HomeMy WebLinkAboutSubsoil Study for Foundation 02.29.16G&Et~h
HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 22, SUN MEADOW F.STATES
ANTONELLI LANE
GARFIELD COUNTY
COLORADO
JOB NO. 116 013A
FEBRUARY 29, 2016
PREPARED FOR:
JUPITER OTERO
1229 STANDING DEER DRIVE
SILT, COLORADO 81652
iupiterotero@hobnail.com
Parker 303-841-i 119 • Culnrado Springs 719-633-5562 • S1kcrrhomc 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ - I -
PROPOSED CONSTRUCTION .................................................................................... -I -
SITE CONDITIONS ....................................................................................................... -2 -
FIELD EXPLORATION ................................................................................................. -2 -
SUBSURFACE CONDITIONS ...................................................................................... -3 -
FOUNDATION BEARING CONDITIONS .................................................................. -3 -
DESIGN RECOMMENDATIONS ................................................................................ - 4 -
FOUNDATIONS ........................................................................................................ -4 -
FLOOR SLABS····································································································*·····-S -
UNDERDRAil'1SYSTEM .......................................................................................... -6 -
SURFACE DRA™AGE ............................................................................................. - 6 -
LIMITATIONS ............................................................................................................... -7 -
FIGURE 1 -LOCATlON OF EXPLORATORY BORINGS
FIGURE 2 -LOGS OF EXPLORATORY BORJNGS
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 through 6 -SWELL-CONSOLIDATION TEST RESULTS
TABLE I -SUMMARY OF LABORATORY TEST RESULTS
Job No . 116 013A
PURPOSE AND SCOPE OF STUDY
This report presents the results of n subsoil study for a proposed residence to be located
on Lot 22, Sun Meadow Estates, Antonelli Lane (County Road 216), 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 professional services to Jupiter Otero dated January
25, 2015. We previously perfonned a preliminary geotechnical study for the Sun
Meadows Estates development and presented our findings in a report dated March 28,
2000, Job No . 100 169
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 sweII 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
The residence will be a one story wood frame structure located on the lot as shown on
Figure 1. Ground floors will be structurally supported over crawlspace in the living area
and slab-on-grade in the garage. Grading for the structure is expected to be relatively
minor with cut depths between about 3 to 5 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 . 116 Ol3A
SITE CONDITIONS
The lot was vacant and covered with about 1 V2 to 2 feet of snow at the time of our field
exploration. The terrain is relatively flat with a strong slope down to the southeast where
there is a moderately steep slope down along the eastern portion of the Jot which slopes
towards a creek further to the east. Elevation difference across the proposed building site
is estimated at about 2 or 3 feet. There are shallow drainage ditches along the roads
which border the lot. About 2 to 3 feet deep cuts were observed on the Jot adjacent to
road areas apparently for construction of the roadways. Vegetation on the site consisted
of grass and sagebrush.
FIELD EXPLORATION
The field exploration for the project was conducted on February 11, 2016. Two
exploratory borings were drilled at the locations shown on Figure 1 to evaluate the
subsurface conditions. The borings were drilled at the locations requested by the client
prior to the site plan being developed. The borings were advanced with 4-inch diameter
continuous flight augers powered by a truck-mounted CME45B drill rig. Graded access
up the road cut slope for the truck rig was provided by the client. The borings were
logged by a representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with 1 ll. inch l.D . spoon sampler and 2-inch
California 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 0·1586. 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.
Job No 116 013A
-3-
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils encountered, below about !h foot of organic topsoil, consisted of about 13 to
16 feet of stiff/medium dense, silt and sand overlying medium dense, silty to very silty
and typically clayey sand silty sand that extended down to the maximum depths drilled of
30Yi feet. The silty to very silty sand soils contained gravelly zones or layers and possible
cobbles.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and density, and percent finer than sand size gradation analyses. Results
of swell-consolidation testing performed on relatively undisturbed drive samples,
presented on Figures 4 through 6, indicate generally low compressibility under conditions
of natural moisture content and low light loading. The samples showed a low to moderate
collapse potential when wetted under a constant light (1,000 psf) surcharge and moderate
to high compressibility when loaded after wetting. The laboratory testing is summarized
in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils
(below the moist topsoil) were slightly moist.
