HomeMy WebLinkAboutSubsoil Study for Foundation DesignJ>'· . ,
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HEPWORTH-PAWLAK GEOTECHNICAL
April 24, 2006
Jordan Architecture
Attn: Brad Jordan
P.O. Box 1031
Glenwood Springs, Colorado
H epworth-Pawlak Geocechnical, Inc
5020 County Road 154
Glenwood Sprtn[!S, Colc rado 8 160 1
Phone:970-945-7988
Fax : 970-945-8454
email : hpgeo@hpgeotech.co m
Job No. 106 0245
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 22,
Springridge Place Subdivision, Springridge Drive, Garfield County,
Colorado.
Dear Mr. Jordan:
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 Jordan Architecture, dated March 15,
2006. 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
development of the subdivision and presented our findings in a report dated June 22,
1004, Job No. JOI 126.
Proposed Construction: The proposed residence will be single story wood frame
structure over a walkout basement level located in the proposed building area shown on
Figure 1. The attached garage and basement floors will be slab-on-grade. Cut depths are
expected to range between about 3 to 8 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 was vacant and covered with up to about 1 ~ feet of snow at
the time of our field exploration. There is an irrigation ditch to the northwest of the
building area. The building area is located on top of a north-south trending ridge and is
relatively flat. There is a steep slope down to the east of the building area at a grade of
25% to 35%. The slope down to the west is moderate at a grade of about 10%. There is
about 8 feet of elevation difference across the building area. Vegetation consists of
scattered brush, grass and weeds.
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 ~ foot
Parker 303-841-7119 • Colorado Spring s 719 -633-5562 • Silverthorne 970-468-198 9
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of topsoil and 1 to 2 feet of silty sandy gravel with cobbles, consist of weathered and
fractured to very hard, san dstone b edrock to the backhoe refusal depths of 3 to 5 feet.
Results of swell-consolidation testing performed on relatively undisturbed samples of
weathered sandstone, presented on Figure 3, indicate low compressibility under existing
moisture conditions and light loading and a minor collapse potential when wetted.
Results of gradation analyses perfonned on samples of the fractured sandstone (minus 1 ~
and 3 inch fractions) obtained from the site are presented on Figure 4. No free water was
obseived in the pits at the time of excavation and the soils were slightly moist to moist.
The bedrock was relatively dry.
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 granular soils or bedrock materials designed
for an allowable bearing pressure of 4,000 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 disturb ed soils and rock encountered at the foundation b earing level
within the excavation should be removed and the footing bearing leve1 extended down to
the undisturbed natural grail ular soils or bedrock materials . Excavating more than a few
feet into the bedrock will be difficult and may require rock excavation techniques such as
chipping or blasting. 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 'i s 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 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 granular soil or well broken bedrock, excluding
oversized rock, 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 unres trained 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 s]abs should be compacted to at least 95% of
m aximum standard Proctor density at a moisture content near optimum. Required :fill can
consist of the on-site granular soils or well broken bedrock 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 and where bedrock is shallow that local perched
groundwater can develop during times of heavy precipitation or-seasonal runoff. Frozen
Job No. I 06 0245
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ground during spring runoff can create a perched condition. We recommend below-grade
construction, such as retaining 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 shou14 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 ~ 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.
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 IO feet in unpaved
areas and a minimum slope of 3 inches in the first l 0 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 I and to the depths shown on Figure 2, the proposed type of
construction, and our experience in the area. Otir services do not include detennining 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 encolintered 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 bas 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
Job No.106 0245
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verify that the recommendations have been appropriately interpreted . Significant design
changes may require additional analysis or modifications to the reconunendations
presented herein. We recommend on-s ite observation of excavations and foundation
bearing strata and testing of structural fill by a representative of the geotechnicaJ
engineer.
If you have any questions or if we may be of further assistance, please let us lmow.
Respectfully Submitted,
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
Jordy Z. Adamson, Jr., P .E.
Reviewed by:
Daniel E. Hanlin, P.E.
