HomeMy WebLinkAboutSubsoil Study for Foundation Design 07.08.2020I(l i,#lli#ntË:rnr.i;-"'
An Employca Owncd Compsny
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Ofüce Locations: Denver (HQ), Pa¡ke¡, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDBNCE
LOT 5, MOUNTAIN MEADOWS AT PRINCE CREEK
MEADO\ry COURT
PITKIN COUNTY, COLORADO
PROJECT NO.20-7-328
JULY 8,2020
PREPARED FOR:
GARRET CONSTRUCTION
ATTN: TODD CERRONE
38923 CRYSTAL BRIDGE DRIVE
CARBONDALE, COLORADO 81 623
todd@.garretconstruction.com
TABLE OF CONTENTS
PT]RPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION .......
SITE CONDITIONS
FIELD EXPLORATION.
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOLINDATIONS
FOUNDATION AND RETAINTNG WALLS .........
FLOOR SL48S........
TINDERDRATN SYSTEM .......
SURFACE DRAINAGE...........
LIMITATIONS..........
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORTNGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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5
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Kumar & Associates, lnc. @ Project No. 20-7-328
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for aproposed residence to be located on Lot 5,
Mountain Meadows at Prince Creek, Meadow Court, Pitkin County, Colorado. The project site
is shown on Figure l. 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 Garret Construction dated June 8, 2020.
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 analyzed to
develop recommendations for foundation types, 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
Development plans for the lot were preliminary at the time of our study. In general, the
proposed residence will be a single-story structure located within the building envelope shown
on Figure l. Ground floor will be structural above crawlspace with a slab-on-grade garage floor.
Mechanical space will be provided in the crawlspace. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 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 at the time of our field exploration. The building envelope is located
downhill, northwest of Meadow Court cul-de-sac as shown on Figure I . The ground surface is
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gently sloping with around 2 to 3 feet of elevation difference across the general building area
Vegetation consists of field grass and weeds.
FIELD EXPLORATION
The freld exploration for the project was conducted on June 10, 2020. 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 truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with l% inch 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-1586.
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
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned 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, below about one foot of topsoil and I to lYzfeet of stiff, sandy silty clay, consist of
dense, silty clayey sand and gravel with cobbles and probable boulders. Drilling in the coarse
granular soils with auger equipment was difflrcult due to the cobbles and boulders down to the
drilled depth of I I feet.
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 l%-inch fraction) of the coarse granular subsoils are shown on Figures 4 and 5.
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.
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FOUNDATION BEARING CONDITIONS
The subsoils encountered at probably excavation depths mainly consist of dense, sandy gravel
with cobbles and possible boulders which are excellent for support of shallow spread footings
and floor slabs. The upper clay soils should be removed from footing bearing areas but can
probably be used for floor slab support which should be further evaluated at the time of
construction.
DESIGN RECOMMENDATIONS
FOLTNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend 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 footing
foundation system.
l) Footings placed on the undisturbed natural granular soils should be designed for
an alluwablc liearing pl'essure of 3,000 psf. Based ou expetienüe, ws expeut
settlement of footings designed and constructed as discussed in this section will
be about I inch or less.
2) The footings should have a minimum width of l6 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 42 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least l0 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.
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The 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 observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOTINDATION 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 backfrll consisting
of the on-site granular soils. Cantilevered retaining structures which are separate from the
residence and can be expected to deflect suff,rciently 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 35 pcf for backf,rllconsisting of the on-site granular soils. Backfill
should not contain organics, debris or rock larger than about 6 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 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 near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95Yo of the maximum 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
backflrll 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 eafth pressure against
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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 450 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fillplaced against
the sides of the footings to resist lateral loads should be a granular material compacted to at least
95o/o of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be to support lightly loaded slab-on-grade
construction. 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. A minimum 4-inch layer of relatively
well graded sand and gravel (such as road base) should be placed beneath slabs as a leveling
course and for support. This material should consist of minus 2-inch aggregate with at least 50%
retained on the No. 4 sieve and less than lZYo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture 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
precipitation or seasonal runoff. Frozen ground during spring runoffcan 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. An
underdrain should not be needed for shallow crawlspace provided the footing bearing level is
down into the relatively free draining granular soils.
Kumar & Associates, lnc. @ Project No. 20-7-328
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Where drains are provided, the drains should consist of drainpipe placed in the bottom of the
wall backfill sumounded above the invert level with free-draining granular material. The drain
should be placed at each level of excavation and at least 1 foot below lowest adjacent frnish
grade and sloped at a minimum lYo to a suitable gravity outlet. Free-draining granular material
used in the underdrain system should contain less than 2%opassing 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%feetdeep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
l) 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 95o/o 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.
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 l0 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 fabric and capped with about 2 feet of the on-site frner graded
soils to reduce surface water infrltration.
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 area at this time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this report are based upon the data obtained
Kumar & Associates, lnc. @ Project No. 20-7-328
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from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
presence, pr.evention 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 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. Vy'e 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 been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundationbearing shata and testing of structural frll by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, Inc.
Steven L. Pawlak,
Reviewed by:
I
Daniel E. Hardin, P.E.
SLPlkac
U'l5?22
Kumar & Associates, lnc. ô Project No. 20-7-328
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BORING 1
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BORING 2
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APPROXIMATE SCALE-FEET
20-7 -328 Kumar & Associates LOCATION OF EXPLORATORY BORINGS 1Fig.
