HomeMy WebLinkAboutSubsoil Study for Foundation Design 08.07.19lcrt
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Gootechnical and l,låterials ËrEineers
and Envimnmeûhl Sc¡entisb
5020 County Road 154
Glenwood Springs, CO 81601
phone; (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
An Employcc O*ncd Compony www'kumarusa'com
Offiee Localions: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 51,IRONBRIDGE, PHASE rlr
TBD BLUE ITF',RON DRTVE
GARFIELD COIINTY, COLORADO
PROJECT NO. 19-7-454
AUGUST 7,2019
PREPARED FOR:
STEVE MARTIN
C/O STEVE MARTIN INSURANCE GROUP
995 COWEN DRTVE, SUITE 200
CARBONDALE, COLORADO 81623
stmartin@ft.NewYorkl,ife.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..........
PROPOSED CONSTRUCTION
SITE CONDITIONS.........
SUBSIDENCE POTENTIAL
FOTINDATION BEARING CONDITIONS .......
DESIGN RECOMMENDATIONS
FOUNDATIONS .........
FLOOR SLABS
TINDERDRAIN SYSTEM .........
SURFACE DRAINAGE
LIMITATIONS......
FIGURE 1 . LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 AND 5 . SWELL.CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE I. SUMMARY OF LABORATORY TEST RESULTS
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FIELD EXPLORATION -2-
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Kumar & Associates, lnc.Projec{ No. 19-7-454
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on Lot
51, Ironbridge, Phase III, TBD Blue Heron Drive, Garfield County, Colorado. The project site is
shown on Figure 1. The pu{pose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreønent for geotechnical
engineering services to Steve Martin dated July 25,2019.
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, 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 presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed conskuction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a one-story wood frame structure over a crawlspace. The
attached garage will have a slab-on-grade floor. Grading for the structure is assumed to be
relatively minor with cut depths between about 3 to 5 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 and appeared to have had some minor overlot grading,likely during
subdivision development. The surface of the lot slopes gently down to the northeast with about
2 feet of elevation Cifference across the building area. Vegetation consists of grass and weeds.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the lronbridge development.
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 work in the area, several sinkholes were observed
scattered throughout the Ironbridge development. These sinkholes appeff similar to others
associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley.
Sinkholes were not observed in the immediate area of 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 cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot 5l throughout the service life of the proposed residence, in our
opinion, is low and similar to other lots in the area; 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.
F'IELD EXPLORATION
The field exploration for the project was conducted on July 29, 2019 . 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-458 drill rig. The borings were logged by a representative of Kumar &
Associates,Inc.
Samples of the subsoils were taken with l3/sinch and 2 inch I.D. spoon samplers. The samplers
were 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-l586.
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
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shown on the Logs of Exploratory Borings, Figure 2. The samples were refurned 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 up to about Yz foot of gravelly sandy clay and silt fill overlying \Yz to 73 feet
of stiff to very stiff, sandy silt and clay soils. Dense, silty sand and gravel with cobbles was
encountered below the silt and clay at depths of 9 to l3Yz feet. Drilling in the dense granular
soils with auger equipment was difficult due to the cobbles and drilling refusal was encountered
inthe depositat13 and 17 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content, density and gradation analyses. Results of swell-consolidation testing performed on
relatively undisturbed drive samples of the silt and clay soils, presented on Figures 4 and 5,
indicate low to moderate compressibility under conditions of loading and wetting. The samples
had a minor collapse potential (settlement under constant load) when wetted. A gradation
analysis performed on the gravel subsoils (minus lYz-inch fraction) are shown on Figure 6. The
laboratory testing is summarized in Table l.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
F'OUNDATION BEARING CONDITIONS
The upper silt and clay soils have low bearing capacity and senerallv low to moderate
compressibilitv when wetted. Shallow spread footings placed on the natural clay and silt soils
can be used with a risk of settlement as described below. The footing bearing level on Lot 51
should be deepened below existing ground surface so there is no more that 4 feet of silt and clav
soils below the bearing level as a foundation settlement mitigation measure. In sub-excavated
areas below design footing level, the on-site soils could be replaced compacted. Extending the
footing bearing level down or use of a deep foundation such as drilled piers placed on the
underlying gravel and cobble soil could be used to achieve a low settlement risk.
