HomeMy WebLinkAboutSoils Report 06.09.2014•
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HEPWORTH-P/ NLAK GEOTECHNICAL
GRANTOR:
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AARON M. KIDDER
SUBSOIL STUDY
FOR FOUNDATION DESIGN
STATE OF ) PROPOSED RESIDENCE
LOT 12, 'ION MESA SUBDIVISION, FILING 1
COUNTY OF 8099 SAGE MEADOW ROAD
GARFIELD COUNTY, COLORADO
Acknowledged before me this — day of
2014, by AARON M.
KIDDER.
Witness my hand and official seal.
My Commission expires: JQB 114 176A
JUNE 9, 2014
Notary Public
PREPARED FOR:
KATIE MANN
3 SEVEN OAKS ROAD
GLENWOOD SPRINGS, COLORADO 81601
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY -1 -
PROPOSED CONSTRUCTION r 1
SITE CONDITIONS - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 3 -
DESIGN RECOMMENDATIONS r 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS r 5 -
FLOOR SLABS - 6
UNDERDRA1N SYSTEM - 7 -
SURFACE DRAINAGE r 7 -
LIMITATIONS - 8 -
FIGURE 1 -LOCATION OF EXPLORATORY BORING
FIGURE 2- LOG OF EXPLORATORY BORING
FIGURE 3- LEGEND AND NOTES
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 12, Pinyon Mesa Subdivision, Filing 1, 0099 Sage Meadow Road, 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 Katie Mann
dated May 15, 2014. We previously performed preliminary geotechnical engineering
studies for the subdivision development and presented our findings in reports dated
November 11, 2005 and April 10, 2006, Job No. 105 652.
An exploratory boring was drilled on the lot to obtain information on the subsurface
conditions. Samples of the subsurface materials obtained during the field exploration
were tested in the laboratory to determine their classification and other engineering
characteristics. The results of the field exploration and laboratory testing were analyzed
to develop recommendations for foundation types, depths and allowable pressures for the
proposed building foundation. This report summarizes the data obtained during this study
and presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface
conditions encountered.
PROPOSED CONSTRUCTION
Development plans for the lot were not available at the time of our study. The residence
will generally consist of a 2 story, wood frame structure over crawlspace or basement
areas with an attached garage and slab -on -grade floor located within the building
envelope shown on Figure 1. Grading for the residence is assumed to be relatively minor
with cut depths between about 2 to 8 feet. We assume relatively light foundation
loadings, typical of the assumed type of construction.
If building location, grading or loading information changes from that described above,
we should be notified to re-evaluate the recommendations presented in this report.
Job No. 114176A �#z
- 2 -
SITE CONDITIONS
The lot was vacant at the time of our field exploration. The lot is located an the north
(uphill) side of Sage Meadow Road as shown on Figure 1. The ground surface is
relatively flat with a gentle slope down to the south. There is about 2 to 3 feet of
elevation difference across the building envelope. Vegetation consists mainly of sage
brush.
FIELD EXPLORATION
The field exploration for the project was conducted on May 23, 2014. One exploratory
boring was drilled on the lot as shown on Figure 1 to evaluate the subsurface conditions.
The boring was advanced with flinch 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 wore driven into the subsurface materials 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 and hardness of the bedrock. 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 profile encountered in the boring is shown on Figure 2.
The subsoils, below about g foot of organic topsoil, consist of about 2% feet of sandy
clay and silt, and silty sand and gravel with rock fragments to probably cobble size.
Siltstonelclaystone bedrock with sandstone zones or layers was encountered in the boring
at a depth of about 15 feet down to the maximum explored depth of 41 feet below the
lob No. 114176A
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existing ground surface. Nearby borings drilled as part of our previous studies
encountered similar subsurface profile and typically deeper clay and silt soils.
Laboratory testing performed on samples obtained from the boring included natural
moisture content and density, percent finer than No. 200 sieve (silt and clay fraction)
gradation analysis and liquid and plastic limits. The soils were generally too coarse to
obtain undisturbed samples for swell -consolidation testing. The liquid and plastic limits
testing indicate the bedrock is low plasticity. The laboratory testing is summarized in
Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils and
bedrock were slightly moist.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa
subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and Limestone. There is a possibility that
massive gypsum deposits associated with the Eagle Valley Evaporite underlie the lot.
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, several
sinkholes were observed scattered throughout this part of Garfield County. These
sinkholes appear similar to others 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 evidence of
cavities was 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 on Lot 12 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. 114 176A
-4 -
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, the proposed residence can be founded with spread
footings bearing on the natural soils with some risk of differential foundation settlement.
The clay and silt soils encountered to a depth on the order of 3 feet are compressible
when loaded and wetted and should be removed from below foundation and slab areas of
the residence.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
I) Footings placed on the undisturbed natural sand and gravel 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. The
soils could be compressible after wetting under load and there could be
some additional post -construction differential foundation settlement of
about i4 to 1 inch if the bearing soils are wetted. The bearing conditions
should be further evaluated at the time of construction. Precautions should
be taken to prevent post -construction wetting of the bearing soils.
