HomeMy WebLinkAboutSubsoil Study for Foundation Design 11.30.15HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
H:p\H1r1h P.1wl.1k Gcc>1cchnrc.1I. Inc
5020 Cnumy Road 154
Glcn1n>OJ Sprinp, ColorJJu 81601
rlumc: 9i0-9·H 7988
fac: 970-945·8'*;'*
email: hr!!rn®hri:cc1t1:c:h cmn
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 45, PINYON MESA
GARFIELD COUNTY, COLORADO
JOB NO. 115 484A
NOVEMBER 30, 2015
PREPARED FOR:
JULIO SANDOVAL
c/o MICHAEL EDINGER
1331 E. SOPRIS ROAD
BASALT, COLORADO 81621
(michael@aliusde.com)
Parker 303-841-7119 • Colorado Springs 719-633-5 562 • Silvcnhome 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ - 1 -
PROPOSED CONSTRUCTION ...................................•.•••..•......................................... -1 -
snc CONDITIONS ....................................................................................................... - 2 -
SUBSIDENCE POTENTIAL .............................•....••..................................................... -2 -
FIELD EXPLORATION ................................................................................................. -2 -
SUBSURFACE CONDITIONS ...................................................................................... -3 -
FOUNDATION BEARING CONDITIONS .................................................................. -3-
DESIGN RECOMMENDATIONS ................................................................................ -4 -
FOUNDATIONS ........................................................................................................ -4 -
FOUNDATION AND RETAINING WALLS ........................................................... -6-
FLOOR SLABS .......................................................................................................... -7 -
UNDERDRAIN SYSTEM .......................................................................................... -7 -
SURFACE DRAINAGE ............................................................................................. - 8 -
LIM:IT ATIONS ....................................................................... , ......................................... -9 -
FIGURE I -LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 -LEGEND AND NOTES
FIGURES 4, 5 AND 6 -SWELL-CONSOLIDATION TEST RESULTS
TABLE 1-SUMMARY OF LABORATORY TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at
Lot 45, Pinyan Mesa, Cliff Rose Way, Garfield County, Colorado. The project site is
shown on Figure l. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for
geotechnical engineering services to you dated October 9, 2015. We previously
performed preliminary geotechnical engineering studies for the subdivision development
and presented our findings in reports dated November 11, 2005 and April 10, 2006, Job
No. 105 652 .
An exploratory boring was drilled 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, compressibility or swell 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 geotechnical
engineering considerations based on the proposed construction and the subsurface
conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be 11 one and two story wood frame structure above a full
basement and with an attached garage. Basement and garage floors will be slab-on-grade.
Grading for the structure is assumed to be relatively minor with cut depths between about
3 to 9 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.
Job No 115 484A
-2 -
SITE CONDITIONS
The vacant lot is vegetated with grass, weeds and scattered sage brush. The ground
surface slopes moderately down to the north. The Eagle Valley Evaporite fonnation is
exposed on a steep hillside south of the site.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evnporite underlies the Pinyon Mesa
Subdivision. Bedrock was not encountered to the depth drilled of SI feet in the
exploratory boring drilled nt the subject site. These rocks are a sequence of gypsiferous
shale, fine-grained sandstone and siltstone with some massive beds of gypsum and
limestone. There is a possibility that massive gypsum deposits associated with the Eagle
Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain
conditions can cause sinkholes to develop and can produce areas of locaJized subsidence.
