HomeMy WebLinkAboutSubsoils Report for Foundation DesignHEPWORTH - PAWLAK GEOTECHNICAL, INC.
October 3, 1996
Craig Morris
P.O. Box 2364
Aspen, Colorado 81612 Job No. 196 475
Subject: Report Transmittal, Subsoil Study For Foundation resign, Proposed
Residence, Lot E-21, Aspen Glen, Garfield County, Colorado
Dear Mr. Morris:
As requested, we have conducted a subsoil study for the proposed residence at the
subject site.
Subsurface conditions encountered in the exploratory borings drilled in the proposed
building area were variable. At Boring 1 to the southeast, about 3 feet of man -placed
fill overlying stiff silty clays with relatively dense coarse gravels below a depth of 6
feet were found. At Boring 2 to the northwest, the subsoils consisted of about 1/2 foot
of topsoil overlying the dense coarse gravels extending to the maximum depth drilled of
10 feet. The variable subsurface conditions and existing fill are apparently the result of
ground loss and backfilling of the sinkhole previously identified in the southeastern part
of the lot. Groundwater was not encountered in the borings at the time of drilling.
The proposed residence should be founded on spread footings placed entirely on the
natural coarse gravel subsoils and designed for an allowable bearing pressure of 2,000
psf. The site is located in a broad surface depression apparently caused by regional
subsidence and adjacent to the north of a backfilled sinkhole. The foundation should be
heavily reinforced to limit the effects of possible differential settlements.
The report which follows describes our exploration, summarizes our findings, and
presents our recommendations. It is important that we provide consultation during
design, and field services during construction to review and monitor the implementation
of the geotechnical recommendations.
If you have any questions regarding this report, please contact us.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
David A. Young
Rev. By: SLP
DAY/kw
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ... • .......... .
PROPOSED CONSTRUCTION ... .. . • ... ...... ' ..
'
SITE CONDITIONS .... • • . • . .. • • ... ' . ...............
GEOLOGIC CONDITIONS ........ • ... • • . • ... • . • .
FIELD EXPLORATION .... • ...... • . . ..
SUBSURFACE CONDITIONS ............... • . • ... .
FOUNDATION BEARING CONDITIONS ... • • . .
DESIGN RECOMMENDATIONS : .. .
5
FOUNDATIONS ...............................
FOUNDATION AND RETAINING WALLS ..... , ......
• ... • . .. 67
FLOOR SLABS .................. .....................
8
UNDERDRAIN SYSTEM ....... ..........
........ 8
SURFACE DRAINAGE ................ ........'
LIMITATIONS ............................. ..
.........
12
TABLE..................... I ........ .......
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be
located on Lot E-21, Aspen Glen, Garfield County, Colorado. The project site is
shown on Fig. 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 Craig Morris dated September 19, 1996.
A field exploration program consisting of exploratory borings was conducted to
obtain information on 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 subsoil conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a two story wood frame structure possibly over
a basement level. Ground floors will be slab -on -grade. Grading for the structure is
assumed to be relatively minor with cut depths between about 4 to 8 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 reevaluate the recommendations contained in
this report.
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SITE CONDITIONS
The site was vacant and generally non -vegetated at the time of our field work.
The site has been rough graded relatively flat including backfilling of an existing
sinkhole in the southeast part of the lot. Elevation difference is about 1 foot across the
building area and about 2 to 3 feet across the lot. Several feet of man -placed fill was
apparently placed in about the eastern quadrant of the lot to backfill the existing
sinkhole and level the lot. A drainage swale has been constructed along the north side
of the lot that outlets to the west. There is a newly constructed residence to the south
of the site on Lot E-18 and a residence under construction across Buffalo Road on Lot
E-16.
GEOLOGIC CONDITIONS
Bedrock of the Pennsylvanian -age Eagle Valley Evaporite underlies the Aspen
Glen Club. These rocks are a sequence of gypsiferous shale, fine-grained
sandstone/siltstone with some massive beds of gypsum and limestone. There is a
possibility that massive, heavily bedded 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 produce areas of localized subsidence.
During previous work in the area, several broad subsidence areas and smaller size
sinkhole areas were observed scattered throughout the Aspen Glen Club (Chen -
Northern, 1993). The sinkholes appear similar to others associated with the Eagle
Valley Evaporite in areas of the Roaring Fork Valley.
The lot is located near the edge of a broad subsidence area and an existing
sinkhole is located to the south of the building envelope which has now been backfilled.
The building envelope is setback from the sinkhole according to the recommendations
by Chen -Northern. No evidence of cavities was encountered in the subsurface
H-P GEOTECH
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materials during our study; 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 gradual subsidence or sinkholes
will not develop. The risk of ground subsidence in the building envelope of Lot E-21
throughout the service life of the proposed residence is mapped as moderate; 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 field exploration for the project was conducted on September 20, 1996.
Two exploratory borings were drilled at the locations shown on Fig. 1 to evaluate the
subsurface conditions. The borings were advanced with 4 inch diameter continuous
flight augers powered by a truck -mounted Longyear BK-51HD drill rig. The borings
were logged by a representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with 13/a 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, Fig. 2. The samples were returned to our laboratory for review
by the project engineer and testing.