FOUNDATION BEARING CONDITIONS
The soils encountered at the building site tend to settle when they become wetted.
A shallow spread footing foundation placed on these soils will have n risk of settlement
and building distress, especially if the bearing soils become wetted . Sources of wetting
include excessive irrigation near the foundation, poor surface drainage adjacent to
foundation walls and utility line leaks. The magnitude of the settlement would depend on
the depth and extent of the wetting but could be several inches and differential. Spread
footings bearing on a depth of structural fill is commonly used to reduce the risk of
settlement.
JobNo 116013A
. 4 ~
A micro-pile or helical pier foundation system could be used to provide a low risk of
building settlement and distress but will need to extend down into low compressibility
sand and gravel with cobble soils or bedrock.
Spread footings should be feasible for foundation support of the residence with a risk of
settlement. To reduce the risk of differential settlement, we recommend at least 4 feet of
structura1 fill be provided below the footings. The structural fill can consist of the on-site
soils, excluding topsoil and oversized (plus 6 inch) rocks , or a well graded granular
imported material such as road base.
Provided below are recommendat ions for a spread footing foundation system bearing on
structural fill . If recommendations for a micro-pile or helical pier foundation system are
desired, we should be contacted . A deep boring would be needed for the pile or pier
foundation design.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, the building can be founded with spread footings
bearing on at least 4 feet of properly placed and compacted structural filJ soils with some
risk of differential settlement. Precautions should be taken to prevent wetting of the
bearing soils.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on a minimum 4 feet of compacted structural fill 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 I inch or Jess . There
could be additional differential settlement if the bearing soils below the
Job No . 116 013A
-5-
structural fill were to become wetted. The magnitude of the settlement
would depend on the depth and extent of the wetting but may be on the
order of 1 Vz to 2 inches for a limited depth of wetting on the order of 10 to
15 feet.
2) The footings should have a minimum width of 18 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 48 inches below exterior grade is
typically used in this area.
4) Continuous foundation walls should be heavily reinforced top and bottom
to span local anomalies and better withstand the effects of some
differential settlement such as by assuming an unsupported length of at
least 15 feet. Foundation walls acting as retaining structures should also
be designed to resist a lateral earth pressure corresponding to an equivalent
fluid unit weight of at least 50 pcf.
5) The topsoil and the required sub -excavation depth to achieve at least 4 feet
of structural fill should be removed in the footing areas and the exposed
subgrade scarified, moistened to near optimum and compacted. Structural
fill placed below footing areas should consist of the on-site soils, or a
suitable imported granular material evaluated by the soil engineer,
compacted to at least 98% standard Proctor density at a moisture content
within about 2% of optimum. The structural fill should extend at least 2
feet beyond the edges of the footings.
6) A representative of the geotechnical engineer should observe all footing
excavations and observe placement and test structural fill compaction on a
regular basis prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-
on-grade construction with a risk of settlement mainly if the subgrade soils become
Job No . 116 013A
-6-
wetted. We should review the need for structural fill below floor slabs at the time of
construction. Providing structurally supported floors over crawlspace such as is planned
for the living areas of the residence would provide a low risk of floor movement.
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
~inch layer of road base should be placed immediately below floor slabs for support 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 12% passing the No. 200 sieve.
All filJ 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 topsoil and oversized (plus 6 inch) rocks.
UNDERDRAIN SYSTEM
A perimeter foundation drain system around shallow crawlspace areas (less than 4 feet
deep) or around floor slab-at grade areas should not be needed provided positive surface
drainage is provided away from foundation walls and the foundation wall backfill is
adequately compacted.
SURFACE DRAJNAGE
Positive surface drainage is an important aspect of the project to prevent wetting of the
bearing materials. It will be critical to the building performance to keep the bearing soils
dry. The following drainage precautions should be observed during construction and
maintained at all times after the residence has been completed:
Job No . 116 Ol3A
I) 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.
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 IO feet in unpaved
areas and a minimum slope of 3 inches in the first IO feet in paved areas.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at
least IO 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 conclusioos and recommendations submitted in this report are
based upon the data obtained from the exploratory borings drilled at the locations
indicated on Figure l, 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. H 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 borings and variations in the subsurface
conditions may not become evident until excavation is performed. H conditions
Job No . 116 013A
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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
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
HEPWORTH -PAWLAK GEOTECHNICAL. INC.
