HomeMy WebLinkAboutSoils Report 06.09.2008Gtech
HEPWORTH-PAWLAK GEOTECHNICAL
June 9, 2008
Kurt Warnecke
P.O. Box 4481
Basalt, Colorado 81621
1151•0...ith•[',t.I.,4,! .PLL IJ It 11 II..
7'020 I. ',MHO. I+..l.l I ti I
t 0,'111...4,1 till{ne 1 .,I,.,.I,1, •Inl'1
I'I,..ii, ";.,•'1_I ;"1",
RECEIVED
APR 0 4 2018
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
Job No.108 265A
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot M-12,
Midland Point, Garfield County, Colorado
Dear Mr. Warnecke:
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 you dated May 22, 2008. The data
obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report.
Proposed Construction: The proposed residence will be one and two story wood frame
construction above a crawlspace with an attached garage. The residence will be located in
the front part of the lot as shown on Figure 1. Garage floor will be slab -on -grade. Cut
depths are expected to range between about 2 to 4 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 site is located on the downhill side of Midland Point Roacl and
vacant of structures. Vegetation consists of grass and weeds and the ground surface is
relatively flat in the building area. There is a very steep slope down to the Roaring Fork
River terrace northeast of the building arca. The steep slope is vegetated with
I':I11,'I 71It) • l.t+1L I19-03i•;7'h' • II I.1.•1'111,4I'n.` 97'0--16tti•I9,ti'4
f footings at least 36 inches below the exterior grade
-2 -
cottonwood trees and brush. An abandoned ranch two track path passes through the site
near the top of the steep slope.
Subsidence Potential:
Subsidence Potential: Midland Point is underlain by
Pennsylvania Age Eagle Valley Evaporate bedrock. The evaporite contains gypsum
deposits. Dissolution of the gypsum under certain conditions can cause sinkholes to
develop and can produce areas of localized subsidence. Sinkholes were not observed in
the immediate area of the subject lot. Based on our present knowledge of the site, it
cannot be said for certain that sinkholes will not develop.
In our opinion, the risk of
ground subsidence at Lot M-12 is low and similar to other lots in the area
should be aware ofthe potential for sinkhole development.
Subsurface Conditions:
but the owner
The subsurface conditions at the site were evaluated by
excavating two exploratory pits at the approximate locations shown on Figure 1. The
Togs of the pits are presented on Figure 2. The subsoils encountered, below about one
toot of topsoil, consist of slightly silty to silty sandy gravel with cobbles. Results of a
gradation analysis pertbrmed on a sample of sandy gravel (minus 5 inch fraction)
obtained from the site are presented on Figure 3. No free water was observed in the pits
at the time of excavation and the soils were slightly moist to moist.
Foundation Recommendations:
Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread
tootings placed on the undisturbed natural soil designed for an allowable soil bearing
pressure of 2,500 psf for support of the proposed residence. 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 soils.
Exterior tootings should be provided with adequate cover above their bearing elevations
for frost protection. Placement o
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 10
.lith No. 108 2(5A
-3 -
feet. Foundation walls acting as retaining structures 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.
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
basement level 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 12% 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 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 and crawlspace 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
Job No. 108 265 A
-4 -
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 I'h feet deep.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the residence has been completed:
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. Free -draining wall backfill should be
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 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.
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
Job Na 108 265A
-5 -
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 GEOTECHNICAL, INC.
Louis Eller
Reviewed by:
aI). uIST
0....A
.!z,_
l -.
Daniel E. Hardin, P.E.:- °% 2,4443 •
4:.."0,-
ar •• 44
LEE/vad ••!f. ��`,+r�...•*'.a
r 44-j.:
� [h .
rry7rysr� lN�It. ���
attachments Figure I — Location ofExploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Gradation Test Results
Job No.108 265 A
COMMON
AREA
ROARING FORK RIVER
r-
H �o
OLD RANROAD
PIT 2
LOT M-12 •
BUILDING
AREA
PIT 1
■
L
MIDLAND POINT ROAD
APPROXIMATE SCALE
1"=30'
LOT M-13
1i
0)
0
5
10
LEGEND:
- J
T
PIT 1
i +4=70
- J -200=14
PIT 2
TOPSOIL; organic sandy silt and clay, firm, slightly moist, dark brown.
0
5
10
GRAVEL (GM); with cobbles, sandy, silty, medium dense to dense, slightly moist to moist, reddish brown,
subrounded rocks.
Disturbed bulk sample.
Practical digging refusal with backhoe.
NOTES:
1. Exploratory pits were excavated on January 12, 2008 with a 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 Togs 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 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:
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
u_
L
a
a)
0
108 265A
LOGS OF EXPLORATORY PITS
Figure 2
likZ
HYDROMETER ANALYSIS SIEVE ANALYSIS
H� Hq TIME READINGS U.S. STANDARD SERIES 1 CLEAR SQUARE OPENINGS
0 24 MIN. 15 MIN. 60MIN19MIN.4 MIN. 1 MIN, #200 / 100 #50 #30 #16 #8 #4 3/8" 3/4' 1 1/2" 3" 5"6" 8" 100
10
20
30
40
50
60
70
80
90
100
1
7
16,
J
t
F
[
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
CLAY TO SILT
GRAVEL 70 %
LIQUID LIMIT %
Adn
ririr
SAND 16 %
PLASTICITY INDEX
COBBLES
SILT AND CLAY 14 %
SAMPLE OF: Silty Sandy Gravel with Cobbles FROM: Pit 1 at 5 to 6 Feet
90
80
70
60
50
40
30
20
10
0
RCENT PAS IN e;
108 265A
GRADATION TEST RESULTS
Figure 3