HomeMy WebLinkAboutSubsoil Study for Foundation Design 06.3016~tech
HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
Hepworth.Pawlik Geotechnlcal, Inc.
5020 County Road 154
Glenwood Springs, Colonido 81601
Phone: 970-945-7988
Fax: 970-945-8454
EmaU: hpgeo@hpgeotech.com
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 28, CORYELL RANCH
SPIRIT MOUNTAIN DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 116 275A
.JUNE 30, 2016
PREPARED FOR:
PAT & JOHN WEISBART
P.O. BOX 3513
BOULDER, COLORADO 80307
jobn.weisbart<@.gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ -I -
PROPOSED CONSTRUCTION ...................................................................................... I ·
SITE CONDITIONS ....................................................................................................... -2 -
SUBSIDENCE POTENTIAL ......................................................................................... -2 •
FIELD EXPLORATION ................................................................................................. • 2 -
SUBSURFACE CONDITIONS ...................................................................................... -3 -
DESIGN RECOMMENDATIONS ................................................................................. -3 •
FOUNDATIONS ........................................................................................................ -3 -
FOUNDATION AND RETAINING WALLS ........................................................... • 4 •
FLOOR SLABS .......................................................................................................... -6 -
UNDEiillRAJN SYSTEM .......................................................................................... • 6 -
SURFACE DRAINAGE ............................................................................................. -7 -
LIMITATIONS ................................................................................................................ 7 •
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2 -LOGS OF EXPLORATORY BORINGS
FIGURE 3 -GRADATION TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be localed at
Lot 28, Coryell Ranch, Spirit Mountain Drive, Garfield County, Colorado. The project
site is shown on Figure 1. The purpose of the study was to develop recommendations for
the foundation design . The study was conducted in accordance with our proposal for
geotechnical engineering services to Pat and John Weisban, dated June 23, 2016.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the
field exploration were tested in the laboratory to determine their classification and other
engineering characteristics. The results of the field exploration and laboratory testing
were analyzed to develop recommendations for foundation types, depths, and allowable
pressures for the proposed building foundation. This report summarizes the data obtained
during this study and presents our conclusions, design recommendations, and other
gcotechnical engineering considerations based on the proposed construction and the
subsurface conditions encountered.
PROPOSED CONSTRUCTION
Our subsoil study was perfonncd for the purchase of the property and specific plans for
the residence ha\'c not been developed. The house will likely be located on the northeast
portion of the building envelope closest to the Spirit Mountain Drive. We assume the
proposed residence to be a one and two-story, wood frame structure constructed over a
crawlspace with a possible basement. Ground floor will either be slab-on-grade or
structurally supported over crawlspace. Grading for the structure is assumed to be
relatively minor with cut depths of about 3 to l 0 feet. We assume relatively light
foundation loadings, typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
Joh No I U1 275/\
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SITE CONDITIONS
The proposed residence will be situated on Lot 28, Coryell Ranch, which is localed at the
end of Spirit Mountain Drive in Garfield County. The lot is currently vacant and is
bounded by Lot 29 to the west, private property to the cast, Lot 27 to the north, and
Midland Point Subdivision to the south. The proposed building area is generally flat,
with less than I fool of elevation difference. Just south of the lot, an active irrigation
ditch borders the south side of the lot with a steep slope up to Midland Point Subdivision.
There is an active irrigation ditch on the west side of the lot and along the Spirit Mountain
Drive cul-de-sac. Vegetation on the site consists mainly of grass and weeds. The area
has historically been used as irrigated pasture land, and minor grading of the site
previously occurred during subdivision development.
SUBSIDENCE POTENTIAL
Coryell Ranch is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The
evaporite contains gypsum deposits. Dissolution of gypsum under certain conditions can
cause sinkholes to develop and can produce areas of localized subsidence. During
previous work in the area, sinkholes have been observed in the lower Roaring Fork River
Valley. Sinkholes were nol observed in the immediate area of the subject lot, although
the lot was previously graded during subdivision development. Based upon our present
1-.nowledge of the site, it cannot be snid for certain that sinkholes will not develop. In our
opinion. the risk of ground subsidence at Lot 28 is low throughout the service life and
similar to other lots in the area, but the owner should be aware of the potential for
sinkhole development.
FIELD EXPLORATION
The field exploration for the project was conducted on June 28, 2016 . Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface
conditions. The borings were advanced with 4 inch diameter continuous flight augers
powered by a truck-mounted CME-458 drill rig. The borings were logged by a
representative of Hepworth-Pawlak Gcotechnical, Inc .
