HomeMy WebLinkAboutSubsoil Studyrcn
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llmqtåffib,lnc.'
Gætcdrnlc¡lanrl llhbdåls Emlnosr¡ 5020 County Road 154
aú Envlnnm¡nhlsclonllab Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-84s4
email: kaglenwood@kumarusa,com
An Employo¡ Owncd compony www.kuma¡usa'com
Office l¿cations: Denver (HQ), Pæket Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVET}
GARFIELD COUNTY
6OMMUNITY DEVELOPMENT
ST]BSOIL STUDY
FOR Í'OT]NDATION DESIGN
PROPOSED RESIDENCE
LOTL}4,ASPEN GLEITI
349 MIDLAI\D LOOP
GARFIELD COUNTY, COLORADO
PROJECT NO. 19-7-651
NOVEMBER 4, 2019
PREPARED F'OR:
R. TODD S}YAI\K
16223 DEER MOT]NTAIN DRTVE
LITTLETON, COLORADO 80127
rtswank@skvbeam,pom
TABLE OT'CONTEI\ITS
PURPOSE AND SCOPE OF STUDY 1
FOTJNDATIONS 4-
FOIJNDATION AND RETAINING WALLS 5-
FLOOR SLABS 6-
LIMITATIONS.......... .,.-7 -
FIGURE I . LOCATION OF ÐGLORATORY BORINGS
FIGURE 2. LOGS OF EXPLORATORY BORINGS
FIGURE 3 . LEGEND AND NOTES
FIGURE 4 - SIWELL.CONSOLIDATION TEST RESULTS
FTGURE 5 - GRADATION TEST RESULTS
TABLE I. SUMMARY OF LABORATORY TEST RESULTS
Kumæ & Assoclates, lnc, o Project No.19.7-851
PURPOSE A}ID SCOPE OT'STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
L24, Aspen Glen, 349 Midland Loop, Garfield County, Colorado. The project site is shown on
Figure 1. The purpose of the study was to develop recomme,lrdatíons for the foundation design.
The study was conducted in accordance with our proposal for geotechnioal engineering services
to Todd Swank dated Ootober9,2019.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsr¡rface conditions. Samples of the subsoils obt¿ined during the field
exploration were tested in the laboratory to determine thgir classification, compressibility or
swell and other engineering characteristics. The results of the field exploration and laboratory
testing were analyzed to develop recommendations for foundation t¡pes, depths and allowable
pressures for the proposed building foundation. This report summarizes the data obtained during
this study and prese,lrts our conclusions, design recomme,ndations and other geotechnical
engineering considerations based on the proposed constnrction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
Design plans were not available at the time of this study and we understand our findings will be
oonsidered in the purchase of ttre lot. We assume the proposed residence will be a one to two
story süucture. Ground floor could be slab-on-grade or süruotr¡ral above crawlspace. Grading
for the sürrcture is assumed to be relatively minor with cut dçths between about 2 to 12 feet.
Vle assume relatively light foundation loadings, tlpical of the proposed t¡pe of consFuction.
When building location, grading and loading information have been developed, we should be
notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The site was vaoant with 2-inches of snow covering the ground at the time of our field
exploration. The ground was vegetated with gass. The site is located in the valley bottom with
Kumar & Assoclates, lnc. @ Project No.19.7-651
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the terrain gently sloping generally down to the north. There is an artifioial pond on the
southeast boundary of the lot. To our knowledge, the pond is constnrcted with an impervious
liner to prevørt leakage.
SUBSIDENCE POTET{TIAL
Bedrook of the.Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision.
These rooks are a sequ€nce of g¡psiferous shale, fine-grained sandstone and siltstone with some
massive beds of g¡psum and limestone. There is a possibilþ that massive g)fpsum deposits
associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the
gJæsum under cert¿in conditions can cause.sinlúroles to develop and can produce areas of
localized subsidence. During previous work in the area" several sinllholes were observed
scattered throughout Aspen Glen, mainly east of the Roaring Fork River. A small sinkhole was
mapped around 60 to 80 feet east of Lot L24 (inthe area of the existing pond). These sinkholes
appear similar to others associated with the Eagle Valley Evaporite in areas of the middle to
lower Roaring Fork River valley.
Sinktroles weie not observed on the 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 oannot
be said for certain that sinkholes will not develop. The risk qf future ground subsidence on
LotL24 throughout the sen¡ice 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 dosired, we should be
contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on October 31, 2019, Two exploratory
borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions.
The borings were advanced with 4 inch diameter continuous flight augers powered by a üuck-
mounted CME-458 driit rig. The borings \r,ere logged by a representative of Kumar &
Associates,Inc.
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Samples of the subsoils were taken vuith 1% inch and 2inc}r I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 140 pound hammer fatling 30
inches. This test is similar to the standard peneüation test described by ASTM Method D-.1586.
