HomeMy WebLinkAboutSubsoil Study for Foundation Design 03.05.18H-PVKUMAR
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 94S-79S8
Fax (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, FortCollins, Glenwood Springs, Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 58, FILING 2, PINYON MESA
236 PAINTBRUSH WAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 18-7-161
MARCH 5n 2018
PREPARED FOR:
R.ESORT DEVELOPMENT BUILDING, INC
ATTN: CHANCE SOLDOFF
99 ALFIN PLACE
GLEN\ilOOD SPRTNGS, COLORADO 81601
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF S'TUDY .
PROPOSED CONSTRUCTION
SITE CONDITIONS.......
SUBSIDENCE POTENTIAL
FIELD EXPLORATION .....
SUBSURFACE CONDITIONS ......
FOLÏNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOTINDATION AND RETAINING V/ALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SURTACE DRAINAGE...............
LIMITATIONS...
FIGURE 1 . LOCATION OF EXPLORATORY BORING
FIGURE 2 . LOG OF EXPLORATORY BORING
FIGURE 3 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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H-P*KUMAR Project No. 18-7-161
PURPOSE AT{D SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at Lot
58, Filing 2, Pinyon Mesa, 236 Paintbrush'Way, Garfield County, Colorado. The project site is
shown on Figure 1. 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 Resort Development Building, Inc. dated February 14,2018.
An exploratory boring was drilled 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 recoÍrmendations 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 conceptual at the time of our study. The proposed residence
is assumed to be a two-story structure above a walkout basement with an attached garage and
slab-on-grade lower floors. Grading for the structure is assumed to be relatively minor with cut
depths befween about 3 to l0 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 properfy is vacant and vegetated with juniper trees, sage brush, grass and weeds. The front
parr of the lot was cut for the road construction and is devoid of vegetation. The ground surface
in the building envelope slopes moderately dorrrn to the west.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies the Pinyon Mesa
Development. These rocks are a sequence of gypsiferious shale, fine-grained sandstone/siltstone
and limestone with some massive beds of gypsum. There is a possibility that massive gypsum
deposits associated with the Eagle Valley Evaporite underlie portions of the property.
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, sinkholes have been
observed scattered throughout the lower Roaring Fork River valley.
No evidence of subsidence or sinkholes was observed on the property or encountered in the
subsurface materials, however, the exploratory boring was 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 at the
site throughout the service life of the proposed structure, in our opinion is low, however the
o\¡/ner should be 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 field exploration for the project was conducted on February 20,2A78. One exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The
boring was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted
cME-458 drill rig. The boring was logged by a representative of H-P/I(umar.
Samples of the subsoils were taken with 1%-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-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
H.PVKUMAR Project No. 18-7-161
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shown on the Log of Exploratory Boring, Figure 2. The samples were retumed to our laboratory
for review by the project engineer and testing.
SUBSURJ'ACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils, below a thin root zone, consist of mixed sand ærd clay with gravel to about 12 feet
underlain by silty clayey sand and gravel with cobbles to about 30 feet where weathered siltstone
bedrock was encountered to the boring depth of 40 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and gradation analyses. Results of gradation analyses performed on
relatively small diameter drive samples (minus lYr-inchfraction) of the gravelly subsoils are
presented on Figure 4. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils \ryere slightly
moist.
FOUNDATION BEARING CONDITIONS
The upper sand and clay soils encountered at typical shallow foundation depth mainly tend to
settle when they become wetted. A shallow foundation placed on these soils will have a risk of
settlement if the they become wetted and care should be taken in the surface and subsurface
drainage around the house to prevent the soils from becoming wet. It will be c¡itical to the long-
term performance of the structure that the recommendations for surface grading and drainage
contained in this report be followed. The amount of settlement, if the bearing soils become wet,
will mainly be related to the depth and extent of subsurface wetting but may result in settlements
of around 1 to2 inches which could cause building distress. Mitigation methods such as a deep
foundation þiles or piers extending down at least 30 feet below existing ground surface) or
removing and replacing the bearing soils as compacted structural fîll could be used to support the
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proposed house with a lower risk of settlement. If a deep foundation is desired, we should be
contacted to provide additional design recommendations.
