HomeMy WebLinkAboutSoils Report 07.18.2006G&ech
HEPWORTH - PAWLAK GEOTECHNICAL
Hepworth-Pawlak Gcarerhniml, lac.
5020 Cnunry Road 154
Gleamed Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-854
ernafi: bpgeo@hpporerh.cora
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 22, CERISE RANCH
LARKSPUR DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 106 0475
JULY IS, 2006
PREPARED FOR:
WALKER CONSTRUCTION
ATIN: IAN WALJR
600 E. HOPKINS AVENUE, SUI'T'E 203
ASPEN, COLORADO 81611
Parker • 303-841-7119 • Colorado Springs 719-633;5562 *. Si.verth»rrie 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY -1-
PROPOSED CONSTRUCTION -1-
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS = 5 -
FLOOR SLABS -- 6 -
UNDERDRA IN SYSTEM • - 7 -
SURFACE DRAINAGE .. - 7 -
LIMITATIONS - 8 -
FIGURE 1- LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE.1- SUMMARY OF LABORATORY TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located
on Lot 22, Cerise Ranch, Larkspur Drive, 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 Walker Construction dated May 16, 2006.
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 types, depths and allowable pressures for the proposed building foundation.
This report summarizes the data abtained 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
The proposed residence will be a 1 story structure over a walkout basement level with an
attached garage located in the area of the exploratory borings shown on Figure 1. The
basement floor and attached garage will be -slab -on -grade. Grading for the structure will
to be relatively minor without depths between about 3 to 10 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.
Job No. 106 0475
SITE CONDITIONS
Lot 22 was vacant at the time of our field exploration and is looted on the south side of
Larkspur Drive, just west of the cul-de-sac and intersection with Bluestream Court. The
ground surface in the proposed building area is relatively flat with a gentle tb moderate
slope down to the southeast. An abandoned irrigation ditch crosses the south part of the
building envelope. An active irrigation ditch is located just below the building envelope
and was flowing at the time of our field exploration. Vegetation consists of grass and
weeds. Eagle Valley Evaporite is visible on the valley hillside to the north..
SUBSIDENCE POTENTIAL
Bedrock ofthe Pennsylvanian age Eagle Valley Evaporite underlies the Cerise Ranch
Subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siftstone 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. Sinkholes have been observed
scattered throughout the Cerise Ranch Subdivision and appear similar to those associated
with the Eagle Valley Evaporite in areas of the Roaring Fork River valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidience of •
cavities was encountered in the subsurface materials; however, the exploratory borings
were relatively shallow, for foundation design only. Based on our present lmowleclge'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 22 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.
Job No. 106 0475
-3 -
FIELD EXPLORATION
The field exploration for the project was conducted on June 12, 2006. 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 CAME -45 track -mounted drill rig. The borings were logged by a
representative of Hepworth-PawIak Geotechnical, Inc.
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-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 ofthe subsurface conditions encountered at the site are shown on Figure 2.
The subsoils, below about one foot of topsoil, consist mainly of stratified sand, silt and
clay with scattered gravel at Boring 1 overflying relatively dense, silty sandy gravel at a
depth of 25 feet in Boring 1 and 1754 feet in Boring 2.
Laboratory testing performed on samples obtained froin'the borings included natural -
moisture content, density and percent finer than No. 200 sieve size (silt and clay fraction)
gradation analysis. Results of swell -consolidation testing perfo®ed on relatively
undisturbed drive samples of the sandy clay soils, presented on Figures 4, indicate
moderate compressibility under conditions of loading and wetting. The laboratory testing
is summarized in Table 1.
Job No. 106 0475
-4 -
Groundwater was measured in the borings at depths of about 10 feet and 17 feet when
checked on June 28, 2006. The upper soils were slightly moist to moist and wet with
depth.