Thepenefration resistanoe values are an indioation of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penefration resistance values are
shoivn on tÈe Logs of Exploratory Borings, Figure 2, lltø samples were returned to or¡r
laboratory for review by the project engineer and testing.
SUBSURT'ACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about 6 inches of topsoil overlying 3 to 4/z feet of stiffl, sandy, silty clay
rurderlain by dense, slightly silty to silty, sandy gravel with cobbles and boulders to the depths
explored of 6 to 16 feet. Drilling in the coarse granular soils with auge'r equipment was difficult
due to.the cobbles and boulders and drilling refirsal was encor¡ntered in the deposit.
Laboratory testing performed on samples obtained from the borings inoluded natural moistr¡re
çontent and density and gradation analyses. Results of swell-consolidation testing performed on
a relatively r¡ndisturbed drive sample of the clay soil, presented on Figure 4, indicate low to
moderate compressibility under coñditions of loading and wetting. Results of gradation analyses
performed on a small diameter drive samples (minus lYz-inch fraction) of the coaf,se granular
subsoils are shown on Figure 5. Tt¡e laboratory testing is summarized in Tablc 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to.moist.
F'OUNDATION BEARING CONDITIONS
The natt¡ral silty clay encountered in the upper 3Yzto 5 feet of the borings possess low bearing
capacity and low to'moderate compressibility when loaded and wetted. The underlying gavel
soils possess moderate bearing capacity and t1ryically low settlement potential. At assumed
excavation depth we expect the subgrade will expose either materials. Spread footings should be
foasible for fourdation support of the residence. To reduce the risk of differential movement due
to the variable bearing conditions, the footings could be extended down to the natural gravel
soils.
Kumar & Acsociates, lnc. @ Project No.'19'7.651
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface.conditions encor¡ntered in the exploratory borings and the nature of
the proposed constnrotion, we recommend the building be founded with spread footings bearing
on the natural soils.
The design and consûuction criteria presented below should be observed for a spread footing
for¡ndation system.
1) Footings placed on the undistubed natural soils should be designed for an
allowable bearing pressure of 2,000 psf. Footings placed entirely on the natt¡¡al
dense granular soils can be designed for an allowable bearing pressure of 3,500
psf. Based on experience, we expeot settlerhent of footings constucted on the
natural soils will be about 1 inoh or less and could be differential between clay
and gravel bearing soils.
2) Ttre footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings be,neath 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
afea.
4) Çontinuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 10 feet.
Foundation walls acting as rctaining sffr¡ctures should also bo designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section ofthis report.
5) The topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils. The exposed soils in
footing area should then be moistened and compacted.
6) A rqresentative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
Kumal & Assoclates,lnc. o Project No.19.7.651
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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 55 pcf for backfill consisting
of the on-site soils. Cantilevered retaining,struottues which are separate from the residenoe and
oan be expected to deflect sufficiently to mobilize the full active earth pressure condition should
be desigrred for a lateral earth pressure computed on the basis of an equivalent fluid unit weight
of at least 45 pcf for backfitl consisting of the on-site soils. Backfill should not contain organics,
debris or rook larger than about 6 inches.
All for¡ndation and retaining sfructures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, haffic, consfuction 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 upyard sloping backfill surface will
increasg the lateral pressrre imposed on a for¡ndation wall or retaining süuchre. An underdrain
should be provided to pfevent hydrostatic pressure buildup behind walls.
Backfill phould be placed in r¡niform lifts and compacted to at least 90% of the mâximum
standard Proctor donsity at a moisturo content noar optimum. Backfill placed in pavørient and
walkway areas should be compacted to at least 95% of the mærimum standard Proctor density.
Care should bè 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 oould result in dishess to
facilities consüucted on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistancÞ 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 friotion of 0.30 for olay soils and 0.50 for gravel soils. Passive pressure
of compacted backfill against the sides of the footings can be calculated using an equivalent fluid
unit weight of 450 pcf for gravel backfill. The coefficient of friction and passive pressure values
recommended above assume ultimate soil sfrength. Suitable factors of safety should be inoluded
Kumar & Assoclates, lnc, @ Project No, 19.7.651
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in the design to limit the stain which will ooor¡r at the ultimate stength, particularly in the case
of passive resistance. Fill placed against the sides'of the footings to resist lateral loads should be
a granular material compacted to at least 95% of the maximum standard Prootor density at a
moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exolusive of topsoil, are suitable to support lightly loaded slab-on-grade
consügotion. To reduce ttre effects of some differential movement, floor slabs should be
separated from all bearing Walls and columns with expansion joints which allow unreskained
vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. The requirenrents for joint spacing and slab reinforcement should be established by the
designer basedpn 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% retained on the No. 4 sieve
and less fhsrL?% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% ofmaximum
standard Proctor density at a moistue content near optimum. Required fill oan oonsist of the on-
site soils devoid of vegetation, top$oil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during orn exploration, it has been our expøience in
the area and where there are clay soils that local perohed groundwater can develop during times
of heavy precþitation or seasonal runoff. Frozen ground during spring runoffcan create a
perched condition. We recommend belowgrade construction, such as retaining walls,
crawlspace and basement af,eas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
The drains should consist of drainpipe plaoed in the bottom of the wall baokfill sr¡rrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
exoavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum lo/o to
a suitable gravity outlet or dr¡nvell based in the underlying,granular soils. Free-draining granulæ
Kumar & Aosoclate¡, lnc. o ProJec{ No,19.7.651
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material used in the underdrain system should contain less than 2% passing the No. 200 sieve,
less than 50% pæsing the No. 4 sieve and have a ma¡rimum size of 2 inches. The drain gravel
backfill should be at least l% feet deep.
