HomeMy WebLinkAboutSubsoil Study for Foundation Designs~ech
HEPWORTH-PAWLAK GEOTECHNICAL
February 12, 2016
Kirk and Jan Williams
04 Redwing Lane
Carbondale, Colorado 81623
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Job No.116 012A
Subject: Subsoil Study for Foundation Design, Proposed Garage/ADU, Lot 4,
Hawkridge, 04 Redwing Lane, Garfield County, Colorado
Dear Mr. and Mrs. Williams:
As requested, Hepworth-Pawlak Geotcchnical, 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 you dated November 11, 2015 and
received by us on January 22, 2016. 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 perfonned a subsoil study for the
existing residence and reported our findings December 9, 1998, Job No. 198 776.
Proposed Construction: The proposed garage I ADU will consist of a one story living
area above a walkout level garage with an entry and mudroom . The building will be
located about 60 feet north of the residence as shown on Figure I. Ground floor will be
slab-on-grade. Cut depths are expected to range between about 3 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 or foundation loadings arc significantJy different from those
described above. we should be notified to re-evaluate the recommendations presented in
trus report.
Site Conditions: There was about 18 inches of snow cover at the time of our field
exploration. The property is occupied with a one and two story wood frame single family
residence above a partial crawlspace and partial slab-on-grade floor. Vegetation consists
of grass and weeds. The ground surface slopes down to the southwest at a grade of about
10 percent. The building area was graded during construction of the residence and is
relatively flat. An irrigation ditch is located uphill and about 120 feet east of the building
site.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits at the approximate locations shown on Figure 1. The
logs of the pits nre presented on Figure 2. The subsoils encountered. below about 6
Parker 303-841-7119 ° Cnlora<ll1 Springs 719-633 5562 • Silvcrchurnc 970-468-1989
-2-
inches of topsoil, consist of sandy silty clay. Results of swell-consolidation testing
performed on relatively undisturbed samples of sandy silty clay, pre sented on Figures 3, 4
and 5 indicate low compressibility under existing moisture conditions and light loading
and a low to moderate collapse potential when welled . No free water was observed in the
pits at the time of excavation and the soils were 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 garage/ ADU. The soils tend to
compress after wetting and there could be some post-construction fou ndation settlement.
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 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 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. A
representative of the geotechnical engineer should observe all footing excavations prior to
concrete placement to evaluate bearing conditions.
Floor Slabs: The natural on -site soils, exclusive of topsoil. are suitable to support lightly
to moderately loaded slab-on-grade construction. To reduce the effects of some
differential movement, floor slabs should be separated from a11 bearing waUs 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 garage slabs to facilitate drainage. This material
should consist of minus 2-inch aggregate with less tban 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 al 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
also create a perched condition. We recommend below-grade construction, such as
retaining walls and walkout garage areas, be protected from wetting and hydrostatic
pressure buildup by an underdrain system.
Job No.116 012A
- 3 -
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 1 Vz feet deep.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the garage/ADU has been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction. Drying could increase the expansion
potentiaJ of the soils.
2) Exterior back.fill 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 IO feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in pavement and
walkway areas. A swale will be needed uphill to direct surface runoff
around the garage/ADU.
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 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
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.
Job No.I 16012A
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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 infonnation. 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.
U you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Louis Eller
attachments Figure 1 -Location of Exploratory Pits
Figure 2 -Logs of Exploratory Pits
Figures 3, 4 and 5 -Swell-Consolidation Test Results
Table I -Summary of Laboratory Testing
cc: Brian Golden (bg@twoscvcnink.com)
Job No.I 16 012A
116 012A
LOT13
APPROXIMATE SCALE
1 =BO'
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H worth-Powlak Geoledlnlcol
-----I
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Prr2 ~10 11
)//~~ PIT1 I
PROPOSED
( GARAGE/ADU
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./
/
/-
LOCATION OF EXPLORATORY PITS
LOT3
Figure 1
0
5
10
LEGEND:
PIT 1
ELEV.• 6873'
-200 .. 74
WC •104
00 ... 79
·200 -48
PIT2
ELEV.• 6868'
~ TOPSOIL: organic sandy silt and clay, firm, moist. dark brown
0
5
10
D ClAY (CL): sandy, silty to very silty, medium sti ff to stiff , moist, brown upper portion porous and blocky
~ 2· Diameter hand driven liner sample.
NOTES:
1 . Exploratory pits were excavaled on February 5, 2016 with a Cat m1mi-excava1or.
2 . Locations of exploratory pits were meastted approximately by pacing from features shown on the site plan
provided.
3. Elevalions ol exploratory pits were obtained by Interpolation between conlours shown on the site p lan provided
4. The exploratory pit locations and elevations should be considered accurale only to the degree implied by lhe 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 lime
7. Laboratory Testing Resulls :
WC = Water Content (%)
DD = Dry Density (pcij
·200 ... Percent passing No. 200 sieve
116 012A ~
H werth-Pcurlalt Geolecllnlcol
LOGS OF EXPLORATORY PITS Figure 2
-
Moisture Content "" 10.3 percent
Dry Density • 85 pcf
Sample of: Sandy SUty Clay
From: Pit 1 at 4 Feet
0
n~
1 -
,,,..,.., v Compression
l\ c._ upon
'#. 2 wetting
c: \ 0 ·u;
fll 3 QI ._ a. ' E
0 I\ (.)
4
\
5 I
6 \
I
7 I\
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01 1.0 10 100
APPLIED PRESSURE • ksl
116 012A ~
Hmworih-Powlc* C.Otechnlcol
SWELL-CONSOLIDATION TEST RESULTS Figure 3
Moisture Content "' 10.4 percent
Dry Density -79 pcf
Sample of: Very Silly Sandy Clay
From: Pil 1 at 5 Feet
0
i-~ r-.. r--..
1
.......
2 --Compression
~
,__.. ) "' upon
V'
i--' wett ing 3
l v"
4
..,.
5 c
.Q en en ~
Cl. 6 I 11
E \ 0 u
7
8
9
10 \
\ 11
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12 \
\
13 \
\
14 t'I
0 I 1.0 10 100
APPLIED PRESSURE ksl
116012A ~
MM>worth-Palrlolt Geatechnlcol
SWELL~CONSOLIDATJON TEST RESULTS Figure 4
Moisture Content "' 15.0 percent
Dry Density :::: 70 pcl
Sample al: Very Silly Sandy Clay
From: Pit 2 at 5 Feet
0
r---r--r---..
"" 1
1'11> Compression
..,/"' --~ upon
rft 2 ~ i,.;"' welling
c: v L ............
v,....
. Q en en 3 Q) ._
a E
0 I\ CJ
4
5 \
\
6 ~
7 \
8 \
\
9
\
10 I\
01 10 10 100
APPLIED PRESSURE ksl
116012A ~ H1C1Worih-P11wrt~ Geotedlnkol
SWELL-CONSOLIDATION TEST RESULTS Figure s
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116 012A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATIJRAL NATIJRAL GRADATION PERCENT ATTERBERG LIMITS UNCONFINED
MOISlURE DRY GRAVEL SANO PASSING LIQUID PLASTIC COMPRESSIVE
PIT DEPTH CONTI:HT DENSITY NO. 200 LIMIT INDEX STRENGTH SOIL TYPE
(%} (%) SIEVE
(It) {%) CccO (%) (%) (PSf)
1 4 10.3 85 Sandy Silty Clay
5 10.4 79 48 Very Silty Sandy Clay
6 6.1 83 74 Very Silty Sandy Clay
2 5 15.0 70 Sandy Silty Clay