HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.19.18H.PVKUMAR 5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Far (970) 945-8454
Email: hpkglenwood@kumarusa.com
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
Office Loætions: Parker, Glenwood Springs, and Silverthorne, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSEI} RESIDENCE
LOT }7,PI¡[YON MESA
168 CLIFTROSE WAY
GARFTELD COUNTY, COLORADO
PROJECT NO. 18-7-707
DECEMBER 19,2018
PREPARED FOR:
MIGUEL ROJO
P. O. BOX 743
GLEI\I-IVOOD SPRINGS, COLORADO 81602
Mieeieroi oSO0@smail.com
RECEIVED
AU6 fl 2 Zotg
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ................- I -
PROPOSED CONSTRUCTION I
SITE CONDITIONS 2-
SUBSIDENCE POTENTIAL -2-
FIELD E}PLORATION ..........- 2 -
SUBSURFACE CONDITIONS 3-
FOUNDATION BEARING CONDITIONS 3
DESIGN RECOMMENDATIONS ...............4-
FOUNDATIONS.......,.4-
FOUNDATION AND RETAINING Iü/ALLS .................. 5 -
FLOOR SLABS 6-
SURFACE DRAINAGE
LIMITATIONS 8-
FIGURE I - LOCATION OF ÐOLORATORY BORTNG
FIGURE 2 - LOG OF ÐGLORATORY BORING
FIGURES 3 and 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1. SUMMARY OF LABORATORY TEST RESULTS
H.PV(uIVIAR Project No. 18-7-707
PURPOSE AIID SCOPE OF STUDY
This report prese,nts the results of a subsoil study for a proposed residence to be located on Lot
37, Pinyon Mesa, 168 Clifhose Vfay Garfield CountS Colorado. The project site is shown on
Figure l. The putpose of the study was to develop recommendations for the foundation design.
The study was conducted in accordance with our agreemeirt for geotechnical engineering
services to Miguel Rojo dated Nove,rnber 26,2018. Hepworth-Pawlak Geotechnical (Now H-P/
Kumar) previously performed preliminary geotechnical engineering studies for the subdivision
development and presented our findings in reports dated November 11, 2005 and April 10,2006,
Job No. 105 652.
A field exploration program consisting of an exploratoryboring was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obt¿ined during the field
exploration were tested in the laboratory to determine their classifi.cation, compressibility or
swell and other ørgineering characteristics. The results of the field exploration and laboratory
testingwere anal¡zed to develop recoÍrmendations for foundation t¡pes, 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
Building designs have not yet been developed. We underst¿nd that this study is for the sale of
the property. The proposed residence will likely be â on€: or two-story wood-fra¡ne structure
over a crawlspace or basement level with an attached garage. Ground floor will be slab-on-grade
or structural over cìrawlspace. Grading for the structure is assumed to be relatively minor with
cut depths between about 3 to 10 feet. We r¡ssume relatively light foundation loadings, tlpical of
the proposed type of construction.
When building location, grading and loading information have been developed, we should be
notified to re-evaluate the recommendations presented in this report.
H.PMI,JMAR Proiect No. 18-7-707
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SITE CONII)ITIONS
The subdivision is located on arclatively flat topographic bench above the Roaring Fork River
valley and below Spring Valley. Vegetation has bee¡r removed from the front northern part of
the site apparently during subdivision development. The southem part of the site is vegetated
with sage brush with juniper and pinyon frees further south and along the eastern edge of the lot.
The front, north, part of the site appears to havebeen cut duringroad development. The ground
surface of the building area appears mostly natural and is moderately sloping down to the north.
There is a rough graded road along the western edge of the lot.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa
Subdivision. These rocks are a sequence of glpsiferous shale, fine-grained sandstone and
siltstone with some massive beds of g¡'psum and limestone. There is a possibility that massive
gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the g)psum under certaiu conditions caû cause si¡kholes to develop and can
produce a¡eas of localtzed subsidence.
