HomeMy WebLinkAboutSoils Report 01.27.2011GLIO—trtech
Jailiii W g 1 1AWLAK GEOTECHNICAL
ECOS
Attn: Caleb Edelman
6690 Highway 82
Glenwood Springs, Colorado 81601
Subject:
Dear Caleb:
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Job No.111 008A
Subsoil Study for Foundation Design,' Proposed Retaining Walls, 6690
Highway 82, Garfield County, Colorado
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design
of foundations at the subject site. The study was conducted in accordance with our
agreement for geotechnical engineering services to ECOS dated January 18, 2011. The
data obtained and our reconunendations based on the proposed construction and
subsurface conditions encountered are presented in this report. Evaluation of potential
geologic hazard impacts to the subject site is beyond the scope of our study.
Proposed Construction: The proposed site retaining walls will be located in the general
area shown on Figure 1. The walls will create a flat area for storage and parking. Cut
depths are expected to range between about 10 to 4 feet. Foundation loadings for this
type of construction are assumed to be relatively light and include lateral overturning
from unbalance earth loading.
If development conditions or foundation loadings are significantly different from those
described above, we should be notified to re-evaluate the recommendations presented in
this report.
Site Conditions: The proposed retaining wall site is located uphill, behind the existing
buildings on the property. The property consists of hilly terrain with a moderate to steep
slope down to the west. The proposed retaining wall area has been graded relatively flat
with recent till derived from uphill, on-site cut. An access drive into the graded area is
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located along the south property line. A hillside drainage channel is located just uphill to
the northeast as shown on Figure 1. Vegetation in natural areas consists of a pinon and
juniper forest with sage brush. A few inches of snow covered the site at the time our field
exploration.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating 2 exploratory pits at the approximate locations shown on Figure 1. The
backhoe to dig the pits was provided by the client. The logs of the pits are presented on
Figure 2. The subsoils encountered, below about 31/2 to 7 feet of recent fill materials,
consist of medium dense, silty sandy gravel, cobbles and boulders. The natural gravel
soils are colluvial deposits derived from Maroon Formation that forms the east side of the
Roaring Fork River valley. Results of gradation analyses performed on samples of the fill
and natural gravel soils (minus 3 to 5 inch fraction) obtained from the site are presented
on Figure 3. The laboratory test results are summarized in Table 1. No free water was
observed in the pits at the time of excavation and the soils were moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, spread footings placed on the
undisturbed natural granular soil and designed for an allowable maximum (toe) bearing
pressure of 2,500 psfcan be used for support of the proposed walls with a risk of
movement and distress. The soils tend to compress after wetting and there could be some
post -construction settlement and rotation of the walls. The amount of movement will
mainly depend on the depth of wetting and precautions should be taken to prevent water
from entering the bearing soils. The wall footings should be a minimum width of 24
inches. The existing fill and loose disturbed soils and existing fill encountered at the
foundation bearing level within the excavation should be removed and the footing bearing
level extended down to the undisturbed natural granular soils. Exterior footings should be
provided with adequate cover above their bearing elevations for frost protection.
Placement of footings at least 36 inches below the exterior grade is typically used in this
area. Continuous foundation walls should he reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 15 feet. The retaining
Job No.111 008A
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walls should be designed to resist a lateral earth pressure based on an equivalent fluid unit
weight of at least 40 pcf for the on-site soil granular soil as backfill. A sliding coefficient
of 0.45 and passive earth pressure of 400 pcf can be used to resist lateral load on the
walls. The sliding coefficient and passive earth pressure values are for ultimate loading
conditions and appropriate safety factors should be taken. The backfill materials should
consist of granular soil excluding organics and rocks larger than about 6 inches, and
compacted to at least 95% of standard Proctor density at near optimum moisture content.
Underdrain System: Although free water was not encountered during our exploration, it
has been our experience in mountainous areas 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 the retaining walls be protected
from hydrostatic pressure buildup by an underdrain system.
The underdrain should consist o f slotted 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 1'/2 feet deep.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the walls have been completed:
1) Inundation of the foundation excavations 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, slab and footing areas and to at least 90% of the maximum
standard Proctor density in .landscape areas. Free -draining wall backfill
Job No.111 OOSA
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should be capped with at least 2 feet of the on-site, finer graded soils to
reduce surface water infiltration.