Sinkholes were not observed in the subdivision, but geologically young sinkholes are
locally present in the evaporite region between Glenwood Springs and Carbondale and we
are aware of three relatively recent sinkholes developing in the lower part of the Roaring
Fork River valley. Based on our current understanding of the evaporite sinkhole process,
the areas in western Colorado (including the project site) where cvaporite is shallow, have
the potential for sinkhole development. The boring drilled at the subject site was
relatively shallow, for foundation design, and no voids or cavities were encountered to the
drilled depth of 51 feet. The risk of future ground subsidence on Lot 45 throughout the
service life of the proposed residence, in our opinion, is low; however, the owner should
be aware of the potential for sinkhole development. If further investigation of possible
cavities in the bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on November 3, 2015. One
exploratory boring was drilled at the location shown on Figure l to evaluate the
Job No 11 S 484A
.3.
subsurface conditions. The boring was advanced with 4 inch diameter continuous flight
augers powered by n truck-mounted CME-45B drill rig. The boring was logged by n
representative of Hepworth·Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with n 2 inch I.D. spoon sampler. The sampler was
driven into the subsoils at various depths with blows from n 140 pound hammer fnlling 30
inches. This test is similar to the standard penetration test described by ASTM Method D·
1586. The penetration resistance values are nn indication of the relative density or
consistency of the subsoils. Depths at which the samples were taken nnd the penetration
resistance vnlues are shown on the Log of Exploratory Boring, Figure 2. The samples
were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2.
The subsoils consist of about 6 inches of organic topsoil overlying stiff to very stiff
slightly clayey sandy silt down to the depth drilled, 51 feet.
Laboratory testing performed on samples obtained from the boring included natural
moisture content, density and percent finer than sand size gradation analyses. Results of
swell·consolidation testing performed on relatively undisturbed drive samples, presented
on Figures 4, 5 and 6, indicate low compressibility of the sandy silt soil under light
loading and a low to moderate collapse potential (settlement under constant load) when
wetted. The samples showed high compressibility under increased loading after wetting.
The laboratory testing is summarized in Table I.
No free water was encountered in the boring at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The sandy silt soils encountered at typical shallow foundation depth tend to settle when
they become wetted. A shallow foundation placed on the upper sandy silt soils will have
Job No 11.S 484A
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a high risk of settlement if the soils become wetted and care should be taken in the
surface and subsurface drainage around the house to prevent the soils from becoming wet.
It will be critical to the long tcnn perfonnance of the structure that the recommendations
for surface drainage and subsurface drainage contained in this report be followed. The
amount of settlement, if the bearing soils become wet, will mainly be related to the depth
and extent of subsurface wetting. We expect that initial settlements will be less than I
inch. If wetting of the shallow soils occurs, additional settlements of 2 to 3 inches could
occur. Settlement in the event of subsurface wetting wiU likely cause building distress
and mitigation methods such as deep compaction, a deep foundation such as piles or piers
or a heavily reinforced mat foundation (on the order of 2 feet thick) should be used to
support the proposed house. If a deep foundation or mat foundation is desired, we should
be contacted to provide further design recommendations.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, the building can be founded with spread footings
bearing on compacted structural fill with n risk of settlement, mainly if the underlying
soils become wetted, and provided the risk is acceptable to the owner. Control of surface
and subsurface runoff will be critical to the Jong-term performance of a shallow spread
footing foundation system. The garage footing areas should be sub-excavated down
about 8 to 10 feet below existing ground surface and the excavated soil replaced
compacted back to design bearing level but to a depth of at least 6 feel below footing
bearing level. We recommend the basement area footing grade be sub-excavated at least
3 feet and the excavated soil replaced compacted back to design bearing level.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
lob No . II S 484A
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1) Footings placed on a minimum 6 feet of compacted structural fill for the
garage nnd at least 3 feel of compacted structural fill for the basement
level of the residence should be designed for an allowable bearing pressure
of l ,200 psf. Based on experience, we expect initial settlement of footings
designed and constructed as discussed in this section will be about 1 inch
or less . Additional settlement of about I inch could occur if the silt and
clay soils below the bearing level become wetted. A Y, increase in the
allowable bearing pressure can be taken for toe pressure of eccentrically
loaded (basement wall) footings.
2) The footings should have a minimum width of 24 inches for continuous
walls and 2 feet for isolated pads .
3) Exterior footings and footings beneath unheated areas should be provided
with adequate soil cover above their bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typically used in this area.