Sinkholes were not observed in tbe subdivisionbut geologically young sinkholes are locally
present in the evaporite region betweeri Glenwood Springs and Carbondale and we are aware of
several sinkhole collapses in this region during the past 10 years. No evidence of cavities was
encountered in the subzurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. Based on or¡r present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinlúroles will not develop" The risk of
fi¡ture ground subsidence on [,ot 37 throughout the serr¡ice life of the proposed residencen in our
opinion, is low; however, the owner should be made aurare of the potential for sinkhole
development. If ñrttrer 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 November 29,2A18. One exploratory
boring was drilled at the location shown on Figure I to evaluate the subsurface conditions. The
H.PVXUMAR Project No. 18-7-707
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boring was 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 H-P/I(umar.
Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampl€r was driven
into the subsoils at various depths with blows from a 140 pound ha¡nmer falling 30 inches. This
test is similar to the standard penetration test described by ASTM Method D-1586. The
penetration ¡esistance values are an indication of the relative density or consistency of the
subsoils and hardness of thebedrock. De,pths at which the samples were taken and the
penetration resistance values are shown on the Log of Exploratory Boring Figure 2. \\e
samples were retumed to our laboratory for review by the project engineer and testing.
SUBSURFACE COI\DITIONS
A graphic log of the subsurface conditions e,ncountered at the site is shown on Figwe 2. The
subsoils consist of about 18 f€et of sligþtly clayey sand and silt overlying bedrock of the Eagle
Valley Evaporite Formation. Weathered waporite was encountered in the boring from a depth
of 18 feet to Z\Yzfeet where crystalline g)lpsum was encountered down to a depth of 23 feet.
Siltstone of the Eagle Valley Evaporite Formation was encountered at a depth of 23 feet down to
the maximum depth explored of 25 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and percent finer than sand size gradation analyses. Results of swell-
consolidation testingperformed on relativelyundisturbed drive samples of the sand and silt,
presented on Figures 3 and 4, indicate low to moderate compressibilityunder conditions of
loading and wetting and a low to moderate collapse potential (settle'lnent under constant load)
when wetted under a constant light surctrarge. The laboratory testing is summarized in Table l.
No free water was encountered in the boring at the time of drilling and the subsoils were sligbtly-
moist.
FOT]NDATION BEARING CONDITIONS
The subsoils encountered on the lot generally consist of sand and sitt and are t¡pically known to
be compressible when wetted under load. Lightly loaded spread footings can be used for support
of the proposed reside,nce provided that some risk of movement and distress is acceptable to the
}I-PVI(I'JMAR Froject No. 18-7-707
4
owner. A heavily reinforced mat for¡ndation would help to make the structure more rigid and
better able to resist difÏerential settlement. Compacting the bearing soils to a depth ol'at least
5 feet below shallow footings would help to reduce the settlement risk. Another alternative is a
deep foundation system that extends thebearing level down to dense, relatively incompressible
granular soils or bedrock. If the deep foundation altemative is selected, we should be contacted
to provide additional recommendations.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratoryboring and the nature of
the proposed construction, we recoûrmend the buildingbe founded with spread footings or a mat
bearing on the natural soils or compacted structural fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Basement level footings or a mat slab placed on the undisturbed nafural granular
soils should be designed for an allowable bearing pressure of 1,500 psf. Shallow
footings placed less than l0 feet below the ground surface should be placed on at
least 5 feet of compacted structural fill. The structural fill can consist of the on-
site silt and sand soils compacted to at least 98% of the maximum standard
Proctor density at a moisture content near optimum. Based on experience, \tre
expect settlement of footings designed and constructed as discussed in this section
will be about I inch or less. There could be additional differential foundation
settlement on the order of 1 to 2 inches ormore depending on the depth of any
subsurface wetting. Precautions should be taken to prevent post-construction
wetting of the bearing soils.
2) The footings should have a minimum width of 20 inches fo¡ continuous walls and
2 feet for isolated pads.