FOUNDATION BEARING CONDITIONS
Based on the subsoil conditions encountered in the borings, a spread footing foundation
bearing on the upper Band, silt and clay soils appears feasible with some risk of
differential settlement. A deep foundation (such as driven piles) which extends down to
the relatively dense gravel subsoils could be used to provide a moderate load capacity and
a low settlement risk. We should be contacted if a deep foundation is proposed.
DESIGN RECOMMENDATIONS
FOUNDATIONS
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 upper natural soils. The excavation should be kept relatively
shallow to avoid potential groundwater and soft soil impacts.
'The design and constriction criteria presented below should be observed for a spread
footing foundation system.
• 1) Footings placed on the upper natural soils 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 about 1 inch or less. There could be about 'A inch of
additional differential settlement if the bearing soils become wetted. _
2) The footings ah.ould have a minin nn width. of 18 inches for continuous
walls and 2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be lirovided
with adequate soil cover above their bearing elevation for frost protection.
Job No. 106 0475 • G h
-5 -
Placement of foundations at least 36 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 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 of this report.
5) The topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the natural soils. The exposed
souls in footing area should then be moisture adjusted to near optimum and
compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAIN NG WALLS
Foundation walls and regaining structures which are laterally supported and can be
expected to undergo only a slight amount of deflection should be designed for a later-a.I
earth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf
for backfill 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 earthpressure condition should be .designed for a lateral earth pressure computed
on the basis of an equivalent fluid unit weight of at least 45 pcf far backfill consisting of
the on-site soils. Backfill should not contain vegetation, topsoil or oversized rock. •
All foundation andretaining• structures should be -designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and
equipment The pressures recommended above assni ne 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 urnderdrain should be provided to prevent hydrostatic pressure
buildup behind walls:
Job No. 106 0475
-6 -
Back ill 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
wallcway areas should be compacted to at Ieast 95% of the maximum standard Proctor
density. Care should be taken not to overcompact the baclftll or use large equipment
near the wall, since this could cause excessive lateral pressure on the wall. Some
settlement of deep foundation wall bark ilt should be expected, even if the material is
placed correctly, and could result in distress to facilities constructed on the barlrfill.
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 hottor_ns of the footings can be
calculated based on a coefficient of friction of 0.35. Passive pressure of compacted
baclffill against the sides of the footings can be calculated using an equivalent fluid unit
weight-of300•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. Fill placed against the sides of the footings to resist lateral loads
should be compacted to at least 95% of the.maximum standard Proctor density at a
moisture content nearoptinium.
FLOOR SLABS
The natural ori -site soils, exclusive of topsoil, are suitable to support lightly loaded slab -
on -grade construction. The soils are compressible and slab settlement and/or distress is
possible, especially if the subgrade 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 unrestrainedvertical movement 11oor slab
control joints should be used to reduce damage due to shrinkage ung, 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 free-
d -raining 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.
Job No. 106 0475
-7 -
All fill materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor density at a moisture content near optimum. Required fill can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock,
UNDERDRAIN SYSTEM
Although free water was encountered in the borings below the probable depth of
excavation for the basement level, 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 reconunend
below -grade construction, 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 bacldlll
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
grade and sloped at a minimum 1% to a suitable gravity outlet. Free -draining granular
material used in the u nderdrain system should contain lass than 2% passing this No. 200
sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The
drain gravel baokfilt should be at least 1% feet d la .111 :. , such as
20 or 30 mil PVC liner, should be placed bene eth 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 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 95% of the maximuun standard Proctor density in
Job No. 106 0473
-8 -
pavement and slab areas and to at least 90% 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 10 feet in paved areas.
Free -draining wall back ill should be capped with about 2 feet of the on-
site finer graded soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill
5) Irrigation sprinkler heads and landscaping which requires regular heavy
irrigation, such as sod, 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 from the exploratory borings drilled at the locationt
indicated on Figure 1, the proposed type 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
conceded 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 byothers of our information. As the
project evolves, we should provide continued consultation. and field services during
Job No. 106 0475
-9 -
construction to review and monitor the implementation of our recommendations, and to
verify that the recommendations have been appropriately interpreted. Siccant design
changes may.require additional..analyeis 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,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Trevor L. Knell, P.E.