4) Continuous foundation walls should be heavily reinforced top and bottom
to span local anomalies such as by assuming an unsupported length of at
least 14 feet. The foundation should be configured in a "box like .. shape to
help resist differential movements . Foundation walls acting as retaining
structures should nlso be designed to resist lateral earth pressures as
discussed in the "Foundation and Retaining Walls" section of this report.
5) The topsoil and any loose or disturbed soils should be removed below the
building area. The exposed soils in footing areas after sub-excavation to
design grades should then be moistened and compacted. Structural fill
should consist of low permeable soil (such as the on-site silt and clay soils)
compacted to at least 98% standard Proctor density within 2% of optimum
moisture content. The structural fill should extend laterally beyond the
footing edges equal to about ~ the fill depth below the footing.
6) A representative of the geotechnical engineer should evaluate the
structural fill as it is placed for compaction and observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
Job No I IS 484A
-6 -
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be
expected to undergo only a slight amount of deflection should be designed for a lateral
earth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf
for backfill consisting of the on-site fine-grained soils. Cantilevered retaining structures
which are separate from the residence and can be expected to deflect sufficiently to
mobilize the f uU active earth pressure condition should be designed for a lateral earth
pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for
backfill consisting of the on-site fine-grained soils.
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. The buildup of water behind a wall or an upward
sloping backfill 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 at least 95% of the maximum 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 expected, even if the material is
placed correctly, and could result in distress to facilities constructed on the backfill.
The lateral resistance off oundation 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 of the footings can be
calculated based on a coefficient of friction of 0.35. Passive pressure of compacted
Job No I IS 484A
Moisture Content === 4.4 percent
Diy Density = 102 pcf
Sample of: Slightly Clayey Sandy Sill
From: Boring 1 at 2 Feet
0
-t-r--. I--""' !"or-,~
1
~ Compression
i.---l/ ... upon ~ i..-
t.,..L-
/_ __... ..... welling 'it 2
c 0 c;;
Ill 3 ! a. (\ E
0
0
4
5 \
6 I\
\
I
7 \
' B \
\
l
9
0.1 1.0 10 100
APPLIED PRESSURE ksf
115 484A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4
Hcwrth-Pcnirlak GeolllChnlcGI
Moisture Content .... 5.2 percent
Dry Density ... 94 pcl
Sample of: Sandy Silt
From: Boring 1 at 5 Feel
0
-
1 Compression -......
c:::' upon
c wetting
"ii. 2
'\ c:
0 a;
Cl)
3 Q> a. \ E
0
0 4
5 \
~
6 \
\
7 \.
\ 1b
a
01 1.0 10 100
APPLIED PRESS liAE ksf
115 484A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 5
w..;worth-Pawtdl Geolechnlc:al
Moisture Content = 6.8 percent
Dry Density .. 93 pcf
Sample of : Sandy SRt
From: Boring 1at15 Feel
0
~ ---r-..... i"'..... 1
~ .....
2 ~"
j ) Compression
v i-~ upon
3 v )1 -[.... i.. ·welling
'ff. ~ v i...... .s {. v
(I) en 4 Q) ._ a.
E 0 u 5
0
6 \
\ 7
8 \
g
'
10 ~
11 \
' 12 <\
13 \
14 ~
1
I )
15
0 .1 1.0 10 100
APPLIED PRESSURE -ksf
115 484A ~
Hmworth-Pawlak Gealec:hnlcal
SWELL-CONSOLIDATION TEST RESULTS Figure 6
HEPWORTH-PAWLAK GEOTECHNICAl, INC.
TABLE 1 Job No. 115 484A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL GRADATION ATIERBEllG LIMITS UNCONFINED
MOISTURE NATURAL PERCENT COMPRESSIVE
BORING DEPTH ORY DENSITY GRAVEL SAND PASSING NO. UQUIDUMIT P1.ASTIC SOIL TYPE CONTENT INDEX STRENGTM
(") (") lOOSIEVE
(ft) '"' (pcfJ I"> '") (PSFI
1 2 4.4 102 Slightly Clayey Sandy Silt
5 5.2 94 Sandy Silt
10 8.3 99 89 Slightly Clayey Sandy Silt
15 6.8 93 Sandy Silt
20 5.4 105 51 Sandy Silt with Shale
Fragments
LEGEND:
TOPSOIL; organic sandy slit and clay, firm , slightly moist, brown.