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SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on
Fig. 2. The subsoils at Boring 1, located in the southeast area of the residence,
consisted of about 3 feet of man -placed fill overlying sandy silty clay to a depth of
about b feet where relatively dense coarse gravels were encountered to the depth drilled
of 71h feet. The fill was generally firm and the clay was stiff to very stiff and
calcareous. At Boring 2, located in the northwest area of the residence, the subsoils
consisted of about lh foot of organic topsoil overlying by relatively dense coarse
gravels to the maximum depth drilled of 10 feet. Drilling in the dense gravels with
auger equipment was difficult due to the cobbles and boulders and drilling refusal was
encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included
natural moisture content and density. Results of swell -consolidation testing performed
on a relatively undisturbed drive sample of the natural clay soils, shown on Fig. 4,
indicate the clays are slightly compressible under conditions of loading and wetting.
The laboratory testing is summarized in Table I.
No free water was encountered in the borings at the time of drilling and the
subsoils were slightly moist to moist.
FOUNDATION BEARING CONDITIONS
Subsurface conditions encountered on the lot are variable and apparently the
result of ground loss and backfilling of the sinkhole located in the southeastern part of
the lot. The natural clays encountered in Boring 1, generally have low compressibility
but bearing on different materials will increase the risk of differential settlement and
distress to the building. The underlying gravels appear relatively dense and should
have low settlement risk to lightly loaded footings.
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsoil conditions encountered in the exploratory borings and
the nature of the proposed construction, we recommend the building be founded with
spread footings bearing entirely on the natural coarse gravels below the existing fill and
clay soils. The foundation should be heavily reinforced to limit the effects of possible
long term differential settlement related to the regional subsidence conditions.
The design and construction criteria presented below should be observed for a
spread footing foundation system.
1) Footings placed on the undisturbed natural coarse gravels should be
designed for an allowable soil bearing pressure of 2� O000sf. Based on
experience, we expect initial settlement under building loads of footings
designed and constructed as discussed in this section will be less than
1 inch.
2) The footings should have a minimum width of 16 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 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
15 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) All existing fill, topsoil, clay soils and any loose or disturbed soils
should be removed and the footing bearing level extended down to the
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relatively dense natural coarse gravels. This may require overexcavation
for shallow, non -basement, footing areas.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION 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 40 pcf
for backfill consisting of the on -site granular 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 35 pcf for backfill
consisting of the on -site granular soils. The wall backfill should not contain rocks
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 a moisture content near optimum. Backfill in
pavement and walkway areas should be compacted to at least 95 % 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
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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 0.5.
Passive pressure of compacted backfill against the sides of the footings can be
calculated using an equivalent fluid unit weight of 400 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 sand and gravel
compacted to at least 95 % of the maximum 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. The existing fill at the site could be of variable
density and should be removed below slab areas. To reduce the effects of some
differential movement, non-structural 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 inches of
free -draining gravel should be placed beneath basement level slabs to act as a leveling
course and 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.
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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 sands and gravels devoid of vegetation, topsoil and
oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our
experience in the area that local perched groundwater may 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
basement levels and retaining walls, 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 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 grade and sloped at a minimum 1 % to a suitable gravity outlet. 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 drain gravel backfill should be at least 11/z feet deep.
SURFACE DRAINAGE
. Surface water should not be allowed to pond on the property which could
aggravate the potential for subsurface subsidence. 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.
-- ---- H-P GEOTECH
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 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 6 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in paved areas.
Granular wall backfill should be capped with about 2 feet of the on -site
clayey soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of
all backfill.
LMTATIONS
This study has been conducted in accordance with generally accepted
geotechnical engineering principles and practices in this area at this time. We make no
other warranty either expressed 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 Fig. 1, the proposed type of construction and our
experience in the area. 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
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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.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
David A. Young
Reviewed By:
Steven L. Pawlak, P.E.
DAY/kw
REFERENCE
Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat
Design, Aspen Glen Development, Garfield County, Colorado, prepared for
Aspen Glen Company dated May 28, 1993, Job No. 4 112 92.
H-P GEOTECH
l
LO T E 26
SED
,NCE
BUILDING
ENVELOPE
f
BORING I
/ j
1
LOT E 21
j LOT E 22
%RIES
APPROXIMATE SCAL j BENCH MARK -PROP.
let 30, CORNER PIN;ELEV. _
/ 100.0', ASSUMED-
.
BUFFALO
— PAWOCATION OF EXPLORATORY80RINGS Fig. I
19g 475�HEPWORTH
GEOTECHNICAL, "INC.
105
100
L 90
BORING 1 BORING 2
ELEV. = 99.4' ELEV. = 100.6'
ASSUMED APPROXIMATE MAIN FLOOR LEVEL
n V.
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rq.
WC-17.9
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32112
DD=110
b _.
