HomeMy WebLinkAboutSubsoil Study for Foundation DesignI
I
c h
HEPWORTH -PAWLAK GEOT ECHN IC/\L
July 30, 2014
Revised September 2, 2014
Greg Vening
4 77 County Road 112
Carbondale, Colorado 81623
(greuv7@g ma il .com)
//,I'll'• nh-l ' 111 l.1 k l ~l "1,,h 1J1•. d. Inc.
i '~l'l .t11m 1 R1•1'l l i i
l •k-1rn""'-l :::111 11 ~-. l ·, /.,, i.f,, -.f ,,l'I
11 ... n c. >i l)·''-l; i •i:,s
I· I\: 'J(l1.'.).j ) •• '-l .j'.J
l·n uil· hr~t:''''h r~i..tikt..h ."'' 111
Job No. 114 271A
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 90,
Filing 7, Elk Springs, Gossamer Road, Garfield County, Colorado
Dear Mr. Vening
As requested, Hepworth-Pawlak Geotechnical, Inc. perfonned 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 July 8, 2014 . The data
obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report. Hepwo11h-Pawlak Geotechnical, Inc.,
previously perfonned a preliminary geotechnical study for Filings 6 through 9, Elk
Springs (fonnerly Los Amigos Ranch PUD) and reported our findings on February 14,
1997, Job No . 197 617.
Pl'Oposed Constmction: The proposed residence will be one story wood frame
construction over a walkout basement level with an attached garage . The residence will
be located in the upper, northwest part of the building envelope. Ground 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 vacant lot is located on a rolling upland mesa vegetated with sage
brush, grass and weeds. The ground surface in the building area slopes moderately down
to the east to a drainage easement and dry swale.
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 one
foot of topsoil , consist of basalt gravel , cobbles and boulders in a sandy silt and clay
matrix . A sandy clay lay er was observed in Pit l overlying the basalt rock . Results of
Park~r 30 )-8 41 -7 1 19 " Colnrn dn Spr in g" 7 19-6 33 -5 562 • Si k e rrl wrn 1..' 97 L1-468-I 989
)
- 2 -
swell-consolidation testing performed on a relatively undisturbed sample of the sandy
clay, presented on Figure 3, indicate low to moderate compressibility under conditions of
loading and wetting. Results of a gradation analysis performed on a sample of basalt
gravel (minus 3 inch fraction) obtained from the site are presented on Figure 4. 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
footings placed on the undisturbed natural granular soil designed for an allowable soil
bearing pressure of 2,000 psf for support of the proposed residence. Settlements are
expected to be less than 1 inch. Footings should be a minimum width of 16 inches for
continuous walls and 2 feet for columns. Utility trenches and deep cut areas below about
3 feet may require rock excavating techniques such as chipping or blasting. Topsoil,
sandy clay and loose or 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 basalt rock soils. Voids created from boulder removal at footing
grade should be filled with a structural material such as road base compacted to at least 98
percent of standard Proctor density at a moisture content near optimum or with concrete.
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 50 pcf for the on-site
soil as backfill excluding rocks larger than about 6 inches.
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 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.
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, basement 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 I foot below lowest adjacent finish
Job No.114 271A
- 3 -
grade and sloped at a minimum I% 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 1 Y, 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, fmer 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. A swale will be needed uphill to direct surface runoff
around the residence. ·
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.
Limitations: This study has been conducted in accordance with generally accepted
geotecbnical 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 repo1t 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
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, arid to
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications to the recommendations
Job No. l14 27!A
- 4 -
presented herein. We recommend on-site observation of excavations and foundati on
bearing strata and testing of structural fill by a representati ve of the geotechnical
engineer.
If you have any questions or if we may be of further assi stance, please let us know.