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BORING 1
EL. 100'
BORING 2
EL. 102.5'
o 15/ 12 12/ 12
WC=11.5
-200=79
36/12
0
23/12
WC=6.9
+4=50
-200=25
5 43/6,5o/ 4.5
WC=3.1
+4=44
-200= 1 1
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WC=2.1
+4=64
-200=9
40/6,50/4.5 10
15 15
20-7 -328 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
TOPSOIL; ORGANIC SANDY SILT AND CLAY, SCATTERED GRAVEL, BRowN.
CLAY (CL); SILTY, SANDY, SCATTERED GRAVEL, VERY STIFF, SLIGHTLY MOIST, BROWN, LOW
PLASTICITY.
ffi
SAND AND GRAVEL
DENSE, STIGHTLY M
(ou);
0tsT,
PROBABLE COBBLES AND BOULDERS, SILTY, SLIGHTLY CLAYEY,
MIXED BROWN.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
I DR|VE SAMPLE, 1 3/8-|NCH l.D. SPLIT SPoON STANDARD PENETRATTON TEST.
n1 /i' DRIVE SAMPLE BLOW COUNT. INDICATES THAT 21 BLOWS OF A 140-POUND HAMMERLI/ IL FALLING 30 INcHES wERE REQUIRED To DRIVE THE SAMPLER 12 INcHES.
NOTES
THE EXPLORATORY BORINGS WERE DRILLED ON JUNE 10, 2O2O 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 ELEVATION 1 OO" ASSUMED.
4. THE EXPLORATORY BORINC 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 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 RETAINED ON NO. 4 SIEVE (ASTM D6915);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM 01140).
20-7 -328 Kumar & Associates LEGEND AND NOTES Fig. 3
.e
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100
90
80
70
60
50
40
50
20
t0
HYDROMETER ANALYSIS SIEVE ÀNALYSIS
TIME READINCS
2¿ HRS 7 HRS
U.S. STANOARD SERIES CUÂR SOUARE OPENINGS
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------+--F-----t---l------
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SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
0
to
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10
50
60
70
80
90
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.oo I .oo2 .o05 .o09 .ott .o57 ,.125 1.75 9,5 t9
DIAMETER OF S
CLAY TO SILI COBBLES
GRAVEL 30 % SAND 45 %
LIQUID LIMIT PLASTICIW INDEX
SAMPLE OF: Silty Cloyey Sond wlih Grovel
SILT AND CLAY 25 %
FROM:BoringtO2.5'
fd
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100
90
80
70
ao
30
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50
20
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20
50
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.425
OIAMETER OF PARTICLES IN
CLAY TO SILT COBBLES
GRAVEL 64 % SAND
LIQUID LIMIT
SAMPLE OF: Slightly Sllty Sondy Grovel
27%
PLASTICITY INDEX
SILT AND CLAY 9%
Th.s. l.d r.lull! opply only lo lh.
somplas vhlch w.r. l.shd. Th.
lcrllng rcporl lhqll nol bc roproduc.d,
rxcrpl ln lull, wllhoul lhc srlllcn
opp.gvql ol Kumqr & Asroclqbr, lnc,Sl.v. qnqlyllt lrtllng lt p.rtormld l¡
occordonc. wlth ASIM 06915, ASÍM D7928,
ÀSTM C136 ond/or ASTM Dll40.
FROM:BorlnglOl0'
HYDROMETER ANÀLYSIS SIEVE ANALYSIS
TIME REAOINCS
24 HRS 7 HRS I
U.S, SIANOÀRD SERIES CLEAR SOUARE OPENINCS
SAND GRAVEL
FINE MEDTUM lCOrnSE FINE COARSE
20-7 -328 Kumar & Associates GRADATION ÏEST RESULTS Fig. 4
:
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2
100
90
80
70
50
50
40
50
20
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20
50
40
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60
70
80
90
ä
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.009 .ot9 .125 r52IN MI M RS
CLAY TO SILT COBBLES
GRAVEL 11 % SAND 45 %
LIQUID LIMIT PLASTICITY INDEX
SAMPLE OF: Slightly Silty Sond ond Grovel
SILT AND CLAY 11 %
FROM:Boring2O5'
Thrta l.sl r.sulls qpply only lo lhr
romplcs whlch w.r. la3lcd, Th.
l.sllng rcporl ¡hqll nol b. rcp¡oduccd,
cxc.pl ln lull, wlthoul lh. wrlll.n
opprovol of Kumqr & Alsoclqlos, lnc.
Sl.vc onolylh l.tllñE lr plrform.d ln
qccordqnco wlih ASTM D6915, AslM 07928.
ASIM C136 ond/or ASIM Dtl,to,
SIEVE ANALYSISHYDROMETER ANALYSIS
IIME REAOINGS
2,1 HRS 7 HRS
U.S. STANDARD SERIES CLEAR SQUARE OPEilINGS
\fA' tfL' t 1fa'
__ l_,__t__,.1_1-a_+_ rI _
SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
Fig. 520-7 -328 Kumar & Associates GRADATION TEST RESULTS
I(tT Hffiflffifffi*r''3;""*TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.20.7.328SOIL TYPESilty Clayey Sand withGravelSlightly Silty Sandy GravelSandy Silty ClaySlightly Silty Sand andGravellosflUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEXt%lATTERBERG LIMITS(%)LIQUID LIMITPERCENTPASSING NO.200 stEVE259791l452745306444GRADATIONSAMPLE LOCATIONDEPTHBORINGNATURALDRYDENSITYNATURALMOISTURECONTENTSAND(%)GRAVEL("/"16.9125llI-tal/L/20I15I2