Reviewed by.
Steven L. Pawlal, P.E.
T KJksw
Job No. 106 0475
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106 0476 jy~Gtech I LOCATION OF EXPLORATORY BORINGS I FIGURE 1
8380
8375
6370
.�... 6365
6360
6355
6350
BORING 1
ELEV.=6381'
BORING 2
ELEV.=6373'
15/12
WC=15.8
DDm111
5/12
15/12
WC=,3b.0
OD= 93
Note: Explanation of symbols Is shown on Figure 3.
6385
6380
6375
6370
6385 _
6360
106 0475
G6istec„
EPWORTHPAWLAK GEOTECF4NICAL
LOGS OF EXPLORATORY BORINGS
FIGURE 2
LEGEND:
2 TOPSOIL; silty day, soft, slightly moist, Tight brown, organic.
7
51/12 the California or SPT sampler 12 Inches.
CLAY AND SILT (CL -ML); sandy, medium stiff to hard, slightly moist to moist with depth, tan and brown to grayish
brown.
SAND AND CLAY (SC -CL); silty, sandy to very sandy, scattered gravel, medium dense/stiff, slightly moist to wet
below the groundwater, tan to brown.
GRAVEL (GM); sandy, silly, medium dense to dense, wet. brown to grayish brown.
Relatively undisturbed drive sample; 2 -Ind LD. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 Inch I.D. split spoon sample, ASTM D-1586.
Drive sample blow count; Indicates that 51 blows of 140 pound hammer falling 30 inches were required to drive
0 16 Free water level €n
boring and number of days following drilling measurement was taken,
--D. Depth boring caved when measured on June 28, 2008.
NOTES:
1. Exploratory borings were drilled on June 12, 2006 with 4 -inch diameter continuous flight power sugar.
2. Locations of exploratory borings were measured approximately by pacing from building envelope corners shown on
Figure 1.
3. Elevations of exploratory borings were obtained by interpolation between contours shown on Figure 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 an the exploratory boring Togs represent the approximate boundaries between
material types and transitions may be gradual.
6. Water level readings shown on the logs were made at the time and under the conditions Indicated. Fluctuations in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content ( 96 )
DD - Dry Density ( pot)
-200 Percent passing No. 200 sieve
106 0475
Gvaech
PrWQ1ill+PAWLAK SIEDTCHNEAL
LEGEND AND NOTES I FIGURE 3
1
w
8 4
5
0.1
r
Moisture Content = 15.8 percent
Dry Density = 114 pci
Sample of: Sandy Clay
From: Boring 2 at 5 Feet
Nk\,
L
` No movement
upon wetting
1.0 10
APPLIED PRESSURE (ksf )
100
Moisture Content = 30.0 percent
Dry Density = 93 pof
Sample of. Sandy Clay
From: Boring 2 et 15 Feet
No mavrnent
upon wetting
L
0.1
106 0475
1
1.0 10
APPLIED PRESSURE ( kat
I -fl PWURTFEPAWLAKdE:]0 ECFMFFCAI_
1
fi
SWELL-CONSOUDAT1ON TEST RESULTS
i00
FIGURE 4
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No.106 0475
SAMPLE LOCATION
BORING
NO.
DEPTH
NATURAL
MOISTURE
CONTENT
•c+� r%a
1❑
NATURAL
DRY
DENSITY
(Dcn
GRADATION
GRAVEL SAND
PERCENT
PASSING
NO. 200
SIEVE
ATTERI ERG LIMITS
UQUID
LIMIT
Hsy
UNCONFINED
PLASTIC i COMPRESSIVE
INDEX STRENGTH
1961 (PSF)
SOIL
TYPE
14.5
108
63
2
5
Clayey silt and sand with
graves
15.8
114
15
30,❑
93
Sandy clay
Sandy clay