2) Footings should have a minimum width of 20 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 14
feet. Foundation walls acting as retaining structures should also be
Job No, 114176A N�
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designed to, resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) The topsoil, clay and silt soils and loose or disturbed soils should be
removed and the footing bearing level extended down to undisturbed
natural granular soils. The exposed soils in footing areas should then be
moistened and compacted prior to constructing footings. Foundation
concrete should contain sulfate resistant cement and be air entrained.
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 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 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 soils. Backfill should not contain topsoil, vegetation or oversized rock.
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 retaining walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill 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
Job No. 114176A
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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 fooling on the foundation materials and passive earth pressure
against the side of the footing. Resistance tosliding at the bottoms of the footings can be
calculated based on a coefficient of friction of 0.40. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 350 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
ofpassive 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.
FLOUR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -
on -grade construction. The upper clay and silt soils are compressible when wetted and
preferably should be removed to limit the risk of slab settlement and distress mainly if the
subgradc 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 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 95% of
maximum standard Proctor density at near optimum moisture content. Required fill can
Job No. 114 176A G Loch
_7_
consist of the on-site soils or imported granular soils (such as road base) devoid of
vegetation, topsoil and oversized rock.
UNDEIZDRAIN 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 retaining walls and
basement areas, be protected from wetting and hydrostatic pressure buildup by a
perimeter underdrain system. The drain system should not be constructed around garage
areas or shallow crawlspaces 4 feet or less in depth. Backfill should be adequately
compacted and the surface sloped to drain away from the residence,
Where provided, the drains should consist of drainpipe 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 or sump and pump. 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 1% feet deep.
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 maximum standard Proctor density in
pavement and slab areas and to at least 90% of the maximum standard
Proctor density in landscape areas.
Job No 114 176A
-8-
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 I2 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 backfill should be 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.
5) Sprinkler heads and landscaping which requires regular heavy irrigation,
such as sod, should be 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 assumed 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 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
Job No. 114 176A
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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,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Steven L. Pawlak, P.E.
Reviewed by:
Daniel E. Har
S LP/ksw
tn, P.E.
lob No. 114 176A
cG61Stech
APPROXIMATE SCALE
1' 20'
LOT 11
14 176A
OPEN SPACE
LOT 12
BORING 1
•
L. BUILDING ENVELOPE r.1
1
LOT 13
SAGE MEADOW ROAD
H
31.pworth—Fairlok Geo tech/1[ml
LOCATION OF EXPLORATORY BORING
Figure 1
25
30
35
40
114176A
BORING 1
12/12
WC=10.6
00=67
200=67
23/12
102/10
WC=2,9
-200=24
100/5
WC=6.5
1wL=23
P1=3
86/12
50/1
50/8
NOTE: Explanatlon of symbols Is shown on Figure 3.
Gagtech
Hera worth—PcrA4k G•atorlrnkal
LOG OF EXPLORATORY BORING
0
5
10 _
15
20
Q.
0
25
30
35
40 1.16
Figure 2
LEGEND;
® TOPSOIL; organic sandy silt and clay, soft, brown.
CLAY AND SILT (CL -ML); sandy, stiff, slightly moist, light brown.
SAND AND GRAVEL (SM -GM); silty, possible cobbles, medium dense, slightly moist, mixed brown, calareous.
SILTSTONE/CLAYSTONE BEDROCK; sandstone layers/zones, hard, slightly moist, Tight brown, Eagle Valley
Evaparite.
2
Sn
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 -1586.
23/12 Drive sample blow count; indicates that 23 blows of a 140 pound hammer falling 30 Inches were
required to drive the California or SPT sampler 12 inches.
NOTES:
1. The exploratory boring was drilled an May 23, 2014 with a 4 -inch diameter continuous flight power auger.
2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan
provided.
1 The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth.
4. The exploratory boring location and elevation should be considered accurate only to the degree Implied by the
method used.
5. The Ilnes 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. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pal)
-200 = Percent passing No. 200 sieve
LL a Liquid Limit (%)
PI at Plasticity Index (90
114176A
11
tiipwownt•PAWLAK GLOTicHIU AL
LEGEND AND NOTES
Figure 3
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE OCATIDN
BORING
DEPTH
Ltd
NATURAL NATURAL
MOISTURE DRY DENSITY
CONTENT
GRADATION
GRAVEL SAND
(%) 1%)
ATEERB
PERCENT
PASSING NO. UQU10 UNIT
200 SIEVE
1
2 10.6 87
67
(Kl
10 2.9
15 & 20
combined
24
6.5
4
23
Job No. 114 176A
RG LIMITS
UNCONFTNEO
SOIL OR
BEOROCKTYPE
PLASTIC
INDEX
(%}
COMPRESSNE
STRENGTH
(PSII
Sandy Silt and Clay
_
Silty Sand and Gravel
3
Siltstone/Claystone