Respectfully Submitted,
Louis E. Eller
Reviewed by:
LEE/ksw
attaclunents
Job No .114 271 A
~tech
114 271A
APPROXIMATE SCALE
1' = 80'
LOT88
LOT90
r;--ITT1 I
I • PIT 2
I • PIT3
I I I BUILDING
ELOPE _J
---------. ---f}A~NI
_. -~w..i~GE. e~s-- - - - - - - - ------
I
I
I
I
~. LOCATION OF EXPLORATORY PITS
He worth-Pawfak Geoteehnlcal
LOT91
Figure 1
1i5
Cl>
LL
a
Cl>
0
0
5
10
LEGEND:
PIT 1 PIT 2
WC=11.9
DD=93
PIT3
' - -+4=92
.
-~ -200=3
TOPSOIL; organic sandy silt and clay, firm, slightly moist, dark brown.
CLAY (CL); sandy, gravelly, stiff, moist, brown, low plasticity.
0
5
10
BASALT GRAVEL, COBBLES, AND BOULDERS (GM); in a sandy silt and clay matrix, dense, slightly moist,
light brown, calcareous.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
i Practical digging refusal on boulders.
NOTES:
1. Exploratory pits were excavated on July 14, 2014 with a Case 9030B trackhoe.
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 logs 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:
WC = Water Content (%)
DD = Dry Density (pol)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
1i5
t'e
.c a
Q)
0
114271A ~ech LOGS OF EXPLORATORY PITS Figure 2
HEPWORTH·PAWLAK GKOTEC::HNICAL
Moisture Content = 11.9 percent
Dry Density = 93 pc!
Sample of: Sandy Silty Clay
From: Pit 1 at 2 Feet
~
'if. 1 ~
z
0
Cii 0 z ..:
0.. ---.... x w \ "--.. ' 1 z \ "' 0
Cii \.) en w 2 er:
0.. ,/ :2
0
(.) ,
•' Ex pan: ion
~ -upo 1
wetti g
0.1 1.0 10 100
APPLIED PRESSURE ( ks!)
114 271A ~ech SWELL-CONSOLIDATION TEST RESULTS FIGURE 3
HEPWORTH-PAWLAK GEOTECHNICAL
• HYDROMETER ALYSIS SIE'v'EANALY:-;IS
24tR.
TIME READINGS I U.S. STANDARD SERIES I CLEAR SQUARE OPENINGS I
7HR
45 MIN. 1SMIN. 60Mli'I. 19MIN. . ""· '"" ..oo 1100 ... "'' "' .. .. "" ,, ,,,,. • .. • 0 100
10 90
20 eo
30 70
Cl 40 so CJ UJ z z ~ ~ a: 60 50 ~ ~ UJ UJ u u a: a: UJ UJ 60 40 Cl. a.
70 30
.. 20
90 10
100 0
.001 = .005 .009 ,QUI 1'.!7 .01• .... """ ""' 1.18 ..,, .... 9.5 12.5 19.0 37.5 "' 127152 ""
DIAMETER OF PARTICLES IN MILLIMETERS
I """ I '"'""' I CLAY TO SILT I I COAASE I "'""'" ""' '-tiaJLN ,., COARSE
Gravel 92 % Sand 5 % Silt and Clay 3 %
Liquid Limit % Plasticity Index %
Sample of: Sandy Gravel From: Pit 2 at 2 1/2 Feet to 3 Feet
114271A ~ech GRADATION TEST RESULTS FIGURE 4
HEPWORlH-PAV.UK GEOTECHNICAL
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 114 271A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NA1URAL NA1URAL GRADATION
PERCENT
ATTERBERG UMITS UNCONANED
MOIS11JRE DRY GRAYa SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR PIT DEPTI-1 CONTENT DENSITY (%) (%) NO. 200 LIMIT INDEX STRENGTI-1 BEDROCK 1YPE
(ft) 1%) (nrf\
SIEVE (%\ (%\ !PSFl
1 2 11.9 93 Sandy Silty Clay
2 2'h - 3 92 5 3 Sandy Gravel
.