DESIGN RECOMMENDATIONS
FOLTNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils at basement level with a risk of movement. Compacted structural fill should be
used for shallow depth footings, such as for the lower walkout side of the building and at the
ga¡age.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the natural soils at basement level or on compacted structural
fill in shallow cut areas should be designed for an allowable bearing pressure of
1,500 psf. The walkout side of the basement and garage footing areas should be
sub-excavated down about 6 to 1 0 feet below existing ground surface and the
excavated soil replaced with compacted structural fill back to design grade. The
sub-excavated areas should extend down at least 3 feet below the footing bearing
2)
level. Based on experience, v/e expect initial settlement of footings designed and
constructed as discussed in this section will be about I inch or less. Additional
settlements of about %to l inch could occur if the bearing soils are wetted. A %
increase in the allowable bearing pressure can be taken for toe pressure of
eccentrically loaded (retaining wall) footings.
The footings should have a minimum width of 20 inches for continuous walls and
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
3)
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of foundations at least 36 inches below exterior grade is typically used in this
area.
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.
The foundation should be configured in a box like shape to help resist differential
movements. Foundation walls acting as retaining structures should also be
designed to resist lateral earth pressures as discussed in the "Foundation and
Retaining Vy'alls" section of this report.
5) The topsoil, sub-excavation depth and any loose or disturbed soils should be
removed below the foundation area. The exposed soils in footing areas after sub-
excavation should then be moistened and compacted. Structural fill should
consist of low permeable soil (such as the on-site sandy clay to clayey sand soils)
compacted to at least 98% of standard Proctor density within 2% of optimurn
moisture content. The structural fill should extend laterally beyond the footing
edges equal to at least Yzthe fill depth below the footing.
6) A representative of the geotechnical engineer should evaluate the fill placement
for compaction and observe all footing excavations prior to concrete placement.
FOLT¡{DATION AND RETAINING V/ALLS
Foundation walls and retaining structues 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 fine-grained 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 least 45 pcf for backfill consisting of the on-site fine-grained 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
pressures recoflrmended above assume drained conditions behind the walls and a horizontal
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backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a forurdation 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 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
backfill 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 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 absume 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, Fill placed against
the sides of the footings to resist lateral loads should be 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 topsoii, can be used to support lightly loaded slab-on-grade
construction with a settlement risk similar to the foundation if the underlying soils are wetted.
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
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based on 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 than 2%
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 soils devoid of vegetation and topsoil.
TINDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where clay soils are present, 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 below-grade construction, such as retaining walls and
basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
systern. An underdrain should not be provided around slab-at-grade garage and shallow
crawlspace areas to help limit potential wetting of bearing soils from shallow water sources.
'Ihe 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 and at least 1 foot below lowest adjacent finish gradc and sloped at a minimurn 1o/o to
a suitable gravity outlet or sump and pump. Free-draining granular material 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 size of 2 inches. The drain gravel backfill should be at
least i %feet deep. An impervious membrane such as 30 mil PVC should be placed beneath the
drain gravel in a trough 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 dwing construction and maintained at all
times after the residence has been completed:
H-PryKUMAR Project No. 18-7-161
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1) Inundation ofthe foundation excavations and underslab areas should be avoided
dwing 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 10 feet in unpaved areas and a minimum slope of 3
inches in the first i0 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with at least 2 feet of the on-site soils to
reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill. Graded surface swales should have a minimum slope of 3%.
5) Landscaping which requires regular heavy irrigation should be located at least l0
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.
