HomeMy WebLinkAboutSoils Report for Foundation Design 10.16.2015_p 1 Icpworrh rim! tl. Gcutcchnrc,tl, Eng
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L.-,e-cDtech Glenl4'( Springs, Color-do 81601
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Phone. 970 945 7938
HEPWORTH-PAWLAK GEOTECHNICAL Fax 970 945 5454
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE (MODEL 1947)
LOT 253, IRONBRIDGE DEVELOPMENT
EAGLE CLAW CIRCLE
GARFIELD COUNTY, COLORADO
JOB NO. 113 471R
OCTOBER 16, 2015
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 81612
iecclestone@pb1111.net
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1
BACKGROUND INFORMATION - 1 -
PROPOSED CONSTRUCTION - 2 -
SITE CONDITIONS
SUBSIDENCE POTENTIAL - 2
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
ENGINEERING ANALYSIS - 4 -
DESIGN RECOMMENDATIONS - 5 -
FOUNDATIONS - 5 -
FOUNDATION AND RETAINING WALLS - 6 -
NONSTRUCTURAL FLOOR SLABS - 7 -
UNDERDRAIN SYSTEM - 8 -
SITE GRADING - 8 -
SURFACE DRAINAGE - 9 -
LIMITATIONS - 10 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 LEGEND AND NOTES
FIGURE 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 253, Ironbridge Development, Eagle Claw Circle, 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 proposal for geotechnical engineering services to Aspen Signature Homes of
Ironbridge, LLC dated September 18, 2015. We previously conducted a preliminary
subsoil study for residences in the Villas North and South Parcels and presented our
findings in a report dated February 28, 2014, Job No. 113 471A.
A field exploration program consisting of an exploratory boring was conducted during the
preliminary subsoil study 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 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.
BACKGROUND INFORMATION
The proposed residence is located in the existing Ironbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted subsurface exploration and
geotechnical evaluation for development of Villas North and Villas South Parcels, Job
No. 105 115-6, report dated September 14, 2005, and performed observation and testing
services during the infrastructure construction, Job No. 106 0367, between April 2006
and April 2007. The information provided in the previous reports has been considered in
the current study of Lot 253.
Job No. 113471R
-2 -
PROPOSED CONSTRUCTION
The proposed residence will be a two-story, wood frame structure with structural slab
foundation and no basement or crawlspace. A post -tensioned slab foundation is expected
at this time. Grading for the structure is assumed to be relatively minor with cut and fill
depths on the order of a few feet or less. 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 proposed residence is located at the west side of the Villas South Parcel and
overlooking the 18'h Green. The natural terrain prior to development in 2006 sloped
down to the east at about 5 to 7% grade. The subdivision in this area was elevated by
filling on the order of 15 to 20 feet above the original ground surface to create a relatively
level building site with a moderate slope down along the west side to the 18`h Green.
Vegetation consists of grass and weeds.
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with
sinkholes and localized ground subsidence in the Roaring Fork River valley. A sinkhole
opened in the cart storage parking lot east of the Pro Shop located to the north of the
Villas South parcel in January 2005. Other irregular bedrock conditions have been
identified in the affordable housing site located to the west of the Villas North parcel.
Indications of ground subsidence were not observed in the Villas development area that
could indicate an unusual risk of future ground subsidence, but localized variable depths
of the debris fan soils encountered by the previous September 14, 2005 geotechnical
study in the Villas development area could be the result of past subsidence. In our
lob No. 113 47112.
-3 -
opinion, the tisk of future ground subsidence in the Villas North and South project area is
low and similar to other areas of the Roaring Fork River valley where there have not been
indications of ground subsidence.
FIELD EXPLORATION
The field exploration for the Villas project was conducted between December 24, 2013
and January 2, 2014 and consisted of drilling ten exploratory borings. The previous
exploratory Boring 1 was drilled on the adjacent Lot 252 just northwest of Lot 253 as
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 -45B drill rig.
The boring was logged by a representative of Hepworth-Pawlak 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 Log of Exploratory Boring, Figure
2. The samples were returned to our laboratory for review by the project engineer and
testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at Boring 1 is shown on Figure 2.
The subsoils encountered, consist of about 18 feet of mixed sand, silt and clay with gravel
fill overlying about 19 feet of alluvial fan deposits consisting of roughly stratified, sandy
silt and silty sand with gravel overlying dense, sandy clayey gravel and cobbles with
probable boulders at a depth of 47 feet and to the drilled depth of 51 feet. The fill soils
are medium dense and slightly moist to moist, and the underlying natural alluvial fan soils
are loose to medium dense and relatively dry to slightly moist. The underlying river
lob Na 113 471R Giegrytech
-4 -
gravel alluvium was encountered below the alluvial fan deposits in the other borings
drilled in the Villas South parcel.
Laboratory testing performed on samples obtained from the boring included natural
moisture content and density and gradation analyses. Results of swell -consolidation
testing performed on a relatively undisturbed drive sample of the natural sandy silt soils,
presented on Figure 4, indicate low compressibility under light loading and a low collapse
potential (settlement under constant load) when wetted. The laboratory testing is
summarized in Table 1.