17112
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ASSUMED APPROXIMATE BASEMENT
n0. DD-113
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FLOOR LEVEL
o
Note: Explanation of symbols is shown on Fig. 3.
105
100
95 ___.1 uu
90 __j
196475 HEPWORTH - PAWLAK LOGS OF EXPLORATORY BORINGS Fig- 2
GEOTECHNICAL, INC.
LEGEND:
FILL; man -placed sandy silty clay, firm, moist, red -brown.
TOPSOIL; organic sandy silty clay with roots, soft, moist, dark red -brown.
CLAY ( CL ); low plasticity, silty, sandy, stiff to very stiff, moist, red -brown, moderately calcareous.
U
GRAVEL ( GP - GM ); with cobbles and boulders, sandy, slightly silty, dense, slightly moist
reddish brown, rounded
Relatively undisturbed drive sample; 2-inch I.D. California liner sample.
l:
86
Drive sample; standard penetration test (SPT), 1 3/8-inch I.D. split spoon sample, ASTM D-15
I
Drive sample blow count; indicates that 32 blows of a 140-pound hammer falling 30 inches were required
32/12 to drive the California or SPT sampler 12 inches.
IPractical rig refusal
NOTES:
mber 20, 1996 with a flinch diameter continuous flight power auger.
1. Exploratory borings were drilled on Septe
2. Locations of exploratory borings were measured approximately by pacing from features shown on
the site plan provided.
easured by instrument level and refer to the Bench Mark
3. Elevations of exploratory borings were m
shown 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 transitions may be gradual.
6. No free water was encountered in the borings at the time of drilling.
Fluctuations in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
196 475 HEPWORTH - PAWLAK LEGEND AND NOTES
3
GEOTECHNICAL, INC.---u--
I Moisture Content = 14.8 percent
,T7
' pry Unit Weight = 113 pcf
I ` Sample of: Sandy Silty Clay
` } From: Boring 1 at 5 Feet
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HEPWORTH - PAWLAK SWELL- CONSOLIDATION TEST RESULTS
GEOTECHNICAL, INC.
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HEPWORTH-PAWLAK GEOTECHNICAL, INC. 50Glenwood Springs, CO 81601
November 18, 1996 Fax 970 945-8454
Phone 970 945-7988
Craig Morris
c/o Demco Construction
Attn: Gary Hisel
0187 Fairway Lane Job No. 196 475
Glenwood Springs, Colorado 81601
Subject: Observation of Excavation, Proposed Residence, Lot E-21, Aspen Glen,
Garfield County, Colorado.
Gentlemen:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on November 15 and 18, 1996 to evaluate the soils
exposed for foundation support. The findings of our work and recommendations for
the foundation design are presented in this report. We previously conducted a subsoil
study for design of foundations at the site and presented our findings in a report dated
October 3, 1996, Job No. 196 475.
The proposed construction is generally similar to that discussed in our previous report.
The building will be two stories of frame construction over a basement level in the
western portion. The ground floor levels of the middle and eastern portions will be
slightly above the existing ground surface. The basement and garage floors will be
slab -on -grade. The middle portion of the building will have the lower level floor
structurally supported over crawlspace.
At the time of our visit to the site on November 15, the foundation excavation was
nearly complete but snow was limiting progress. When we returned on November 18,
the excavation was complete and had been cut in several levels from 1 to 8 feet below
the adjacent ground surface. The cuts were about 7 to 8 feet deep in the basement area
to the west and 1 to 5 feet deep in the crawl space and garage areas to the east. The
soils exposed in the bottom of the excavation consisted of silty to clean sandy gravel
with cobbles. The clean gravels and cobbles were in the deeper cuts for the basement
and contained scattered small boulders. The silty gravels and cobbles were in the
shallower cuts and along the eastern and southeastern building wall lines where the
footing areas had been cut to just below the silty clays. Depth to the course gravels at
the site was generally shallow, deepening rapidly to the east -south east towards the
previously mapped sinkhole. Up to about 4 to 8 inches of loose disturbed soils were
observed in the footing areas from the excavation process. No free water was
encountered in the excavation and. the soils were slightly moist to moist. A shallow
depth of water was flowing in the drainage ditch along the north side of the lot.
The soil conditions exposed in the excavation are consistent with those previously
encountered on the site and suitable for support of spread footings designed for the
recommended allowable bearing pressure of 2,000 psf. Loose and disturbed soils
should be compacted or removed in the footing areas to expose the undisturbed natural
Craig Morris
November 18, 1996
Page 2
coarse gravels. The bearing soils should be protected from freezing until backfilled for
frost cover. Surface drainage should divert water away from the sinkhole area on the
east and southeast sides of the building. Other recommendations presented in our
previous report which are applicable should also be observed.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and the previous subsurface exploration at the
site. Variations in the subsurface conditions below the excavation could increase the
risk of foundation movement. We should be advised of any variations encountered in
the excavation conditions for possible changes to recommendations contained in this
letter.
If there are any questions or if we may be' of further assistance, please let us know.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
David A. Young
Reviewed by:
Steven L. Pawlak, P.E.
DAY/ro
H-P GEOTECH