HomeMy WebLinkAboutSoils Report 03.18.2016Gwt
HEPWORTH-PAWLAK GEOTECHNICAL
March 18, 2016
Clearwater Construction Management
Attn: Jason Anthony
P.O. Box 6236
Snowmass Village, Colorado 81615
(jantleon v @ ccmaspen.com)
Hepworth Pawl d; Genrechnical, Inc.
5020 County Road 154
Glenwood Sprin;-:. Colorado 81001
Phone. 970-945-7988
Fax: 970.945-8454
em,rd; hpgen$hpl;rotech.cnrn
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Job No.116 066A
Subject: Subsoil Study for Foundation Design, Proposed Butter Residence, Lot 5,
Roaring Fork Preserve, 6 Silver Spruce Drive, Garfield County, Colorado
Dear Mr. Anthony:
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 Clearwater Construction Management
dated March 11, 2016. The data obtained and our recommendations based on the
proposed construction and subsurface conditions encountered are presented in this report.
Hepworth-Pawlak Geotechnical, Inc. previously conducted a preliminary geotechnical
study for the development and presented our findings in a report dated November 30,
2000, Job No. 100 627.
Proposed Construction: The proposed residence will be one and two story wood frame
construction with slab -on -grade floor and located on the site as shown on Figure 1. The
attached garage will also have a slab -on -grade floor. Cut depths are expected to range
between about 2 to 3 feet. Foundation loadings for this type of construction are assumed
to be relatively light and typical of the proposed type of construction.
If building 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 lot is vacant and accessed by an existing gravel drive. There was
about two inches of fresh snow cover at the time of our field exploration. Vegetation
consists of grass and weeds. Previous trees were destroyed in a wildfire in April 2008.
The ground surface is relatively flat with a slight slope down to the south and west. An
abandoned irrigation ditch is located just north of the building site. An existing dry pond
is located on the south side of the building area. Eagle Valley Evaporitc Formation is
exposed on the hillside south of County Road 100.
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silvcrthorne 970-468-1989
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Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating three exploratory pits at the approximate locations shown on Figure 1. The
logs of the pits are presented on Figure 2. The subsoils encountered, below about one
foot of topsoil, consist of silty sandy gravel with cobbles and small boulders. A thin silty
sand layer was encountered in Pits 2 and 3 between the topsoil and gravel. Results of a
gradation analyses performed on a sample of silty sand and sandy gravel (minus 3 -inch
fraction) obtained from the site are presented on Figures 2 and 3. No free water was
observed in the pits at the time of excavation and the soils were slightly moist to moist.
An exploratory pit dug on Lot 5 in August, 2000 encountered groundwater at a depth of 7
feet and groundwater level is generally known to be seasonally shallow.
Subsidence Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite
underlies the Roaring Fork Preserve 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 portions of 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 the lower Roaring Fork River valley. These sinkholes appear similar
to others associated with the Eagle Valley Evaporite in areas of the Eagle Valley.
Sinkholes were not observed in the immediate area of the subject lot or within the
subdivision. No evidence of cavities was encountered in the subsurface materials;
however, the exploratory pits were 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 5
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. 1f further
investigation of possible cavities in the bedrock below the site is desired, we should be
contacted.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread
footings placed on the undisturbed natural gravel soil designed for an allowable soil
bearing pressure of 2,500 psf for support of the proposed residence. The upper sand soils
tend to compress under loading and should be removed to limit foundation settlement.
Footings should be a minimum width of 16 inches for continuous walls and 2 feet for
columns. Loose and disturbed soils encountered at the foundation bearing level within
the excavation should be removed and the footing bearing level extended down to the
undisturbed natural gravel 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 be reinforced top and bottom to span Iocal anomalies such as by assuming
an unsupported length of at least 10 feet. Foundation walls acting as retaining structures
(if any) should be designed to resist a lateral earth pressure based on an equivalent fluid
unit weight of at least 45 pcf for the on-site soil as backfill. A representative of the
geotechnical engineer should observe all footing excavations prior to concrete placement
Job No 116 066A
Gecrrbech
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to evaluate bearing conditions and adequate removal of the upper topsoil and silty sand
soils.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly
loaded slab -on -grade construction. 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
interior slabs to facilitate drainage. This material should consist of minus 2 -inch
aggregate with less than 50% passing 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 ]east 95% of
maximum standard Proctor density at a moisture content near optimum. Required fill can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: It is our understanding the finished floor elevation at the lowest
level will be at or above the surrounding grade. Therefore, a foundation drain system is
not required. 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.
If the finished floor elevation of the proposed structure has a floor level below the
surrounding grade, we should be contacted to provide recommendations for an underdrain
system or making the foundation and floor slab watertight. All earth retaining structures
should be properly drained.
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 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.
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 5 feet from the building.
Job No.116 066A
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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 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 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
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 GEOTECFINICAL, INC.
Louis Eller
Reviewed by:
Steven L. Pawlak, P.E.
LEE/ksw
attachments Figure 1— Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Gradation Test Results
Job No.116 066A
LOT 6
LOT 4
BUILDING SETBACK LINE
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PIT 1 /` \\ \\ ♦` ^ N\
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1 LOT 5
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9,>D 7,▪ 9
APPROXIMATE SCALE
1"=80'
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PIT 2
PROPOSED
RESIDENCE
PIT 3 ■
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NOTES:
PIT 1
ELEV.= 6620'
r +4=69
-200-1
PIT 2
ELEV. = 6620'
WC -15.1
DD -100
-200=.36
PIT 3
ELEV.= 6619.5'
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TOPSOIL; organic sand, silt and clay, firm, moist, black, upper 6 inches disturbed.
SAND (SM); silty to very silty, medium dense, moist, reddish brown.
GRAVEL (GP -GM); with cobbles and small boulders, sandy to very sandy, dense, mast, light brown,
rounded rock.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
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10
1. Exploratory pits were excavated on March 15, 2016 with a Deere 50c mini -excavator.
2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits were obtained by Interpolation between contours shown on the site plan provided.
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 (%)
DD = Dry Density (pcf)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
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HYDROMETER ANALYSIS SIEVE ANALYSIS 1
24 1-97 MR TIME READINGS U 5 STANDARD SERIES 1 CLEAR SQUARE OPENINGS !I
0 45 M! . 15 MIN. 60MIN19MIN 4 MIN. 1 MIN #200 #100 #50 #30 #16 #8 #4 3/8` 3/4' 1 1/2' 3' 5'5' 8' 100
10
20
30
40
5o
60
70
80
90
100
001 002 005 009 019 037 074 150
300 600 1 18 2 36 4 75 9.5 19.0 37 5 76 2 152 203
12.5 127
DIAMETER OF PARTICLES IN MILLIMETERS
GRAVEL 69 %
LIQUID LIMIT %
SAMPLE OF: Sandy Gravel
COBBLES
SAND 30 % SILT AND CLAY 1 %
PLASTICITY INDEX %
FROM: Pit 1 at 3 to 4 Feet
90
80
70
50
40
30
20
10
0
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