LIMITATIONS
This study has been conducted in accordânce with generally accepted geotechnical engineering
principles and practices in this area atthe time of this study. '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 boring drilled at the location indicated on Figure l,
the proposed type of constmction 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 ofpractice 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 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.
H.PVKUMAR Project No. 1B-7-161
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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
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
H-P+KUMAR
Steven L. Pawlak, P.E
Reviewed by:
L
Daniel E. Hardin, P.E.
SLPlkac
H-PÈKUIMAR Projec{ No. 18-7-161
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APPROXIMATE SCALE.FEET
18-7-161 H-PryKUMIAR LOCATION OF EXPLORATORY BORING Fig, I
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BORING 1 LEGEND
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5
m
SAND AND ctÂY (SC-CL); stLTY, SCATTIREO cRAvEL T0
GRAVELLY, MEDIUM DTNSE/VERY STIFF, SL|GHTLY MO|ST, cRAy
22/12
2t/12
WC=5.6
*4=42
-200=29
16/12
WC=8,5
-200=62
82/12
\{C=5.1
DD=1 l3
*1=43
-200=37
30/6, 50/5
30/12
11 /12
Y'lÇ=27.2
-200=9.|
!4ND AND cRAvtL (sc-GC); StLTy, cLAyty, C0gBt-ts, MtDtUM
DENSI, SLIGHTLY MOIST, EASALT ROCK.
ffi yq¡r¡enrD sTLTSToNE; MEDTuM HARD tvrTn HARD LÀyERs,
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zlstlGHTLY MolsT, LIGHT BR0WN.
10
15
2A
25
50
55
40
DRIVE SAMPLT, 2-II,¡CH I.D. CALIFORNIA LINTR SAMPLE.
NOTES
THT TXPLORATORY BORING WAS DRILLED ON FTBRUARY 20,20I8 WITH A 4-INCH OIAMTTTR CONTINUOU$-FUGHT POWER
AUGER.
THT LOCATION OF THT EXPLORATORY BORING WAS IIEASURTD
APPROXIMATELY BY PACING FROM FEATURTS SHOWN ON THE
SIÏT PLAN PROVIDED.
3. THT TLEVATION OF THT EXPLORATORY BORING WÂS NOT
MEASURED AND THT LOG OF THE TXPLORATORY BORING IS
PLOTTTD TO DEPTH.
1. ÏHT EXPLORÀTORY BORING LOCAÏON SHOULO BT CONSIOERED
ACCURATI ONLY TO THE DTGREE IMPLIED SY THE METHOD
USED.
5. THT LINTS ETTWTEN MATERIALS SHOWN ON THE TXPLORATORY
BORING LOG REPRESENT THT APPROXIMATT BOUNOARITS
BEÏWTTN MATIRIAL TYPTS AND THE TRANSITIONS MAY 8I
GRÂDUAL.
6. GROUNDWATTR WAS NOT TNCOUNTERED II{ THE EORING AT THE
TIME OF DRILLING.
7. LÂBORATORY TEST RTSULTS;
WC = WATTR c0NTtNT (x) (ASTM Ð 2216);
DD = DRY DENstTy (pct) (mru o 2216):
+4 = ptRcENTAGE RtTAtNtD 0N N0. 4 SttvE (lsru o +zz);
-200 = PTRGENTAGE PASS|NG N0. 200 SIEVE (ASTM D tr10).
oRtvE sAMpLE, 1 3/S-|NCH t.0. spltT sp00N STANDARD
PTNETRATION TEST.
22¡p ÙRIYE SAMPL"E BLOW C0UNT. IND|CATES THAT 22 BLOWS 0F, A I4O-POUND HAMMTR FALUNG 30 INCHES WERT REQUIRTD
TO DRIVT THT SAMPLTR f2 INCHTS.
--+ OEPTH AT WHICH BORING cAvED.
¡
2.
1 8-7- 1 61 H-PVKUTVIAR LOG OT EXPLORATORY BORING fî9. 2