SURFACE DRAINAGE
T?re following drainage precautions should be observed during consfrt¡ction and maintained at all
times after the residence has been completed:
1) Inundation of the foundation exoavations and underslab areas should be avoided
during consüuction.
2) Exterior backfill should be adjusted to ne,ar optimum moisture and compacted to
at least 95% of themaximum standard Proctor density in pavement and slab areas
and to at least 90% of the ma¡rimum standard Proctor density in landscape af,eaÉ¡.
3) The ground surface surrounding the exteriorof the building should be sloped to
drain away from the foundation in all directions. rWe recommend aminimum
slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inohes in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and oapped with about 2 feet of the on-site finer graded
. soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least 5
feet from foundation walls.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this arca atthis time. rWe make no waranty either express or implied.
The oonclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory borings drilled at the looations indicated on Figute 1, the proposed t¡pe of
consbr¡ction and our experience in.the area" Our sen¡ioes do not include determining the
presenoe, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the fr¡tr¡re. If thq çlient is conoemed about MOBC, then a professional in this special field of
Kumar &Associates, lnc. @ Project No. 19.7,651
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practice should be consulted. Our findings inolude interpolation and exfrapolation of the
subsurface,conditions identified at the exploratory borings and variations in the subsurfaoe
conditions may not become evident until excavation is performed. If conditions encountered
during constuction appeàr different from those desoribed in this report, we should be notified so
that re-evaluation of the recommendations maybe made.
This report has been prepared for the exolusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the projeot evolves, we
should provide continued consultation and field sert/ices during consüuction to review and
monitor the implementation of our rec¡mmendations, and to veriff that the recommendations
.
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recornmendations presented herein. 'We recommend on-site observation
of exc¿vations and foundation bearing shat¿ and testing of sfructural fill by a represelrtative of
the geotechnical engineer
Respectñrlly Submitted,
Kumar & Associates, fnc,
Shane J. Robat, P.E.
Reviewed by:
Steven L.
SJR/kac
Kumar & Associates, lnc. @ Project No. 19.7.051
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1 9-7-651 Kumar & Associates LOCATION OF EXPLORATORY BORINGS 1Fig.
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1 9-7-651 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2
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T0PSOIL; SANÐY, SILTY, CLAY, FIRM, SLIGHTLY MOIST, DARK BROWNI, ORGANICS.
CLAY (CL); SANDY, SILTY, STIFF, SLIGHTLY MOIST, BROWN.
ffi ONAU.I- (GM); SANDY, SLIGHTLY SILTY TO SILTY, COBBLES, POSSIBLE BOULDERS, DENSE,
[F]sLroHrlv Morsr ro MotsT, MtxED BRoWN.
DRIVE SAMPLE, 2-INCH I.D. CALIFCIRNIA LINER SAMPLE
I DR|VE SAMPLE, 1 5/8-INCH LD. SPLIT SPOON STANDARD PENETRATION TEST.
E^/' DRIVE SAMPLE BLOW COUNT. INDICATES THAT 50 BLOWS OF A 140-P0UND HAMMER""/ - FALLTNG 50 tNcHES wERE REQUIRED To DRtvE THE SAMPLER 4 tNcHEs.
I PRACTICAL AUGER REFUSAL.
NOTES
1, THE EXPLORATORY BORINGS WERE DRILLED ON OCTOBER 51,2019 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 AND REFER
TO BORING 1 AS 1OO',
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE MEÏHOD 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 Ð2216)t
DD = DRY DENSITY (pcf) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4. SIEVE (ASTM D6913);
-200= PERCENTAGE PASSING N0. 200 SIEVE (ÄSTM 01140).
1 9-7-651 Kumar & Associates LEGEND AND NOTES Fis. 3
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SWELL-CONSOLIDATION TEST RESULTS Fig. 41 9-7-651 Kumar & Associates
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IIYDROMETER ANALYSIS SIEVE ANALYSIS
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CLAY TO SILT COBBLES
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LIQUID LIMIT PLASTICITY INDEX
SAMPLE OFr Slightly Sllty Sondy Grovel
SILT AND CIAY 12 %
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GRADATION TEST RESULTS Fis, 5Kumar & Associates1 9-7-651
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