HomeMy WebLinkAboutSubsoil Study for Foundation Design 02.11.15.. <~tech
HEPWORTH-PAWLAK GEOTECHNICAL
February 11, 2015
Luis Carrera
11741 County Road 320
Rifle, Colorado 81650
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Job No.115 029A
Subject: Subsoil Study for Foundation Design, Proposed Residence, 11743 County
Road 320, Garfield County, Colorado
Dear Luis:
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 January 29, 2015. 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 a double wide modular
structure at the same location as the previous trailer on the site as shown on Figure 1. A
basement level with slab-on-ground floor is proposed. Cut depths could range between
about 2 to 6 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, location 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 proposed building site was previously developed with a mobile
home which was apparently removed for the new home development. Existing concrete
walkway and patio slabs are located along the south side of the previous trailer location.
The ground surface slopes gently down to the north through the building site then
becomes steeper into a small drainage channel located near the eastern property line
approximately shown on Figure 1. Vegetation consists of grass in the yard area of the
house site with willows and cottonwood trees along irrigation ditches in the area.
Subsurface Conditions: The subsurface conditions at the site were evaluated by drilling
one exploratory boring at the approximate location shown on Figure I. The log of the
boring is presented on Figure 2. The subsoils encountered, below about Y2 foot of topsoil,
consist of stiff and moist to very moist to wet and soft with depth sandy silty clay.
Results of swell-consolidation testing perfonned on a relatively undisturbed sample of the
upper clay, presented on Figure 4, indicate moderate to high compressibility under
conditions of loading and wetting. Results of liquid and plastic limits and unconfined
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compressive strength testing indicate the clay soils have low plasticity and soft to medium
stiff consistency. The laboratory test results are summarized in Table I. Free water was
encountered in the boring at the time of drilling and at a depth of about 4Y2 feet the day
after drilling.
Foundation Recommendations: The upper soils encountered to a depth of about 2 to 3
feet below existing ground surface can be used for spread footing foundation support.
Deeper excavation could encounter unsuitable soft and wet soils with a high risk of
settlement. Excavation to below the groundwater level for below grade construction is
not recommended. Spread footings placed on the upper natural soil can be designed for
an allowable soil bearing pressure of 800 psf. The soils tend to compress when loaded and
there could be about 1 inch of post-construction foundation settlement. Footings should
be a minimum width of 20 inches for continuous walls and 2 feet for columns. Loose
disturbed soils and existing fill encountered at the foundation bearing level within the
excavation should be removed. Stabilization of the bearing surface such as by geogrid
and base course aggregate could be needed in footing areas. 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. The grade around the building can be elevated for frost cover and drainage but
should not be more than 2 feet above existing ground surface to not excessively load the
bearing soils which could cause additional foundation settlement. Continuous foundation
walls should be heavily reinforced top and bottom to span local anomalies such as by
assuming an unsupported length of at least I 4 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 55 pcf for the on-site soil as backfill.
Slab-on-Grade: The natural soils near existing ground surface, exclusive of topsoil, can
be used 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
relatively well graded sand and gravel, such as road base, should be placed beneath slabs
for subgrade support. 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 low moisture content soils devoid of vegetation, topsoil and debris.
Underdrain System: Free water was encountered at relatively shallow depth in the
exploratory boring and the water level could rise or perched groundwater could develop
during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can create a perched condition. We recommend below-brrade construction, such as
retaining walls and crawlspace areas, be protected from wetting and hydrostatic pressure
buildup by an underdrain system.
Joh No.115 029A
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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 I 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 l 'l'l 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.
5) Landscaping which requires ret:.JUlar heavy irrigation should be located at
least 5 feet from the building.
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 boring drilled at the
location indicated on Figure 1 and to the depth 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 boring 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
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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 need further assistance, please let us know.
Respectfully Submitted,
SLP/ksw
Attachments: Figure I -Location of Exploratory Boring
Figure 2 -Log of Exploratory Boring
Figure 3 -Legend and Notes
Job No.115 029A
Figure 4 -Swell-Consolidation Test Results
Table I -Summary of Laboratory Test Results
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115 029A
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APPROXIMATE SCALE
1" = 40'
EXISTING
STABLE
EXISTING TRAILER
(RECENTLY REMOVED)
• BORING 1
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117 43 COUNTY
ROAD320 EXISTINGD·
GARAGE I
PRIVATE GRAVEL ROAD
LOCATION OF EXPLORATORY BORING
TOCOUNlY
ROAD320
Figure 1
BORING 1
0 0
10/12
WC-17.6
DDw110
1 -200~68 -5 -5 -5/12
WCr 19 7
DD-109
-200=86
UC=650
10 4/12 10
Qi
we 19.5
Q) 0 DD-105 al u.. --200 -79 -u..
I -I .c LL-27 .c a Pl::13 a Q) Q)
0 15 15 0 6/12
20 2/12 20
25 25
NOTE: Explanation of symbols is shown on Figure 3.
115 029A ~ He worth-Pawlak Geotochnlcal
LOG OF EXPLORATORY BORING Figu re 2
LEGEND:
TOPSOIL; organic sandy silt and clay, brown.
CLAY (Cl); silty, sandy, scattered gravel, stratified sand lenses, stiff to soft with depth, moist to wet with depth,
light brown to brown, low plasticity.
Relatively undisturbed drive sample; 2-inch 1.0. California liner sample.
10112 Drive sample blow count; indicates that 1 O blows of a 140 pound hammer falling 30 inches were
required to drive the California sampler 12 inches.
0,1 Free water level in boring and number of days following drilling measurement was taken.
Depth at which boring had caved when checked on February 5, 2015.
NOTES:
1. The exploratory boring was drilled on February 4, 2015 with a 4-inch diameter continuous flight power auger.
2. The exploratory boring location was measured approximately by pacing from features shown on the site plan
provided.
3. The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth.
4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the
method used.
5. The lines between materials shown on the exploratory boring log represent the approximate boundaries between
material types and transitions may be gradual.
6. Water level readings shown on the log were made at the time and under the conditions indicated. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC= Water Content(%)
OD = Dry Density (pct}
-200 = Percent passing No. 200 sieve
LL = liquid Limit (%)
Pl = Plasticity Index (%)
UC = Unconfined Compressive Strength (psf)
115 029A ~ H epworth-Powlok Geotechnlcol
LEGEND AND NOTES Figure 3
Moisture Content = 17.6 percent
Ory Density = 110 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 2 1/2 Feet
0
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i.;'upon
* 2 "' }" wetting
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0 ()
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0.1 1.0 10 100
APPLIED PRESSURE -ksf
115 029A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4
Heoworth-Pawlak Geotechnlcal
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 115 029A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL GRADATION ATIERBERG LIMITS UNCONFINED
MOISTURE NATURAL PERCENT COMPRESSIVE GRAVEL SANO PLASTIC SOIL OR BORING DEPTH CONTENT ORY DENSITY PASSING NO. LIQUID LIMIT STRENGTH
(%) (%) 200SIEVE INDEX BEDROCK TYPE
{It) {%) {pcf) (%) (%) (PSF)
1 21h 17.6 110 68 Sandy Silty Clay
5 19.7 109 86 650 Sandy Silty Clay
10 19.5 105 79 27 13 Sandy Silty Clay