HomeMy WebLinkAboutSubsoils Report for Foundation DesignI(+A Kumar & Associatc, lnc.@
Geotechnical and Materlals Engineers
and Environmental Scientists
5020 CountY Road 154
Glenwood SPrings, CO 81601
Phone: (970) 945-7988
fax: (970)945-8454
email: kaglenwood@kumarusa'com
offrce locations: Denver (He), parker, colorado Springs, Fort collins, Glenwood springs, and Summit county' colorado
SUBSOIL STTJDY
FOR FOTJNDATION DESIGN
PROPOSED RESIDENCE
LOT IlS,IRONBRrDGE
227 SILYFR MOUNTAIN DRIVE
GARFIELD COUNTY, COLORADO
PROJECT No.21-7-851
DECEMBERT'202I
PREPARED FOR:
BRANDON COWTIEY
571 RIVER BEND WAY
GLENWOOD SPRTNGS, COLORADO 81601
(cowhevbrandon@ smail'com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL'
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOI-TNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS .'".....".
FOt]NDATIONS
FOI-INDATION AND RETAINING WALLS ...
FLOOR SLABS
LTNDERDRAIN SYSTEM ..'"..''"..
SITE GRADING.......
SURFACE DRAINAGE
LIMITATIONS.........
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - GRADATION TEST RESULTS
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Kumar & Associates, lnc. o Project No. 21'7'851
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 118, Ironbridge, 227 SllvetMountain Drive, Garfield county' colorado' The project site is
shown on Figure 1. The pulpose 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 Brandon cowhey dated october 28,2021' Hepworth-Pawlak
Geotechnical (now Kumar & Associates) previously conducted a preliminary subsoil study for
Lotsl08tollsandpresentedthefindingsinareportdatedDecember6'2002'JobNo'
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
bu'ding 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 plans for the residence had not been developed at the time of our study' In general' the
proposed building will be in the upper part of the lot and be a I 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 6feet. 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
andweedsinthebuildingarea'Adrainageswaleroughlyfollowsthesouthwestemsideofthe
lot
Kumar & Associates, lnc' @
Project No. 21'7'851
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SUBSIDENCE POTENTIAL
Bedrock of the pennsylvan ianageEagre varley 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
associatedwiththeEagleValleyEvaporiteunderlieportionsofthelot.Dissolutionofthe
gypsum under certain conditions can cause sinkholes to develop and can produce areas of
rocalized subsidence. During previous studies for Ironbridge and other deveropments, broad
subsidence areas and sinkholes have been observed including sinkholes in the central to northern
parts of Ironbridge. These sinkhores 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 10t 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
knowredge of the subswface conditions at the site, it cannot be said for certain that sinkholes will
not deverop. The risk of future ground subsidence on Lot 1 1g 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 freld exploration for the project was conducted on November l'2021' 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 were logged by a representative of Kumar &
Associates
Samples of the subsoils were taken with a l3/s rnchl'D' spoon sampler' The sampler was driven
into the subsoils at various depths with blows from a r40 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 anindication of the relative density or consistency of the
subso's. 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 refirrned 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 nil to about t/zfoot of topsoil (andt/zfoot of sandy clay in Boring 2) overlying
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Verydense,slightlysiltysandygravelandcobbleswithboulders.Drillinginthecoarsogranular
soils with auger equipment was difficurt due to the cobbles and bourders and drilling refusal was
encountered in the dePosit'
Laboratory testing performed on samples obtained from the borings incruded natural moisture
contentandgradationanalyses.Resultsofagradationanalysesperformedonsmalldiameter
drive sampres (minus r%-inchfraction) of the coarse granular subsoils are shown on Figure 3.
Nofreewaterwasencounteredintheboringsatthetimeofdrillingandthesubsoilswere
slightly moist.
FOUNDATION BEARING CONDITIONS
The natural gravel and cobbre so's encountered below the topsoil and clay soils are suitable for
support of spread footing foundations with moderate bearing capacity and relatively low
settlementpotential.Alltopsoilandclaysoilsshouldberemovedfrombeneaththeproposed
building area. At typical foundation depths for the general proposed type of construction' we
expecttheexcavationwillbedownintothegravelandcobblesoils.
DESIGN RECOMMENDATIONS
FOI.'NDATIONS
considering the subsurface conditions encountered in the exploratory borings and the nature of
theproposedconstruction,werecommendthebuildingbefoundedwithspreadfootingsbearing
on the natural granular soils'
The design and construction criteria presented below should be observed for a spread footing
foundation sYstem.
