HomeMy WebLinkAboutSoils Report 07.13.2016H -PKU MAR
Geotechnlcal Engineering I Engineering Geology
Materials Testing 1 Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 72, SPRING RIDGE RESERVE PUD, PHASE 4
TBD HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 16-7-121
JULY 13, 2016
PREPARED FOR:
CRAIG WHITLOCK
P.O. BOX 1757
GLENWOOD SPRINGS, COLORADO 81602
(sitewvest@msn.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 2 -
FOUNDATION BEARING CONDITIONS - 3 -
DESIGN RECOMMENDATIONS - 3 -
FOUNDATIONS - 3 -
FOUNDATION AND RETAINING WALLS - 4 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 AND 5 - 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 at
Lot 72, Spring Ridge Reserve, TBD Hidden Valley Drive, Garfield County, Colorado.
The project site is shown on Figure 1. The purpose of the study was to develop
recommendations for the foundation design. The study was conducted in accordance
with our agreement for geotechnical engineering services to Craig Whitlock dated June
21, 2016. Hepworth-Pawlak Geotechnical, Inc. (now H-P/Kumar) previously performed a
preliminary geotechnical study for the subdivision development and reported their
findings on June 22, 2004, Job No. 101 126.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock 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.
PROPOSED CONSTRUCTION
Building plans for the proposed residence are conceptual. Typical construction in the area
consists of one and two story wood frame structures above a basement or crawlspace with
an attached garage. Ground floors are typically slab -on -grade. Grading for this type of
structure is assumed to be relatively minor with cut depths between about 3 to 9 feet. We
assume relatively light foundation loadings, typical of the proposed type of construction.
When building location, grading and loading information have been developed, we
should be notified to re-evaluate the recommendations presented in this report.
-2 -
SITE CONDITIONS
The property is vacant and vegetated mostly with grass and weeds in the proposed
building area. There are scattered stands of scrub oak uphill of the building site. The
ground surface slopes down to the west at a grade of about 10 percent in the building site
and about 30 to 40 percent uphill to the east. An abandoned irrigation ditch crosses the
site just below and parallel to the rear building envelope line where sandstone bedrock
crops out.
FIELD EXPLORATION
The field exploration for the project was conducted on July 1, 2016. Two exploratory
borings were drilled at the locations shown on Figure I to evaluate the subsurface
conditions. The borings were advanced with 4 inch diameter continuous flight augers
powered by a truck -mounted CME -45B drill rig. The borings were logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with a 2 inch LD. spoon sampler. The sampler was
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
Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for
review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils, below about 6 inches of topsoil, consist of about 4 to 18 feet of sandy silty
clay overlying sandstone bedrock. Drilling in the sandstone with auger equipment was
difficult due to its hardness and drilling refusal was encountered in the deposit.
-3 -
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 of the
clay soils, presented on Figures 4 and 5, generally indicate low to moderate
compressibility under conditions of loading and wetting. A sample tested from Boring 1
at 2' feet showed a minor swell potential when wetted and the sample from Boring 2 at
21/2 feet showed a low collapse potential when wetted. The laboratory testing is
summarized in Table 1.
No free water was encountered in the borings at the time of drilling or when checked 3
days later and the subsoils were slightly moist.
FOUNDATION BEARING CONDITIONS
The clay soils have variable low to moderate compressibility or expansion under
conditions of loading and wetting. Sandstone bedrock was encountered in Boring 2 at a
depth of 4' feet with minor compressibility potential and niay be exposed in the building
excavation. Good drainage away from the structure will be critical to the long term
performance of the foundation.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, we recommend the building be founded with spread
footings bearing on the natural soils below topsoil or on bedrock. The soils exposed in
the building excavation should be further evaluated to settlement/heave potential and the
need for ground treatment to limit foundation movement at the time of construction.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1)
4
Footings placed on the undisturbed natural soils or bedrock should be
designed for an allowable bearing pressure of 1,500 psf.
The exposed soils
tend to compress or expand when wetted and there could be on the order of
1 inch or more of post -construction settlement of the foundation if the
bearing soils become wet. Care should be taken to reduce the risk of
future building distress by providing good surface drainage away from the
foundation with roof downspouts where provided that drain well away
from the foundation.
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 36 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 such as by assuming an unsupported length of at
least 12 feet. Foundation walls acting as retaining structures should also
be designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) The topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the undisturbed natural soils or
bedrock. The exposed soils in footing area should then be moistened and
compacted.
