HomeMy WebLinkAboutSubsoil Study for Foundation Design 08.13.09~tech
HEPWORTH -PAWLAK GEOTECHNICAL
August 12, 2009
Dave and Kammy Young
3285 County Road 241
New Castle, Colorado 81647
Hepworth-Pawlak Gcotcchnic;i[, Inc.
5020 County Road 154
Glcnwnocl Springs, Colorado 8160 I
Phone: 970-945-7988
fox: 970-945-8454
cmai I: hpgcn@hpgcotcch.com
Job No. 108 621A
Subject: Subsoil Study for Foundation Design, Proposed Residence and Garage,
3285 County Road 241, Garfield County, Colorado
Dear Mr. and Mrs. Young:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design
of foundations at the subject site. The study was conducted in general accordance with
our agreement for geotechnical engineering services to you dated November I 0, 2008 and
received by us on December 2, 2008. Percolation testing was not requested at this time.
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 two story wood frame
construction above a full basement and about 5000 square feet in size. The residence will
be located in the general area shown on Figure 1. Basement floor will be slab-on-grade.
The existing residence and garage will be razed prior to construction. Cut depths are
expected to range up to about 9 feet for the residence and 4 feet for the garage.
Foundation loadings for this type of construction are assumed to be relatively light and
typical of the proposed type of construction. We understand the septic system will be
located southwest or northeast of the building area.
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 property is occupied with an existing residence, guest house,
garage and outbuildings. The existing residence is one story above a basement.
Vegetation consists of grass and weeds with scattered cottonwood and aspen trees. The
ground surface in the building area is relatively flat with a slight slope down to the south
and west. A pond is located downhill and west of the proposed residence.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating three exploratory pits in the building area and one profile pit in the septic
disposal area at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered in the building area, below about 6 to 12
-2-
inches of topsoil, consist of medium dense silty sandy gravel with scattered cobbles and
occasional boulders. About 3 feet of topsoil was observed overlying the sandy gravel in
the Profile Pit. Results of a gradation analysis perfonned on a sample of silty sandy gravel
(minus 3 inch fraction) obtained from the site are presented on Figure 3. No free water
was observed in the Pits 1, 2 and 3 in the building area at the time of excavation and the
soils were slightly moist to moist. Groundwater was encountered in the Profile Pit at a
depth of 5 feet at time of excavating.
Foundation Bearing Conditions: The natural granular soils are adequate for support of
spread footing foundations. All new footings should bear on the natural granular soil
below all topsoil and man-placed fill material from previous site development. The
existing fill should be evaluated for slab support at the time of excavation.
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 of2,500 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 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 soils. Exterior footings should be provided with adequate cover
above their bearing elevations for frost protection. Placement of footings at least 42
inches below the exterior grade is recommended 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 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. The existing fill should be evaluated for slab support
at the time of excavation. 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 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 devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration in
the building area, it has been our experience in mountainous areas that local perched
groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen
Job No. 108 621A
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ground during spring runoff can create a perched condition. We recommend below-grade
construction, such as retaining walls 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 of2 inches. The
drain gravel backfill should be at least I Vi 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
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.
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. 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. 108 621 A
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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
engmeer.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
HEPWORTH -PAWLAK GEOTECHNICAL, INC.
10.~J
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Reviewed by:
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attachments
cc: Kirk & Associates Architecture -Attn: Wayne Kirk
Job No. 108 621A
PROFILE
PIT.
APPROXIMATE SCALE
1" = 40'
EXISTING
POND
EXISTJNGO GUEST
HOUSE
108 621A
PROPOSED
BUILDING
AREA I
PIT3 •
I
TOCOUN/ ROAD241
/:
EXISTING
DRIVEWAY
heNCH MARI< FINISH FLOOR OF
RESIDENCE; ELEV.= 100.0', ASSUMED.