3) The ground surface surrounding the walls should be sloped to drain away
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 pavement and walkway areas. A swale will be needed uphill to
direct surface runoff around the walls.
4) Landscaping which requires regular irrigation should not be located within
10 feet of the walls. Consideration should be given to the use of xeriscape
to prevent potential wetting of soils below the foundation 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 pits excavated at the
locations indicated on Figure 1 and to the depths shown on Figure 2, 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 pits 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 at once so 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
Job No.1 l 1 008A
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bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
HEPWORTH - PAWLAK GEOTECHNICAL, .INC.
Steven L. Pawlak, P.E.
Reviewed by:
Daniel E. Hardin, P.E.
SLP/jsm
attachments Figure 1 — Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Gradation Test Results
Table 1 — Summary of Laboratory Test Results
cc: Kurtz and Associates — Attn: Brian Kurtz, PE
Job No.J 11 008A,
Gtech
APPROXIMATE SCALE
1"= 30'
6040
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6p
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6045
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PROPOSED
RETAINING WALL
AREA
NOTE: Topography doesn't reflect current grading in retaining wall area.
6p6O
PIT 1
6045
■
PIT 2
7
6070
6055
6050
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LEGEND:
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PIT 1
PIT 2
WC=1.7
+4=88
-200=4
FILL; loose silty sandy gravel, cobbles and boulders, with some tree branches and sage brush.
GRAVEL (GM); silty sandy gravel, cobbles and boulders, medium dense, moist, red.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
0
6
10
NOTES:
1. Exploratory pits were excavated on January 20, 2011 with a John Deere backhoe.
2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits were not measured and the logs of exploratory pits are drawn to depth.
4. The exploratory pit 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 pit Togs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the pits at the time of excavating. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
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111 008A
GeStech
HEPWORTH•PAWLAK GEOTECHNICAL
LOGS OF EXPLORATORY PITS
Figure 2
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HYDROMETER ANALYSIS
TIME READINGS
SIEVE ANALYSIS
U.S. STANDARD SERIES r CLEAR SQUARE OPENINGS
45 MIN. 1EL 5 MIN. 60MIN19MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8" 3/4" 1 1/2" 3" 5"6"
0
10
20
30
40
50
60
70
80
90
100
001 .002 .005 .009 012 .037 .074 .150 .300 .900 1 16 2.36 4,75
DIAMETER OF PARTICLES IN MILLIMETERS
8"
100
•
•
•
•
-H
CLAYTO SILT
GRAVEL 75 %
.V.ND
1 1197.UM 1 COARSE
9 5 12,5 19.0
35507.
375 762 152 203
127
FINE 1 CC;Pa=
COBBLES
SAND 19 % SILT AND CLAY 6 %
LIQUID LIMIT % PLASTICITY INDEX
FROM: Pit 1 at 5 to 6 Feet
SAMPLE OF: Sandy Gravel with Cobbles (Fill)
Some Wood Debris
90
60
70
60
5o
40
30
20
10
0
HYDROMETER ANALYSIS I
SIEVE ANALYSIS I
TIME READINGS U.S. STANDARD SERIES 1 CLEAR SQUARE OPENINGS
45
MIN. 15 MIN. 60MIN19MIN.4 MIN. 1 MIN. 4200 #100 #50 430 #16 #8 44 3/8" 3/4" 1 1/2' 3" 5'6" 8"
0 100
10
20
30
40
50
60
70
80
90
100 100
1
f
90
80
IN1 ��
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70
cn
60
50 W
U
40 W
0
30
20
10
0
.001 .002 .005 .009 .019 .037 .074 .150 300 .600 1.18 2.36 4 75 9.512 519.0 37.5 76.2 1252 203
DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT
05N1)
FILE 1 PI70lM 1 COnP•
cry}1�L
1 COA6SE
COBBLES
GRAVEL 88 % SAND 8 % SILT AND CLAY 4 %
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF: Sandy Gravel FROM: Pit 2 at 4 to 4 z Feet
111 008A
G7',tech
HEPWORTH-PAWLAK GEOTECHNICAL
GRADATION TEST RESULTS
Figure 3
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