HomeMy WebLinkAboutSoils ReportHEPWORTH-PAWLAK GEOTECHNICAL
August 22, 2014
Franz and Susi Zedlacher
c/o Scot Broughton Architects, LLC
Attn: Scot Broughton
P.O. Box 4096
Basalt, Colorado 81621
(sbarchitectsco@mae.com)
Hepworth-Pawbk Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970.945-7988
Fax. 970-945-8454
email: hpgeo@hpgeocech.com
Job No.114 295A
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 8,
Filing 8, Elk Springs, Garfield County, Colorado
Dear Mr. & Mrs. Zedlacher:
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
proposal for geotechnical engineering services to Scot Broughton Architects, LLC dated
May 16, 2014. 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 performed a preliminary geotechnical
study for Filings 6 through 9, Elk Springs (formerly Los Amigos Ranch PUD) and
reported our findings on February 14, 1997, Job No. 197 617.
Proposed Construction: The proposed residence will be one and two story wood frame
construction above a partial walkout basement and partial craw[space with an attached
garage. The residence will be located on the site as shown on Figure 1. Basement and
garage floors will be slab -on -grade. Cut depths are expected to range between about 3 to
9 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 vacant lot is located on a rolling upland mesa above the Roaring
Fork River valley. Vegetation consists of sage brush, grass and weeds. The ground
surface slopes down to the south at a grade of about 9 percent. The building footprint and
driveway alignment were flagged in the field at the time of our field exploration.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating three exploratory pits in the building area and two exploratory pits along the
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorrie 970-468-1989
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driveway alignment at the approximate locations shown on Figure 1. The Iogs of the pits
are presented on Figure 2. The subsoils encountered, below about one foot of topsoil,
consist of basalt cobbles and boulders in a calcareous sandy silt and clay matrix. A thin
sandy clay layer was observed in Pits 2 and 5 overlying the basalt rock. Digging in the
coarse rocky soils with a backhoe was difficult due to the cobbles and boulders in the
highly calcareous soil matrix and digging refusal was encountered in the deposit. Results
of swell -consolidation testing performed on relatively undisturbed samples of the sandy
clay and matrix soils, presented on Figures 3 and 4, indicate low to moderate
compressibility under existing moisture conditions and light loading. No free water was
observed in the pits at the time of excavation and the soils were slightly moist.
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 soil designed for an allowable soil bearing
pressure of 2,000 psf for support of the proposed residence. The clay and silt matrix soils
soften when wetted and there could be differential settlement if the bearing soils are
wetted_ Footings should be a minimum width of 16 inches for continuous walls and 2
feet for columns. Utility trenches and deep cut areas below about 3 feet may require rock
excavating techniques such as chipping or blasting. 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 soils. Voids created
from boulder removal at footing grade should be filled with a structural material such as
road base compacted to at least 98 percent standard Proctor density at a moisture content
near optimum or with concrete. 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 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 a lateral earth pressure based on an equivalent fluid unit
weight of at least 50 pcf for the on-site soil as backfill, excluding rock larger than about 6
inches.
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 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.
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
Job No. 114 295A
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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 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 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. Free -draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site, finer
graded soils to reduce surface water infiltration.
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 pavement and
walkway areas. A swale may be needed uphill to direct surface runoff
around the residence.
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 10 feet from the building. Consideration should be given to the use
of xeriscape to limit 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 I 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
Job No.114 295A
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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,
HEPWOR- PAWLAK GEOTECHNICAL, INC.
Louis E. Eller
Reviewed by:
Steven L. Pawlak, P.E.
LEE/ksw
attachments Figure 1— Locatio xploratory Pits
Figure 2 — Logs of Exploratory Pits •
Figures 3 and 4— Swell -Consolidation Test Results
Table 1— Summary of Laboratory Testing
Job No.114 295A
APPROXIMATE SCALE
\\‘1"=60'
s
/
/
/
r
8915
-�
r.
PIT 1 r�----------- - 8910
' r
PIT 2
------:8905
I
�-
114 295A
r
H
i
-
•
i
/ LOT 8
PIT 3
_ FILING 8 r --
8900
- - - _ 8895
_ ___-- ''"
----- _8890
ij,-S'°i91/UGS
ORS
-------- 888,5
....................•
Hepworth-Pawlek GGeeoleehnicnl
LOCATION OF EXPLORATORY PITS I Figure 1
8
u_
0
0
5
10
LEGEND:
ti
—7
i
T
PIT 1 PIT 2 PIT 3 PIT 4
ELEV. - 8911° ELEV.= 8907' ELEV.= 8900 5' ELEV.= 8907' ELEV.= 8904.5'
PiT 5
WC=8.7
DD= 100
WC=10.2
DD =78
TOPSOIL; organic sandy clay and silt, firm, slightly moist, brown.
CLAY (CL); sandy, silty, stiff, slightly moist, brown.
we=11.4
DD=71
-200=73
LL=41
P1=14
0
WC=8.4 _
DD=96
-200=91
LL=33
PI=13
5 —
BASALT COBBLES AND BOULDERS (GM); in a sandy silt and clay makrix, medium dense, slightly moist, light
brown, highly calcareous.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
Practical digging refusal with backhoe.
NOTES:
1. Exploratory pits were excavated on August 13, 2014 with a Cat 416 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 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 logs 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)
114 295A
MEPWORTM.PAWLAlC GEOTECHNICAL.
-200 = Percent passing No. 200 sieve
LL = Liquid Limit (%)
PI = Plasticity Index (%)
LOGS OF EXPLORATORY PITS
Figure 2
m
AIM
Compressions 96
Moisture Content = 8.7 percent
Dry Density = 100 pct
Sample of. Sandy Silty Clay
From: Pit 2 at 1 Feet
0.1
14 295A
I-1
1.0
APPLIED PRESSURE - ksi
Compression
upon
wetting
10 100
Hepworth -Paw lok Geeetechnlool
1 SWELL -CONSOLIDATION TEST RESULTS
Figure 3
1
Compression %
0
1
2
3
4
5
6
7
8
9
10
11
12
Moisture Content = 10.2 percent
Dry Density = 78 pcf
Sample of Sandy Silt and Clay Matrix
From- Pit 2 at 3 Feet
Compression
upon
'Wetting
0,1 1.0
14 295A
Hepworth-pawlok GGeeotaehnicoi
APPLIED PRESSURE - ksf
10
100
SWELL -CONSOLIDATION TEST RESULTS 1 Figure 4
Job No. 114 295A