LOCATION OF EXPLORATORY PITS Figure 1
~
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0
5
10
LEGEND:
PIT 1
ELEV.= 99.1'
-, +4 •65
-.J -200 •14
PIT2
ELEV."" 98.0'
WC =7.8
,........,...,, " DD -=92
-200 •46
-, +4•55
-.J ·200 =23
PIT3
ELEV.= 100.7'
TOPSOIL; organic sand and silt with gravel, firm, slightly moist, brown .
PROFILE PIT
I
-.J
0
5
10
GRAVEL (GM); sandy, silty, with scattered cobbles and occasional boulders, medium dense, slightly moist to
moist, moist to wet with depth at Profile Pit, light brown, subangular and subrounded rocks.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
Free water level in pit at time of excavating .
NOTES :
1. Exploratory pits were excavated on July 21, 2009 with a Caterpillar 420E 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 measured by instrument level and refer to the Bench Mark shown on Figure 1.
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 (pcQ
+4 = Percent retained on the No . 4 sieve
-200 = Percent passing No . 200 sieve
~
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0
108 621A LOGS OF EXPLORATORY PITS Figure 2
.
• • 1 HYDROMETER ANALYSIS $[EVE ANALYSIS
I TIME READINGS I U.S. STANOARO SERIES I CLEAR SQUARE OPENINGS I 2~ 7HR 'J/6' 3/4' 11/2' 3' 5'6" 6' 4 IN. 15 MIN 60MINt9MJN .4 MN. 1 MIN . #200 #100 #50 #30 #16 #8 #4
0 100
10 90
0 20 80
UJ (!)
z 30 70 z
~ en en
er:: 40 tll ct
!z I-
50 so a:i UJ
0 0
er:: 60 00 er::
UJ w
a.. a.
70 30
80 20
90 IO
100 0
.llOI .11112 .oo5 .()(If .o1t .Al37 m4 .150 .300 ,11)11 111 2.311 4.75 t.5 12.5 19.0 37.5 'IU 152 203
127
DIAMETER OF PARTICLES IN MIWMETEAS
Cl.'YlO SllT I FINE I ~ ICXW!SE I FINE :r1' COAASE I C08llfS
GRAVEL 65 % SAND 21 % SILT AND CLAY 14 %
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF: Silty Sandy Gravel FROM: Pit 1 at 7 to 8 Feet
HYDROMETER ANALYSIS SIEVE ANALYSIS
7 H TIME READINGS I U.S. STANDARD SER:ES I CLEAR SQUARE OPENINGS I ~g ~-15 Mt. 60MINt9MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8' 3/4' 1 1/2' 3' 5'6' 8'
0 100
10 90
Cl 20 80
UJ (!)
z 30 70 z
~ Ci5 , en
er:: 40 60 ~
I-I-z 50 50 z
UJ w
0 0 a: 60 40 er::
UJ w
a.. a..
70 30
80 20
90 10
100 0
.001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 236 4 75 e.51 2_519.o 37 .5 76 .2 12¥2 203
DIAMETER OF PARTICLES IN MIWMETERS
Cl.'YTO Silt I fllE I ~ 1roMSE I ME!lf..M
GAA~ I COiia.ES FllE I COARSE
GRAVEL 55 % SAND 22 % SILT AND CLAY 23 %
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF : Silty Sandy Gravel FROM: Pit 2 at 7 to 8 Feet
108 621A ~:h
HIEPWOltnl•PAWLAK GEOTECHNICAL
GRADATION TEST RESULTS Figure 3
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 108 621A
SUMMARY OF LABORATORY TEST RESULTS . '
SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT ATTERBERG LIMITS UNCONFINED
MOISTURE DRY GRAVB. SAND PASSING LIQUID PLAS11C WMPRESSIVE SOIL OR PIT DEP'Tlt CONTENT" OENSTTY NO. 200 LIMIT INDEX STRENGTI-1 BEDROCK TYPE (%) (%) SIEVE
(ft) (%) (ocfl (%) (%) (pg:\
1 7-8 LJ, <;'" ;... t I <t Silty Sandy Gravel
2 4 7.8 92 46 Silty Sand
7-8 55 22 23 Silty Sandy Gravel