SILT (ML); sandy, slightly clayey. scattered shale fragments with depth, stiff to very stilf, slightly moist, brown,
calcareous streaks .
Relatively undisturbed drive sample; 2-inch 1.0. California liner sample.
Drive sample blow count; indicates lhat 17 blows of a 140 pound hammer falling 30 inches were
17/12 required to drive the California sampler 12 inches .
NOTES:
1. The exploratory boring was drilled on November 3, 2015 with a 4-inch diameter continuous lllght 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 localion 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 . No free water was encountered in the boring at the time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC -Water Content (%)
DD = Ory Density (pc!)
-200 "' Percent passing No. 200 sieve
115 484A LEGEND AND NOTES Figure 3
-10-
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.
Respectfully Submitted.
HEPWORTH -PAWLAK GEOTECHNICAL, INC.
Louis E. Eller
Reviewed by;
LEE/ksw
Job No. I IS 484A
.
~
CUFF ROSE WAY (
I --
APPROXIMATE SCALE
1· = 30'
r-------1
I I
I I • I I
I I
I
BORING 1 I
I I
I I
I I
I r LOT44
LOT46 l I
I I
I I
I I
I I L _______ .J
LOT45
I I
I I
--
115 484A c~~ch LOCATION OF EXPLORATORY BORING Figure 1
H1Dwarth-Pa.tdl Geotmdlnlcal
BORING 1
0 t7/12 0
WC r 44
DO 102
15/12
WC -52
OD 94
10 15112 10
we s.3
00-.99
·200 89
14/12
WC ,.6.8
00 -93
20 20 23.112
WC -5.4
00 105
·200 51
14112 i ~ 30 u.
' 30 26112 •
~ .c a ii
QI ~ 0
40 40/12 40
50 50/4 50
60 60
NOTE: Explanation or symbols Is shown on Figure 3
115 484A LOG OF EXPLORATORY BORING Figure 2
-7-
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 325 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 which will occur at the ultimate strength. particularly in the case
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.
FLOOR SLABS
The natural on -site soils. exclusive of topsoil. can be used to support lightly loaded slab-
on-grade construction with settlement risk similar to that described above for foundations
in the event of wetting of the subgrade soils. 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 interior slabs to limit capillary
moisture rise. This material should consist of minus 2 inch aggregate with at least 50%
retained on the No. 4 sieve and less than 12% passing the No. 200 sieve.
All fiJI 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 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
Job No. 115 484A
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basement areas, be protected from wetting and hydrostatic pressure buildup by nn
underdrain system. An underdrain should not be placed around shallow footing depth
structures such as the garage area and shallow crawlspace, if provided.
The drains should consist of drainpipe placed in the bottom of the wnll 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 I% to an interior sump of solid casing. 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 Y2 feet deep. An impervious
membrane such as a 20 mil PVC liner should be placed beneath the drain gravel in a
trough shape and attached to the foundation wall with mastic to prevent wetting of the
bearing soils.
SURFACE DRAINAGE
It will be critical to the building performance to keep the bearing soils dry. 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 n 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 paved areas.
Joh No 115 484A
Free~draining wall backfill should be covered with filter fabric and capped
with at least 2 feet of the on-site soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill. Natural vegetation lined drainage swales should have a minimum
slope of3%.
5) Landscaping which requires regular heavy irrigation should be located at
least 10 feet from foundation walls. Consideration should be given to use
of xeriscape to reduce the potential for wetting of soils below the building
caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical
engineering principles nnd practices in this nrea 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, 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 so
that 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, and to
verify that the recommendations have been appropriately interpreted. Significant design
Job No 115 484A