1)Footingsplacedontheundisturbednaturalgranularsoilsshouldbedesignedfor
an allowable bearing pressure "f 3,000 p.L Based on experience, we expect
settlementoffootingsdesignedandconstructedasdiscussedinthissectionwill
be about 1 inch or less'
The footings should have a minimum width of 16 inches for continuous walls and
3)
2 feet for isolated Pads'
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 inchestelow exterior grade is typically used in this
atea.
Continuous foundation walls should be reinforced top and bottom to span local
anomaliessuchasbyassuminganunsupportedlengthofatleast12feet,
2)
4)
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Project No. 21'7-851
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Foundation walls acting as retaining structures should also be designed to resist
lateralearthpressuresaSdiscussedinthe''FoundationandRetainingWalls''
section of this rePort'
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'
FOLINDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amounl of deflection should be designed for a lateral earth pressure
computed on the basis of an equivarent fluid unit weight of at reast 50 pcf for backfirl 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 reast 40 pcf for backfiil 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, traffic, construction materials and equipment' The
pressures recommended above assums drained conditions behind the walls and a horizontal
backfrll surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the raterar pressure imposed on a foundation wa[ 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 themaximum standard Proctor density'
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateralpressure on the wall. Some settlement of deep foundation wall
backfil should be expected, even if the material is praced correctly, and could result in distress to
facilities constructed on the backfill'
The lateral resistance offoundationorretainingwallfootingswillbeacombinationofthe
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
Kumar & Associates, lnc. o Project No, 21'7'851
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coefficient of friction and passive pressure values recommended above assumo ultimate soil
strength. Suitabre factors of safety shourd be included in the design to limit the strain which will
occuf at the ultimate strength, particularly in the case of passive resistance' Fill placed against
the sides of the footings to resist laterar loads should be a granular material compacted to at least
g5o/o of themaximum 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 wails and columns with expansion joints which allow unrestrained
vertical movement. F100r slab control joints should be used to reduce damage due to shrinkage
cracking. The requirements for joint spacing and srab reinforcement shourd be established by the
designer based on experience and the intended slab use' A minimum 4-inch layer of free-
draining gravel shourd be praced beneath basement rever slabs to facilitate drainage. This
material should consist of minus 2-rnch aggregatewith at reast 50% retained on the No. 4 sieve
and less than}Yopassing the No' 200 sieve'
A11filImaterials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum' Required filI can consist of the
on-site gravel soils devoid of vegetation, topsoil and oversized rock.
LTNDERDRAIN 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 berow-grade construction, such as retaining walrs and basement areas' be protected
fromwettingandhydrostaticpressurebuildupbyanunderdrainsystem.
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert rever with free-draining granurar materiar. The drain shourd be placed at each level of
excavation and at reast I foot berow rowest adjacent finish grade and sloped at a minimum 1olo to
a suitable gravity outlet. Free-draining granular materiar used in the underdrain system should
contain less than 2o/o passingthe No. 200 sieve, less than 50% passing the No' 4 sieve and have a
maximum sizeof 2inches. Thedraingravelbackfillshouldbeatleast l%feetdeep'
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided cut and fill
depths are limited. we assume the cut depths for the basement level will not exceed about
Kumar & Associates, lnc, @
Project No. 21'7'851
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10 feet. Fills should be limited to about g feet deep. Embankment fills should be compacted to
at reast 95yo of themaximum standard proctor density near optimum moisture content. Prior to
fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil
and compacting to at least g5o/o of themaximum standard proctor density. The fill shourd be
benchedintotheportionsofthehillsideexceeding20o/ograde.Permanentunretainedcutandfill
slopes should be graded at2horizontar to 1 vertical or flatter and protected against erosion by
revegetation or other means
SURFACE DRAINAGE
The forowing drainage precautions shourd be observed during construction and maintained at all
times after the residence has been completed:
1)lnundationofthefoundationexcavationsandunderslabareasshouldbeavoided
during construction'
2) Exterior backfrll should be adjusted to near optimum moisture and compacted to
atleastg5YoofthemaximumstandardProctordensityinpavementandslabareas
and to at least g0% 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
3inchesinthefirstl0feetinpavedareas'Free-drainingwallbackfillshouldbe
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.