--r;;: (!, ~u...--
Tom C. Brunner, Staff Engineer
Reviewed by:
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PROPOSED
RESIDENCE
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APPROXIMATE SCALE
1·'""' 80'
LOCATION OF EXPLORATORY BORINGS Figure 1
0
5
10
15
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0 20
25
30
35
116 013A
BORING 1
15/12
WC -43
OD 97
·200 84
22/12
we 57
00 -95
26112
we 50
00-105
19/12
14/12
WC 74
OD 103
16112
16112
BORING2
20/12
22112
WC -41
DO J 95
41/12
WC 6.6
DO 105
20/12
WC 49
QO a 103
·200 -42
33/12
18/12
WC =52
DO 105
18/12
Note Explanation of symbols is shown on Figure 3.
LOGS OF EXPLORATORY BORINGS
0
5
10
15
-tf
~ a.
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20 Cl
25
30
35
Figure 2
LEGEND:
TOPSOIL; sandy silt, with roots and organics, moist, loose, dark brawn
SILT ANO SAND (ML-SM); occasionally clayey, stiff to medium dense, slightly moist, tan to light brown.
SAND (SM-SC); silty to very silty, typically clayey, with gravelly zones, possible scattered cobbles,
medium dense, slightly moist, light brown to brown.
Relatively undisturbed drive sample; 2-inch 1.0. California liner sample .
Drive sample; standard penetration test (SPT), 1 3/8inch1.0. split spoon sample, ASTM D-1586.
15/12 Drive sample blow count; indicates that 15 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
NOTES :
1 . Exploratory borings were drilled on February 11, 2016 with 4-inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
provided.
3 . Elevations of exploratory borings were not measured and the logs of exploratory borings are drawn to depth.
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 exploralOfY boring logs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free waler was encountered in the borings at the time of drilling . Fluctuation in water level may occur with lime
7. Laboratory Testing Results :
WC = Water Content (%)
DD = Dry Density (pcQ
-200 = Percent passing No. 200 sieve
116 013A LEGEND AND NOTES Figure 3
Moisture Content = 5.7 percent
Ory Density -95 pcl
Sample of : Very Sandy Silt
From: BOl'ing 1 at 4 ~ Feet
0
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1
~ Compression l----~ ~ ~ ... upon t. -..... ~ welting
fl. 2 i-
c I~ ) 0 ·c;; \ en 3 Q) a \ E 0
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10
01 10 10 100
APPLIED PRESSURE -ks!
116 013A ~
HllClworth-PalllClk Geolechnlcol
SWELL·CONSOLIDATION TEST RESULTS Figure 4
Moisture Content = 7.4 percent
Dry Density J2 103 pct
Sample of Silty Sand
From : Boring 1at19 ~Feet
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1 '-....
11, Compres st on
....-t7 upon
~ 2 ~ ......... "''"wetting
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APPLIED PRESSURE -ksl
116 013A ~ch
HEl'WOA'1'fH'AWLAK GEDIECHMCAL
SWELL-CONSOLIDATION TEST RESULTS Figure 5
Moisture Content -=-6.6 percent
Dry Density -= 105 pcf
Sample of Very Silty Clayey Sand
From: Boring 2 at 9 ~ Feet
0
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upon
'#. 2 wetting
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01 10 10 100
APPUEDPAESSURE·ksf
116 013A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 6
H~lnH-PAWLAK G~NICAL
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116 013A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT
MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY N0.200 LIMIT INDEX STRENGTH BEDROCK lYPE (%) (%)
nu ,.,0, (pcf)
SIEVE l%t (%' (PSF\
1 21h 4.3 97 84 Sandy Silt
4 1h 5.7 95 Very Sandy Silt
9 1h 5.0 105 Very Silty Sand
19112 7.4 103 Silty Sand
2 4 1h 4.1 95 Sandy Silt
9 1h 6.6 105 Very Silty Clayey Sand
14'h 4 .9 103 42 Very Silty Sand
24'h 5.2 105 Silty Sand