ENGINEERING ANALYSIS
The upper 18 feet of soils encountered in the boring located on the adjacent Lot 252,
consist of fill place mainly in 2006 as part of the subdivision development. The field
penetration tests and laboratory tests performed during the study, and review of the field
density tests performed during the fill construction indicate that the structural fill was
placed and compacted to the project specified 95% of standard Proctor density. Debris
fan soils which tend to collapse (settle under constant load) when wetted were
encountered below the fill. The amount of settlement will depend on the thickness of the
compressible soils and their wetted depth. The settlement potential and risk of excessive
building distress can be reduced by compaction of the soils to a certain depth below the
foundation bearing level (as has already been done) and by heavily reinforcing the
foundation to resist differential settlements. The compaction should also extend to below
driveway and utility areas. The compacted soils can consist of the existing structural fill
used to elevate the project site. Foundation levels deeper than 5 feet below the existing
ground surface on this site are not recommended. Relatively deep structural fills will also
have some potential for long term settlement. Proper grading, drainage and compaction
as presented below in the Site Grading and Stuface Drainage sections will help reduce
the settlement risks. A heavily reinforced structural slab or post -tensioned slab
foundation designed for significant differential settlements is recommended for the
building support. As an alternative, a deep foundation that extends down into the
Job No l 13 471R
-5 -
underlying dense, river gravel alluvium and structural floor slabs could also be used to
reduce the settlement risk.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, we recommend the building be founded with a
heavily reinforced structural slab or post -tensioned slab foundation bearing on at least 15
feet of compacted structural 1111 in the Villas South parcel. If a deep foundation system is
considered for building support, we should be contacted for additional recommendations.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) A structural slab or post -tensioned slab placed on at least 15 feet of
compacted structural fill should be designed for an allowable bearing
pressure of 1,500 psf. Post -tensioned slabs placed on structural fill should
be designed fora wetted distance of 10 feet but at least half of the slab
width whichever is more. Settlement of the foundation is estimated to be
about 1 to 1V2 inches based on the long term compressibility of the fill.
Additional settlement between about 2 to 3 inches is estimated if deep
wetting of the debris fan soils were to occur. Settlement from the deep
wetting would tend to be uniform across the building/development area
and the settlement potential of the fill section should control the design.
2) The thickened sections of the slab for support of concentrated loads should
have a minimum width of 20 inches.
3) The perimeter turn -down section of the slab should be provided with
adequate soil cover above the bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typically used in this area. If a frost protected foundation is used, the
perimeter turn -down section should have at least I8 inches of soil cover.
10040, 113471R
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4) The foundation should be constructed in a "box -like" configuration rather
than with irregular extensions which can settle differentially to the main
building area. The foundation walls, where provided, 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 (if any) should also be designed to resist lateral earth pressures
as discussed in the "Foundation and Retaining Walls" section of this
report.
5) The root zone and any loose or disturbed soils should be removed.
Structural fill placed below the slab bearing level should be compacted to
at least 98% of the maximum standard Proctor density within 2 percentage
points of optimum moisture content.
6) A representative of the geotechnical engineer should evaluate the
compaction of the fill materials and observe all footing excavations prior
to concrete placement for 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 at least 50 pcf
for backfill consisting of the on-site soils. Cantilevered retaining structures which are
separate from the buildings 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 40 pcf for backfill consisting of
the on-site 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 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
lob No. 113 471R` t
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retaining structure. An underdrain should be provided to prevent hydrostatic pressure
buildup behind walls. Site walls with a maximum back slope of 2 horizontal to 1 vertical
should be designed for an active earth pressure of at least 60 pcf equivalent fluid unit
weight.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density near optimum moisture content. 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 retaining 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 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 300 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 near
optimum moisture content.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support lightly loaded slabs -on -grade separate
from the building foundation. The fill soils can be compressible when wetted and result
in some post -construction settlement. To reduce the effects of some differential
movement, nonstructural floor slabs should be separated from buildings to allow for
unrestrained vertical movement. Floor slab control joints should be used to reduce
damage due to shrinkage cracking. The requirements for joint spacing and slab
Job No. 113 471R
-8 -
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 subgrade support. This material should consist of
minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 12%
passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor density at near optimum moisture content. Required fill can
consist of the on-site 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 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 recommend below -grade construction, such as grade change site retaining
walls, be protected from welting and hydrostatic pressure buildup by an underdrain
system. An underdrain should not be provided around structural building foundation
slabs and separate slabs -on -grade.
Where installed, 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 feet deep.
SITE GRADING
Extensive grading was performed as part of the existing Villas South development.
Additional placement and compaction of the debris fan soils could be needed to elevate
the site to design grades and reduce the risk of excessive differential settlements and
building distress. In addition, the water and sewer pipe joints should be mechanically
Joh No. 113 471R ch
-9 -
restrained to reduce the risk of joint separation in the event of excessive differential
settlement. Additional structural fill placed below foundation bearing level should be
compacted to at least 98% of the maximum standard Proctor density within 2 percentage
points of optimum moisture content. Prior to fill placement, the subgrade should be
carefully prepared by removing any vegetation and organic soils and compacting to at
least 95% of the maximum standard Proctor density at near optimum moisture content.