s)Landscaping which requires regular heavy inigation should be located at least
5 feet from foundation walls'
LIMITATIONS
has been conducted in accordance with generally
and practices in this area at the time of this study'
implied. The conclusions and recommendations
accepted geotechnical engineering
upon the data obtained from the exploratory borings drilled at the locations indicated on
Figure 1, the Propo sed type ofconstruction and our experlence 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
This study
principles
express or
We make no warrantY either
submitted in this rePort are based
Kumar & Associates, lnc, @
Project No' 21'7'851
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and variations in the subsurface conditions may not become evident until excavation is
performed. If conditions encountered during constrnction appeatr 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 pu{poses. 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 veriff that the recommendations
have bee,n appropriately interpreted. Significant 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, Inc'
Daniel E. Hardin, P
Reviewed by:
Steven L. Pawlak, P.E.
DEH/kac
Kumar & Associates, lnc. @ Project No, 21-?'851
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APPROXIMATE SCALE_FEET
Fig. 1LOCATION OF IXPLORATORY BORINGSKumar & Associates21 -7 -851
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BORING 1
EL. 91 .5
BORING 2
EL. 95.0
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82/8 80/ 12
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LEGEND
ffi roesou: oRGAN|C SILTY SAND W|TH GRAVEL, F|RM, MO|ST, BROWN, ROoTS
hvN
CLAY (CL): SILTY, SANDY, MEDIUM STIFF, SLIGHTLY MoIST' LIGHT BRoWN'
GRAVEL (GM-GP); WITH
DENSE, SLIGHTLY MOIST'
COBBLES AND SMALL BOULDERS, SANDY, SLIGHTLY SILTY' VERY
GRAY-BROWN
0
5
10
I DRTVE SAMPLE, 1 3/8-INCH l.D. SPLIT SPOON STANDARD PENETRATIoN TEST'
WITH A 4-INCH
s0l6 PXIIE,NST\'-.9;]I"*,i'#-l;lt?l?i'1; 'JHJ,u?rP'3I'i'iJ- i
"fi8;ESiND
HAMMER
I eucrrcAL AUGER REFUSAL'
NOTES
I.THEEXPLoRAToRYBoRINGSWEREDRILLED0NNoVEMBERI'2021
oilvEiER coNTlNuous-FLIGHT PowER AUGER'
2, THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY
PACINC FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED'
3'THEELEVATIoNSoFTHEEXPLoRAToRYBoRINGSWEREMEASUREDBYINSTRUMENTLEVEL
nNO REFER TO THE STREET ELEVATION = 1 00'0 FEET'
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED
ACCUUTE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5,THELINESBETWEENMATERIALSSHOWNONTHEEXPLORATORYBORINGLOGSREPRESENT
THE APPROXIUITi-AOUNONiITS 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);
+4 = pERCENTAGE RETAINLD ON NO. 4 SIEVE (ASTM D6915);
_ZQO= PERCENTAGE PASSING NO' 2OO SIEVE (ASTM D1140)'
WC=0.8
+4=60
-200= 1 0
Fig. 2LOGS OF TXPLORATORY BORINGSKumar & Associates21 -7 -851
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SIEVE ANALYSIS
HYDROMETER ANALYSIS
6
ro0
90
80
70
60
50
40
50
20
to
0
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to
20
30
40
so
60
70
80
90
100
=G
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t,t 1.75 3a.l 127
.oo2 .oo9 .125
DIAMETER F PARTICLES IN
CLAY TO SILT
COBBLES
GRAVEL 60 % SAND 50
LIQUID LIMIT
SAMPLE OF: Sllghtly Silty Sondy Grovel
PLASTICITY INDEX
SILT AND CLAY 10 %
FROM: Boring 1 @ 2.5' & 5' Comblned
Thcse l€st rosulls qpply only lo lhc
somoles which wero loslod. The
t*iiio report sholl nol b€ roProduc€d,
."".oi ln full. wllhoul the wrlll€n
ooorLvol qf Kumqr & A9sgclql.s, lnc'
ii"'"" qnoivsi" lesllng ls performed ln
i..*a.n"l wilh AST-M 06913, ASTM D7928'
ASTM Cl56 ond,/or ASTM Dll'10'
GRAVELSAND
COARSEFINEMEDTUM lcornseFINE
Fig. 3GRADATION TEST RESULTSKumar & Associates21 -7 -851