3) The mat edges, exterior footings and footings beneath unheated areas ehould be
provided with adequate soil cover above their bearing elevation for frost
protection. Placement of foundations at least 36 inches below exterior grade is
H-P*l(UlrIAR Project No. 18=7-7û7
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tlpically used in this area. Shallow frost protection can consist of rigid foam
insulation in the shallow mat foundation condition.
Foundations should be designed to be rigid with "box-like" configuration and
isolated footings should be avoided. Continuous foundation walls should be
reinforced top and bottom to span local anomalies such as by aszuming an
unsupported length of at least 14 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 ofthis report.
All existing fill, topsoil and any loose or disturbed soils should be removed and
the footing bearing level extended down to the undisturbed natr¡ral soils. The
exposed soils in footing area should then be moistened and compacted.
A rqnesentative of the geotechnical engineer should observe all footing
excavations prior to concreteplacement 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 55 pcf for backfill consisting
ofthe on-site soils. Cantilevered retaining structures which are separate from the resideirce and
can be expected to deflect sufficientlyto mobilize the full active earth pressure condition should
be designed for a lateral earttr pressure computd on the basis of an equivalent fluid unit weight
of at least 45 pcf for backfill consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressr¡res 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 maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway
areas should be compacted to at least 95% of the maximum standard Proctor densþ. Care
should be taken not to overcompact the backfill or use large equipme,nt near the wall, since this
4)
5)
6)
,-t-PvKt.rMl\R Project No. 18-7-707
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could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backflll should be expected, even lf the rnâterial ts placed côrnectly, and could result in distrcss 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 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 325 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
shength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate sfrengtl¡ particulady in the case of passive resistance. Fill placed against
tho sides of tho footings to resist lateral loads should be compacted to at least95%o of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoilo are suitable to support lightly loaded slab-on-grade
construction. There is a risk of slab settlement and distress if the bearing soils become wetted.
To reduce the effects of some differential movement, floor slabs should be separated from all
beming walls and columns with expansion joints which allow unrestrained vertical movement.
Floor slab control joints should be used to reduce damage due to shrinkage øacking. The
requirerrents for joint spacing and slab reinforcement should be established by the designer
bascd on cxpcricnce and the intended slab use. A minimum 4 inch layer of frce-draining gravcl
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 than 2%
passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Pmctor density at a moistwe content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
mountainous areas that local perched groundwater can develop during times of heavy
H.PVKUMAR Project l'.,lc, 18-?-707
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precipitation or seasonal runoff. Frozen ground during spring runoffcan create a perched
condition. W'e recommend below-grade consfuction, such as retaining walls, crawlspace 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 surrormded above
the invert level with ûee-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 lo/o to
a suitable gavity outlet. Free-draining granular material used in the underdrain system should
contain less than ZYo 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 lt/z fæt deep. An
impervious mer¡brane zuch as 20 mil PVC should be placed beneath the drain gravel in a hough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.
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 themæcimum standard Proctor density in pavernent and slab areas
and to at least 90% of the ma,ximum 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 l0 feet in paved areas. Free-draining wall backfill should be
covered with filter fäbric and capped with about 2 feú of the on-site 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 10
feet from foundation walls. Consideration should be given to use of xeriscape to
reduce the potential for wetting of soils below the building caused by irrigation.
H.PV(Uil¡IAR Frolect No. 18-7-707
-8-
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this arca at this time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this re,port are based upon the data obtained
from the exploratory boring drilled at the location indicated on Figure 1, the proposed tlpe 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 boring and variations in the subsurface
conditions may not become evident until excavation is performd. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations maybe 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 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
ormodifications to the recommendations presented herein. We recommend on-site obserr¡ation
of excavations and foundation bearing strata and testing of structural fill by a represelrtative of
the geotechnical engineer.
Respectfu lly Submitted,
H-P*KUMTAR
1/Øl
Robert L. Duran, E. I.
Reviewed by:
Daniel E. Hardin, P.