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
-5 -
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 50 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
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 placed 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
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 0.35 for clay soils and 0.50 for sandstone
bedrock. 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 of passive resistance. Fill placed against the
-6 -
sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab
on -grade construction with a movement risk. 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 a moisture content near optimum. Required fill can
consist of the on-site soils and well broken bedrock 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 retaining walls, crawlspace
and basement areas, be protected from wetting and hydrostatic pressure buildup by an
underdrain system.
The drains should consist of drainpipe placed in the bottom of the wall backfill
surrounded above the invert level with free -draining granular material. The drain should
be placed at each Ievel of excavation and at least 1 foot below lowest adjacent finish
-7 -
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 11/2 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.
3) The ground surface surrounding the exterior of the building should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in paved areas.
Free -draining wall backfill should be covered with filter fabric and capped
with about 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) 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 conclusions and recommendations submitted in this report are
-8 -
based upon the data obtained from the exploratory borings drilled at the locations
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 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 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,
H-RtKUMAR
Louis E. Eller
Reviewed by:
Steven L. Pawlak, P.E.
LEE/ksw
M-P*KUMAR
APPROXIMATE SCALE
1"=30'
LOT 73
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16-7-121
HIDDEN VALLEY DRIVE
KUMAR
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py',ar lIa;i Tu55r a f GfoisysnmeaG71
LOT 71
LOCATION OF EXPLORATORY BORINGS
Figure 1
a5
LL0
0
5
10
15
20
BORING 1
ELEV.-4= 6454'
r 14/12
/ WC=6.2
DD=100
13/12
WC=8 7
DD=99
/ -200=80
16/12
WC=4 8
DD=115
✓ f
n
23/12
/
r
• /
50/0
BORING 2
ELEV.= 6454'
15/12
WC=3 8
DD= 106
38/12
WC=34
DD -114
-200=28
5' 10
0
5
10
15
20
25 25
16-7-121
Note: Explanation of symbols is shown on Figure 3
H-P---IKUMAR
G,_ca,,,iinicn! E' j r:eedny j Eno r fl ling Gsagv},-
t; a'.£rl,ri4 iH,bnp I Enmraur.lc,Ini
LOGS OF EXPLORATORY BORINGS
aw
OCD
0 -
Figure 2
LEGEND:
2 TOPSOIL; sandy silt and clay, firm, moist, brown.
CLAY (CL); sandy, stiff to very stiff, slightly moist, red, low plasticity.
SANDSTONE BEDROCK; weatered to very hard with depth, slightly moist, red. Maroon Formation.
14/12
T
NOTES:
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample blow count; indicates that 14 blows of a 140 pound hammer falling 30 inches were
required to drive the California sampler 12 inches.
Practical drilling refusal,
1. Exploratory borings were drilled on July 1, 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 obtained by interpolation between contours shown on the site plan provided.
4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the
method used
5. The fines 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 or when checked 3 days later. Fluctuation in
water level may occur with time
7. Laboratory Testing Results:
WC = Water Content (%)
DDT Dry Density (pcf)
-200 = Percent passing No 200 sieve
16-7-121
H-P-KUMAR
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LEGEND AND NOTES
Figure 3
Compression - Expansion %
Compression %
1
0
1
2
3
0
1
2
3
4
Moisture Content = 6,2 percent
Dry Density = 100 pcf
Sample of: Sandy Clay
From: Boring 1 at 2 Y2 Feet
,N\
Expe
upor
wetti
nsic
ig
11
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 4.8 percent
Dry Density -- 115 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 10 Feet
Compression
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
16-7-121
F -� P Ili MAR
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
0
•0
c
0
0
0
a
2
E
0
U
3
4
5
6
7
8
Moisture Content 3.8 percent
Dry Density 106 pcf
Sample or, Sandy Silty Clay
From Boring 2 at 2 1/2 Feet
i
ACI
Compression
upon
wetting
J
N\
01
1:0
APPLIED PRESSURE - ksf
10
100
16-7-121
H -P KUMAR
SWELL -CONSOLIDATION TEST RESULTS
Figure 5
H-P_KUMAR
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 16-7-121
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
re)
NATURAL
DRY
DENSITY(%)
(pci)
GRADATION
PERCENT
PASSING
NO. 200
SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(PSF)
SOIL OR
BEDROCK TYPE
BORING
DEPTH
(ft)
GRAVEL
SAND
(%)
LIQUID
LIMIT
(%)
PLASTIC
INDEX
v
em
1
21/2
6.2
100
Sandy Clay
5
8.7
99
80
Sandy Silty Clay
10
4.8
115
Sandy Silty Clay
2
2'/z
3.8
106
Sandy Silty Clay
5
3.4
114
28
Weathered Sandstone
I