Jnll No I IC. 27SA
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Sampl<:s of the subsoils were taken with a 1¥.. inch 1.0. spoon sampler. The sampler was
driven into the subsoils at various depths with blows from a 140 pound hammer falling 30
inches. This test is sim ilar to the standard penetration test described by ASTM Method
D-1586. The penetration resistance values are an indication of the relative density or
consistency of the subsoils. Depths nt which the samples were taken and the penetration
resistance values arc shown on the Logs of Exploratory Borings , Figure 2. The samples
were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site arc shown on Figure 2.
llte subsoils, beneath about 3 lo 6 inches of organic topsoil, consist of dense , slightly silty
sandy gravel with cobbles and probable small boulders, down to practical drilling refusal
at depths of 4 lo 7 feet. A thin stiff sandy silty clay layer, about 1 foot thick, was
encountered in Boring 2 overlying the dense gravel. Based on our experience in the area,
the dense gravels extend down to considerable depth below expected foundation grades
and groundwater is nlso relatively deep, probably about th e nearby river level.
Laboratory testing performed on samples obtained from Boring I included moisture
content and gradation analysis. Results of the gradation analysis perfom1cd on the minus
I ~~ -inch portion of the coarse granular subsoils arc shown on Figure 3.
No free water was encountered in the borings at the time of drilling. The subsoils were
slightly moist .
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature or the proposed construction . we recommend the building be founded with spread
footings bearing on the natural grnnular soils.
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The design and construction criteria presented below should be observed for a spread
footing foundation system.
I) Footings placed on the undisturbed natural granular soils should be
designed for nn allowable bearing pressure of 2,500 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be less than 1 inch .
2) The footings should have a minimum width of 16 inches for continuous
walls and 2 feet for isolated pads .
3) Exterior footings and footings beneath unheated areas should be provided
with adequate soil co\'er above their bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typicall y used in this area.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomal ics, such as by assuming an unsupported length of at least 10
ICet. Foundation walls acting as retaining structures should also be
designed to resist lateral earth pressures as discussed in the 11 Foundation
and Retaining Walls" section of this report .
5) The topsoil, clay soils and any loose or disturbed soils should be removed
and the footing bearing level extended down to the relatively dense natural
grnnular soils . The exposed soils in footing areas should then be
moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excm ations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINJNG WALLS
Foundation walls and retaining structures that arc lnterally supported and can be expected
to undergo only a slight amount of deflection should be designed for a lateral enrth
pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for
backfill consisting of the on·sitc granular soils . Cantilevered retaining structures that are
separate from the residence and can be expected to deflect sufficiently to mobilize the full
acti\ c Cilrth pressure condition should be designed for a lateral earth pressure computed
Joh No I IC1 :?7SA
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on the basis or an cquivalcnl fluid unit weight of at least 40 per for backfill consisting of
the on-site granular soils. Backfill should not contain topsoil or rocks larger than about 6
inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures, such as adjacent footings, traffic, construction materials and
equipment The pressures recommended above assume drained conditions behind the
walls and a horizontal backfill surface. Buildup of water behind a wall or an upward
sloping back fill surface will increase the lateral pressure imposed on a foundation wall or
retaining structure. An underdrain should be provided to prevent hydrostatic pressure
buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement and
walkway areas should be compacted to al least 95% of the ma.ximum standard Proctor
density. Care should be taken not to overcompact the backfill or use large equipment
near the wall, since this could cause excessive lateral pressure on the wall. Some
settlement of deep foundation wall backfill should be e:-.pected, even if the material is
placed correctly, which could result in distress to facilities constructed on the backfill.
rhe lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure
against the side of the footing. Resistance to sliding at the bottoms or the footings can be
calculated based on a coefficient of friction of 0.50. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 400 pcf. The coefficient of friction and passive pressure values recommended
above assume ultimate soil strength. Suitable factors of safety should be included in the
design to limit the strain that will occur at the ultimate strength, particularly in the cose of
passive resistance. Fill placed against the sides of the footings to resist lateral loads
should be compacted to at least 95% of the maximum standard Proctor density at a
moisture content near optimum.
Joh Nu 11 (1 275"
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FLOOR SLABS
The nalural on-sile soils, exclusive of Lopsoil, are suitable lo 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 that allow
unrestrained vertical movement. Floor slab control joints should be used lo 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 ph1ced beneath
basement level slabs to facilitate drainage. This material should consist of minus 2-inch
aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No.
200 sieve.
All fill materials for support or 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 granular 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
era\\ lspace and basement areas, be protected from welling and hydrostatic pressure
buildup by an undcrdrain system.