JZA/ksw
attachments Figure I -Location of Exploratory Pits
Figure 2 -Logs of Exploratory Pits
JobNo.106 0245
Figure 3 -Swell-Consolidation Test Results
Figure 4 -Gradation Test Results
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APPROXIMATE SCALE
1· = 50' 70 LOT21
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106 0245 LOCATION OF EXPLORATORY PITS Figure 1
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LEGEND:
PJT1
ELEV.= 100'
PIT2
ELEV.= 103'
wc .. 9.s
00-113
+4=65
•200c30
TOPSOIL; sandy silty clay, organics, roots, soft, moist, brown.
PIT3
ELEV.= 101'
0
5
10
GRAVEL ANO COBBLES (GM); silty, sandy, dense, slightly moist to moist, reddish brown, fragments of
sandstone.
SANDSTONE BEDROCK; weathered and fractured to very hard, dry, reddish brown. Maroon Formation.
~ 2• Diameter hand driven liner sample.
f -: Disturbed bu lk sample. ~-.J
T Practical digging refusal with backhoe.
N01ES:
1. Exploratory pits were excavated on March 17, 2006 with a caterpillar 4300 backhoe.
2. Locations of exploratory pl_ts were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits were obtained by Interpolation bet\wen contours shown on the site plan provided and
checked by instrument level. Logs are drawn to depth.
4. The exploratory pit locations and elevations shou ld 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 transltlons may be gradual.
6. No free water was encountered in the pits at the trme of excavating. Fluctuatio n in water level may occur with time .
7. Laboratory Testing Results:
WC = Water Content {%)
DD = Dry Density (pc~
106 0245
+4 = Percent re tained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
LOGS OF EXPLORATORY PITS Figure 2
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Moisture Content = 9.6 percent
Dry Density = 113 pcf 0
Sample of: Weathered Sandstone
From: Pit 2 at 2.5 Feet
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APPLIED PRESSURE · ksf
106 0245 ~ SWELL-CONSOLIDATION TEST RESULTS Figure 3
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I I HYDROMETER ANALYSIS I SIEVE ANALYSIS
TME READINGS U.S. STANDARD SERIES I Cl.EAA SQUARE OPENINGS
~ 7lfl w 3/4" 11/2' 3° 5'6" a+ . 15 MIN. 60MINJDA.IN.4 MN. 1 MW. #200 #100 #50 #30 #16 #8 #4
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.1111 Jm .1115 Jiii .011 lllT .1114 .110 .. .11111 t,111 2.lll UJ t.$1U ~ ,1.1 lU 152 :1111:1
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DIAMETER OF PARTICLES IN MIWMETEAS
CV.YlOSll.T I !!! I
MIC) """M .1CIWISE I -!i:c IXIMSI: I =-.a
GRAVEL 65 % SAND 5 % SILT AND CLAY 30 %
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF: Sandstone Fragments in a Slightly Sandy FROM: Pit 2 at 4 Feet
. ..-..... Slit M::ltrix
HYOftO~ 11:t1 ANALYSIS SIEVE ANALYSIS
~ ~. 7 HR TIME AEAOINGS I I I U.S. STANDARD SERES CLEAR SQUARE OPENINGS ~ • 15 MIN. 60MIN19MIN.4 MIN. 1 MIN. #2()0 #100 #50 #30 #16 #8 #4 'NS' 3/4 " 11/2' 3' 6*6" 8'
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.001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 2.36 4.75 e512.519.o 37.5 76.2 12~52 203
DIAMETER OF PARTJClES IN MILUMETEAS
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GRAVEL 33 % SAND 23 % SILT AND CLAY 44 % ·-LIQUID UMrT % PLASTICITY INDEX %
t SAMPLE OF: Sandstone Fragments in a Sandy Silt FROM ; Pi t 2 at 2}'2 to 4~ Feet .. Matrix
106 0245 o&'&tech
HEPWORn+PAWLAK GnnECHNICAL
GRADATION TEST RESULTS Figure 4