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DESIGN RECOMMENDATIONS
FOI.INDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural clay and silt or granular soils or compacted structural filI with a settlement risk.
The design and conbtruction criteria presented below should be observed for a spread footing
foundation system.
l) Footings placed on up to 4 feet of the undisturbed natural soils or structural fill
should be designed for an allowable bearing pressure of 1,500 psf. Based on
experience, we expect initial settlement of footings designed and constructed as
discussed in this section will be aboutYz to 1 inch or less. Additional differential
settlement up to about 1 inch could occur if the bearing soils are wetted.
2) The footings should have a minimum width of 20 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 typically used in this
atea.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure coffesponding to an equivalent fluid unit weight of at least
55 pcf for the onsite silt and clay soil as backfill.
5) All existing fill, topsoil and any loose or disturbed soils should be removed in
footing areas. The exposed soils in footing areas at the sub-excavated level
should then be moistened and compacted. Structural fill (recompacted on-site silt
and clay soils) should extend laterallybeyond the footing edges at leastYz the fill
depth below the footing and be compacted to at least 98% of the standard Proctor
density at near optimum moisture content. The soils should be protected from
frost and concrete should not be placed on frozen soils.
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6)A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete piacement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction with a risk of settlement if the bearing soils are wetted. Structural fill about 2 feet
deep consisting of the on-site soils Can be placed below slabs to limit the settlement risk. 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.
All fill materials for support of floor slabs should be compacted to at least 95% ofmaximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site silt and clay soils devoid of vegetation and topsoil.
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 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. Shallow crawlspace
should not be provided with an underdrain to help isolate the bearing soils from surface water
infiltration.
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 with the drain invert elevation at least 1 foot below lowest adjacent finish grade and
sloped at a minimum 1o/oto a suitable gravity outlet sump or drywell into the gravel soils. Free-
draining granular material used in the underdrain system 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
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drain gravel backfill should be at least lYz feet deep. An impervious membrane such as 20 mil
PVC should be placed beneath the pipe and drain gravel in a trough shape and attached to the
foundation wall with mastic to prevent wetting of the bearing soils.
ST-IRFACE DRAINAGE
Providing proper surface grading and drainage is very important to the satisfactory performance
of the buitding. 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 avoided
during construction.
2)Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% 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.
The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. 'Vy'e 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 10 feet in paved areas. Free-draining wall backfill (if any)
should be capped with about 2 feet of the on-site soils to reduce surface water
infiltration.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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.
3)
4)
s)
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this areaat this time. 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
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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 concemed 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
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
ofexcavations and foundation bearing strata and testing ofstructural fill by a representative of
the geotechnical engineer.
Respectñrlly Submitted,
Kumar & Associates, Inc.
Daniel E. Hardin, P.E
Reviewed by:
Steven L. Pawlak, P.E.
DEH/kac
Cc: Rock Leonard roçk@so_trrris.nçt
Kumar & Associates, lnc.Project No. 19-7-454
LOT 50
LOT 52
LEGEND:
+BENCHMARK: PAVEMENT GRADE
AT PROPERTY CORNER
ASSUMED lOO FEET.
I
APPROXIMATE SCALE.FEET
EORING 1
19-7 -454 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
ORING I
5'
BORING 2
Ë.L. 102'EL. 1 00
105 105
100 100
15/ 12
13/ 12
WC=5.6
DD=97 14/12
WC=6.5
DD=97Fl¡JLJL
Izo
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95 14/12
WC=6.6
DD=95
-2OO=84
95 l--tdt¡lL!