The fill should be benched into slopes that exceed 20% grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or
flatter and protected against erosion by revegetation or other means. This office should
review site grading plans for the project prior to construction.
SURFACE DRAINAGE
Precautions to prevent wetting of the bearing soils, such as proper backfill construction,
positive backfill slopes, restricting landscape irrigation and use of roof gutters need to be
taken to help limit settlement and building distress. The following drainage precautions
should be observed during construction and maintained at all times after the residence has
been completed:
1) Inundation of the building structural slab foundation excavations should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture content and
compacted to at least 95% of the maximum standard Proctor density in
pavement and nonstructural 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. The slope
should be at least 6 inches in the first 5 feet in unpaved areas and at least 3
inches in the first 10 feet in paved areas. Drain gravel of retaining walls
should be covered with filter fabric and capped with at least 2 feet of the
on-site soils to reduce surface water infiltration. Surface swales in
landscape areas should have a minimum grade of 4%.
Job No. 11347111
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4) Roof gutters should be provided with downspouts that discharge at least 5
feet beyond the foundation and preferably into subsurface solid drain pipe.
5) Landscaping which requires regular heavy irrigation, such as sod, should
be minimized and 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.
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 boring drilled at the location 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
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 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.
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
Joh No. 113 471R
-11 -
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Steven L. Pawlak, P.E
Reviewed by:
Daniel E. Hardin, P.E.
SLP/ksw
cc: Silich Construction—John Silich (john@silichconstruclion.com)
Silich Construction — Eric Lintjer elint'er@silichconstrucIion.cv
Job No 113 471R
APPROXIMATE SCALE
1'=20'
BORING 1
(113 471A)
•
EAGLE CLAW CIRCLE
LOT 252
LTA
5992
5991
5990
LOT 253
PROPOSE&
RESIDENCE ��
FF=5994,96 \
PATIO
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,
l...I., r
�r-
r J� -��
LOT 254
595992
- 5997
-:75p80990
- 5987
CART PATH
PROPOSED
RETAINING
WALL
113471R
FiCAWORTN-PAWLAJC GEOTECHNICAL
LOCATION OF EXPLORATORY BORING
Figure 1
L
BORING 1
ELEV. = 5993'
LOT 252
0
5
10
- 15
- 20
- 25
- 30
- 35
40
NOTE: Explanation of symbols is shown on Figure 3.
32/12
38112
WC -36
DD 123
+4- 58
-200, 13
53112
18/12
WC -58
-200= 66
28/12
WC=46
CID =005
-200 52
33112
24/12
75/4
BOTTOM OF BORING 1
AT 51 FEET
Q
5
10
15
20
25
30
35
40
115 471R
H
Hepworth—Pawlnk Geotsehn[cal
LOG OF EXPLORATORY BORING
Figure 2
LEGEND:
38/12
FILL; mixed clayey silt, sand and gravel with cobbles, medium dense, slightly moist to moist, constructed mainly
in 2006.
SILT (ML); sandy to very sandy, slightly clayey, gravel layers, stiff to very stiff, slightly moist, Tight brown to
brown, slightly calcareous.
GRAVEL AND COBBLES (GM -GP): slightly silty, sandy, probable boulders, dense, moist, brown, rounded river
rock.
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM 0-1586.
Drive sample blow count; indicates that 38 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Depth al which boring had when caved when checked 3 days following drilling.
NOTES:
1. The exploratory boring was drilled on December 24, 2013 with 4 -inch diameter continuous flight power auger.
2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided.,
3. Elevation of the exploratory boring was obtained by interpolation between contours shown on the site plan provided.
4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the
method used.
5. The lines between materials shown on the exploratory boring log represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the boring at the time of drilling or when checked 3 days later. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
113 471R
N
Hepworth—Pawlak Geotnchnleal
LEGEND AND NOTES
Figure 3
Compression %
0
1
2
3
4
Moisture Content a 4.6 percent
Dry Density 105 pc(
Sample of: Very Sandy Clayey Silt
From: Boring 1 at 25 Feet
Compression
upon
wetting
01
10
APPLIED PRESSURE Isf
10
100
113 471R
H
Hepworth—Pawlok Geatechnlco!
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
Job No. 113 471R
SOIL OR
BEDROCK TYPE
Silty Sandy Gravel (Fill)
Sandy Clayey Silt 11
Very Sandy Clayey Silt 11
UNCONFINED
COMPRESSIVE
STRENGTH
(PSFI
I ATTER BERG LIMITS
u
a k
2
J
C X
5
a
PERCENT
PASSING NO.
200 SIEVE
13
t
2
0
a
4
4
C7
O
X
N
Q ]
a
w
00
In
NATURAL
DRY OENSITY
Ipcf I
123
In
0
NATURAL
MOISTURE
CONTENT
(%)
4D
oa
IC
SAMPLE LOCATION
BORING DEPTH
(It)
1 10
20
25