job No. I 14 I 134 c,äFtecrr
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express or impliecl. The conclusions and recommendations submitted in this repoÍ are
based upon the data obtainecl fì'om the exploratory borings drilled at the locations
indicated on Figure 1, the proposecl type of construction and our ex¡rerience in the area.
Our services do not include detennining tlie plesence, prevention or possibility of rnolti or
other biological contaminants (MOBC) cleveloping in the future. If the client is
concerned about MOBC, then a professional in this special freld of practice should be
consultecl. Our findings inclucle 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 perfirnnecl. If conclitioris
encountered during construction appear diff-erent li'om those described in this report, we
should be notified so that re-evaluation of the recommenclations mav be made.
This report has been prepared fol the exclusive use by our client for design purposes. We
are not responsible for technical interpretations by others of our infonnation. As the
project evolves, rve should provide continuecl consultation and field services during
constructicln to revier,v and lnonitor the irnplernentation olour recommendations, and to
veri$, that the recotnmendations have been appropriately interpretecl. Significant clesign
changes may require adclitional analysis or modifications to the recornmendations
¡rresented herein. We recommencl on-site observation of excavations ancl founclation
bearing strata and testing of structural fìll by a representative of the geotechnical
engineer.
Respectflilly Submittecl,
HEPWORTH . PAWLAK GEOTECHNICAL, INC
uis E. Eller
Reviewecl by:
DanielE. Harclin, P.E.
LEE/ljg
oô çla/,
Job No. ll+ I l3A
il,'iL
Geótech
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REFERENCES
chen-Northern, Inc., r99r, Preliminary Geotechnical Engineering sturly, proposed
Aspen Glen Development, Garfield county, colorado, prepared for.Aspen Glen
Company, dated December 20, 1991, Job No. 4 Llz92.
Chen-Northern, Inc., 1993, Geotechnical Engineering Studyfor Preliminary Plat Desígn,
Aspen Glen Development, Ga(ìeld counþ, colorado, prepared for Aspen Gren
Company, datecl May 28, I 993, Job No. 4 112 92.
Job No. tt4 1l3A c$teclr
APPFOXIMATE SCALE
1" : 20',LOT M8
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BORING 2
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LOT M46
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BORING 1
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LOT M45 I LOT M47I
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BENCH MARK: GROUNDAT PROPERTY
CORNEF; ELEV. : 100.0', ASSUMED.
GOLDEN STONE
3A114 11 LOCATION OF EXPLORATORY BORINGS Figure 1
BORING 1
ELEV.:97,9'
BORING 2
ELEV.:93.9'
100 100
9s 55112
WC=2,2
+4:57
-200:8
95
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WC:2.0
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-200:10
90
9A112
85 85
Note: Explanation of symbols ís shown on Figure 3.
3A114 11 LOGS OF EXPLORATORY BORINGS Figure 2
LEGEND:
m
FILL; sandy silt and clay with cobbles and small boulders, medium dense, moist, rnixed brown and red-brown,
GRAVËI, COBBLES AND BOULDERS (GM-GP); sandy, slightly silty, dense, slightly moist, light brown,
subrounded rocks.
I Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-l586.
55/12
Drive sample blow count; indicates that 55 blows of a 140 pound hammer falfing 30 inches were
required to drive the SPT sampler 12 inches.
Practical refusal to auger drilling.
î
NOTES:
1. Exploratory borings were drilled on Aprit 14,2014 with 4-inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
províded.
3. Elevations of exploratory borings were measured by instrument level and refer to the Bench Mark shown on Figure 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 approxímate boundaries between
material types and transitions may be graduaf.
ô. No free water was encountered in the borings at the time of drilling or when checked 43 days later. Fluctuation in
water level may occur with tÍme.
7. Laboratory Testing Results:
WC : Water Content (7")
+4 : Percent retained on the No.4 sieve
-20A : Percent passing No. 200 sieve
LEGEND AND NOTES114113A Figure 3
TIME BEADINGS U.S, STANDARD SERIES? ¡"rR
15 MtN_60MlNr9MrN.4 MlN. 1 MtN. #2A0 *10A #50 #30 #16 #8 #4
CLEAR SQUARE OPENINGS
318' gl4u 1 U2" 3" 5"6'
152
127
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10
20
30
40
50
60
70
80
90
100
r00
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BO
70
5rÌ
40
30
20
t0
0
.m2.001 .005 .0@ .019 .037 .O7A ,tso .3m .600 ¡,,18 2,36 4.7s 9.5 lz.s tg.o 37.5 76.2
DIAMEÏER OF PARÏCLES IN MILLIMETERS
+.-+--_
+-t-+-
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------.,i-I
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7 HB TIME READINGS
15 MlN. 60MtN1gMtN,4 MtN. 1 MlN. #200 #10û
U,S. STANDARO SERIES
#50 #30 #16 #8
CÔBBLES
CLEAR SQUABE OPENINGS
3/8' A/4' 1 1/2" 3u 5'6"
CLAY TO SILT
GRAVEL 57 7o
LIQUID LIMIT O/O
SAMPLË OF: Slightly Sitty Sandy Gravet
SAND 35 % S¡LTANDCLAY 8 %
PLASTICITY INDEX %
FROM: Boring 1 at2åFeet
90
80
CI704ØU)60Í
Fsoñ
()
40fi
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30
24
45
0
10
ô20
LU230
t-
H40
F250
UJOE60
LIJrL
#4 o
100
70
80
s0
100
20
10
0
.001 .AoZ .00s,009 .019 .097 .074 .1s0 .300 .600 1.18 2.96 4.25
DIAMETER OF PABTICLES IN MILLIMETERS
9.q2.519.0 37.5 76.2 1t52 2o3
FINE
t I l.
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-¡+l._+-;--
1-
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CL,AY 1O SILI
GRAVEL 56 %
LIQUID LIMIT
SAMPLE OF
o//o
SAND 34 0/"
PLASTICITY INDEX
coBELrs
SILTAND CLAY 10 O/O
o/o
2at?and 5 Feet CombinedGravelFROM:
3A114 11 GRADAT¡ON TEST RESULTS Figure 4