Izotr
LdJr¡l9047 /12
WC=3.4
*4=40
-200= 1 5
1E/ 12
WC=7.5
DD=97
-200=88
90
50/6
85 85
80 80
19-7 -454 Kumar & Associates LOGS OF EXPLORAÏORY BORINGS Fig. 2
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LEÈEND
X
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FILL: ORGANIC SANDY SILTY CLAY WITH GRAVEL, STIFF, SLIGHTLY MOIST, DARK BROWN.
SILT AND CLAY (ML-CL); SANDY, STIFF TO VERY STIFF, SLIGHTLY MolST, LIGHT BRowN.
t.%
P,:.1
KT
SAND AND GRAVEL (SM-GM); SILTY, DENSE, SLIGHTLY MolsÏ, MIXED BRowN
F
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRTVE SAMPLE, 1 5/8-|NCH LD. SPLIT SPOON STANDARD PENETRATION TEST.
""r.' DRIVE SAMPLE BLOW COUNT. INDICATES THAT 15 BLOWS OF A 140-POUND HAMMER'ol '' FALLTNG go TNcHES wERE REQUTRED To DRtvE THE SAMPLER 12 rNcHES.
I enlcrrclL AUcER REFUsAL.
NOTES
1, THE EXPLORATORY BORINGS WERE DRILLED ON JULY 29, 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.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO THE BENCHMARK ON FIG. 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
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANS|TIONS 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)t
DD = DRY DENSITY (pcf) (ASTM 02216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140).
19-7 -454 Kumar & Associates LEGEND AND NOTES Fig. 5
SAMPLE OF: Sondy Sllt ond Cloy
FROM:Boringl@2.5'
WC = 5.6 ?6, DD = 97 pcf
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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SWELL_CONSOLIDATION TEST RESULTS Fig. 419-7 -454 Kumar & Associates
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SAMPLE OF: Sondy Silt ond Cloy
FROM:Boring2@5'
WC = 6.5 %, DD = 97 pcl
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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19-7 -454 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 5
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ãFig. 6GRADATION TESÏ RESULTSKumar & Associates19-7 -454IøfñE10090ao70EO50¿lO!o20toooto20EO4050æ70so90100EEF152DIAMETER OFINCLAY TO SILTCOBBLESGßAVEL 40 '6 SANDLIQUID LIMITSAMPLE OF: Sllly Sond ond Grovel45%PLASTICITY INDEXSILT AND CI.AY 15 ZFROM:BortnglOl0'Ths. lrêl ..¡ulh opÞly only lo ih.Eomple! whloh u¡r. l¡¡l¡d, Thol.rllng ÞÞgrl lholl not b. rcpÞduo.d,lxcapl ln tull, wllhoul lh. wrlllrnopprcYol ot Kumor & ArtæloLr, lnc.Slry! onolysl! leatlng l! Þorlormod lnocoo¡donco wllh ASll¡ D6913. ASTU D792E.ASIM Cl56 qndlor Asltl Dll¡10,II¡llIIi1-tìIiIII1IIIIIiI::' -:l-Itt.cLuR sou^nE oPEHtxosIiU.S. SANDARD SERIES2¿ HRS 7 H¡SÍuE nmNosSIEVE ANALYSISHYDROMETER ANALYSISCOARSEFINEcoARstMEDIUMFINEGRAVELSAND
l(+rtffi**TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.19-7.454Sandy Silt and ClaySandy Silt and ClaySiþ Sand and GravelSandy Silt and ClaySandy Silt and ClaySOIL TYPE(osl)Ul,lC0ilFlilEDCOilPRESSTvESÏRENGTHG Ltilffs(ololPLASTICl1'¡DEXATTERBEIlolcìLIQUID LII'TÍ5188PERCEI'IIPASSIilG NO.200 $a/E84TK)NSAT{De/.)4540GRÂ(/'lGRAVEL97I{ATURAtDRYDEt'lS¡TYlocft9793973.46.3t.3lø\I'¡ATURAI-MOFTURECONTEilT5.66.6501lfrìDEPTH2y,510I2SAIIPLE LOCATIOI'IBORII¡G