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Frojecf No. 18-7-707
SEWER MANHOLE RIM o
Elevation Assumed - 100 Feet
CLIFFROSE WAY APPROXIMATE SCALE
69.00'
10.0'10.0'
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69.00'
18-7-707 H.P*K|,JMAR
csôræh¡løl É¡O¡¡Mno I EnBl@hg G€dogy LOCATION OF EXPLORATORY BORING Figure 1
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BORING 1
EL. 111'
LEGEND
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SAND AND SILT (SM-MI); SLIcHTLY CLAYTY ìl,lTH DtPTl.t,
SCATTTRTD GRAVTL WTH DEPTH, STIFF TO VTRY STIFF, SLIG}ITLY
MOIST, TAN TO BROWN. GYPSIFFROIIS, ANßI.II.AR ROCK.
12/12
E=t
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WEATHERED TAGLE VALLEY EVAPORITE, MEDIUM HARD, SLIGHTLY
MOIST. LIGHT TAN, GYPSUM CRYSTALS.
5
17 /12
WC=6.4
DD=103
IAGLE VALI."EY EVAPORITT, CRYSTALINE GYPSUM, WHITE, HARD,
SLIGHTLY MOIST.
SILTSToNE (uS); CVpSlrfRoUS, HARD, SLtcHTLy M0IST, BR0WN.
10 DRIVE SAMPLE, z-INCH I.D. CALIFORNIA LINER 5AMPLE.
21 /12
IYC=5.4
DD=104
-200=45
Fl¡l
¡¡Jl¡
Ixt'-o-l¿lo
15
27 /12
WC=10.5
DD='107
NOTEg
1. THE EXPLORATORY BORING WAS DRILLED ON NOVEMBER 29,
2O1E WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER
AUGER.
2. THE LOCATION OF THT EXPLORATORY BORING WAS MEASURED
APPROXIMATELY 8Y PACING FROM FIATURES SHOWN ON THE
SIÏE PI.AN PROYIDED.
20 5. IHE ELEVATION OF THE EXPLORATORY EORING WAS MEASURED
BY HAND LTVEL AND RTFER TO THT SEIVÊR MANHOLE RIM IN
CLIFFROSE WAY AS lOO FEET ASSUMTD.
14/6, 60/6
4. THE EXPLORATORY BORING LOCATION AND ELIVATION SHOULD BT
CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY TI{E
MEÏHOD USED.
25 5. THE L]NES EETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THT APPROXIMATE BOUNDARIES BETWEIN
MATERIAL TYPES AND THT TRANSITIONS MAY BE GRADUAI.
6. GROUNDWATTR WAS NOT ENCOUNTTRTO IN THT BORING AT THI
TIMI OF DRILLING.
7, I"ASORATORY TEST RESULTS:
WC = WATER CONTENT (r) (ASTM D 2216);
DD = DRY DENSITY (pct) (mru D 2216);
-200 = PTRCENTAGE PASSING N0. 200 StEVt (AST[,| D il40).
50/4
50
18-7-7A7 Kumar & Assoclates LOG OF EXPLORATORY BORING Fig. 2
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SAMPLE OF; Sand ond Sill
FROM:Boringl@5'
WC = 6.4 %, DD = 103 pcf
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UNDER CONSTANT PRESSURE
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18-7 -707 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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SAMPLE OF: Sllghtly Gloyey Sond oñd Sllt
FROM: Boring 1 O 15'
WC = 1O.5 %, DD = 107 pcf
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18-7-707 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULÏS Fig. 4
H-P*KUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProjectNo. 1&7-707SOILTYPESand and SiltSand and SiltSlightty Clayey Sand andSiltUNCONFINEOCOMPRESSIVESTRENGTFIlosflPI.ASTICINDEX(o/olLIQUIDLtiltr(o/olPERCENTPASSINGNO.200SIEVE45GRADATIONSAND(wGRAVETØt103NATURALDRYDENSITYfocll104107NAÎURALMOlsTURECONTENTlo/ol6.45.410.5LOCATIONDEPTH(fr)5t015BORINGI