The drain 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 al
each level of excavation and at least I foot below lowcsl adjacent finish grade and sloped
at a minimum I% to a suitable outlet, such as a drywell or sump and pump. Free-draining
granular material used in the underdrain system should contain less than 2°Ai passing the
No. 200 sieve , less than 50% passing the No. 4 sieve and have a maximum size of 2
inches. The drain gravel bad.fill should be at least l ~= feet d«!ep.
Joh No. I 1<1 :?73A
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and
maintoined at all times after the residence has been completed:
I) Inundation of the foundation excavations and under-slab areas should be
a voided during construction .
2) Exterior backfill should be adjusted to near optimum moisture and
compnctcd to at least 95% of the maximum standnrd Proctor density in
pavement and slab areas and to nt least 90~o of the maximum standard
Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the build ing should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of al least 6 inches in the lirst 10 feet in
unpaved areas and a minimum slope of 3 inches in the first I 0 feet in
paved areas . Free-draining wall backfill should be capped with about 2
feet of the on-site liner grained soils to reduce surface water infiltration .
4) Roof downspouts and drains should discharge well beyond the limits of all
backlill.
LIMITATIONS
This study has been conducted in acco rdance with generally accepted geotechnical
engineering principles and practices in this area at this lime. We make no warranty either
express or implied . The conclus ions nnd recommendations submitted in this report are
based upon the dala obtained from the exploratory borings excavated at the locations
indicated on Figure 1. the proposed type of construction and uur 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 borings and \·ariations in the subsurfoce
conditions may not become evident until excavation is performed. If conditions
Job No . 11<> 275A
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encountered during construction appear different from those described in this report , we
should be notified so thal re-evaluation of the recommendations may be made .
This report has been prepared for the exclusive use by our client for design purposes. We
arc 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 mon itor 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 nnd testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
Reviewed by :
Steven L. Pawlak, P.E .
DEI -1/ksw
Joh No . 116 27SA
APPROXIMATE SCALE
1· = 100'
LOT29
116 275A
LOT27
BORING 1 O
BORING2o
LOT28
LOCATION OF EXPLORATORY BORINGS Figure 1
BORING 1 BOAING2
0 0
73/12 1 5617
WC ::.1.7 301/2 iD g +4 =~6 if u. 5 -200 =12 5 I I 73/12 .r:: .s::;
a. a
Q.I a>
0 0
10 10
Note : Explanation ol symbols is shown on Figure 3
116 275A LOGS OF EXPLORATORY BORINGS ~
HEPWOltnt-PAWLAK GltOTECHNlc:AL
Figure 2
LEGEND:
TOPSOIL: roots, organics, sill, c1ayey, sandy, cobbles present, medium sliff, slighlly moisl. brown
CLAY (CL); silty, slight 'y sandy, still, slightly moist, brown.
GRAVEL ANO SAND (GM-SM); silty, cobbles and boulders probable, dense to very dense, slightly
moist, brown .
Drive sample; standard penetration lest (SPl), 1 3/8 inch I D. split spoon sample, ASTM D-1586.
39/12 Drive sample blow count; indicates that 39 blows of a 140 pound hammer railing 30 inches were
required to drive the SPT sampler 12 inches.
T Practical Drilling Refusal
NOTES:
1. Exploratory borings were drilled on June 28, 2016 with 4-inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
provided
3. Eleva!lons of exploratory borings were not measured and the logs of exploratory borings are drawn to depth.
4. The exploratory boring locations should be considered accurate only to the degree implied by the method used
5. The lines between materials shown on the exp oratory boring logs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the borings at the lime of driRing or when checked O days later. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC = Waler Content (%)
+4"" Percent retained on the No. 4 sieve
-200 • Percent passing No. 200 sieve
116 275A c6'tMech
Heoworth-Powlok Ceot1chnlcol
LEGEND AND NOTES Figure 3
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HYDROMETER ANALYSIS 5 EVE ANAL YS:S
I TIME READINGS I U S ST A.'llOAAD SERIES I
O ~~ m1 1~ ~i 60MINl9MIN 4 MIN 1 MIN #200 #100 ISO #30 #16 #8 #4
10
20
30
40
so
70
eo
90
100
001 00? 005 009 01:1 .037 .074 ,150 300 600 1 18 236 4 75
OIAMETEFt OF PAATIClES IN M LLIMETERS
I • ..__\N
COBBLES O % GRAVEL 46 % SANO 42 %
Ct..EAA SQUARE OPENINGS I
318 3/4 1 1/2 3 5"6 a· 100
95 190 375
125
76 2 152 203
127
CA.<'1• I C."OH. cs
In.. I cu.~ .
SILT ANO CLAY 12 %
90
80
70
60
so
30
20
10
0
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF . S lly Sand and Gravel FROM Boring 1 at 2 ~ and 5 Feet Combined
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116 275A GRADATION TEST RESULTS Figure 4