HomeMy WebLinkAboutSubsoil Study for Foundation Design 03.29.2016• I
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HEPWORTH-PAWU\K GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 35, PINYON MESA
GARFIELD COUNTY, COLORADO
JOB NO. 116 060A
MARCH 29, 2016
PREPARED FOR:
SCOTT DILLARD
21 COUNTY ROAD 126
GLENWOOD SPRINGS, COLORADO 81601
(scottdillardrealtor@gmaibsom)
f
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
. '
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ - 1 -
PROPOSED CONSTRUCTION .................................................................................... - I -
Sn'E CONDITIONS ....................................................................................................... -2 -
S'UBSIDENCE POTE'NTIA.L .•••...••....••..•••...••....••..••••..•••..•••....•..•..•••..•.••..•.••...••...••..••.•• -2 -
FIELD EXPLORA.TION ..................................................................................................... - 2 -
SUBSURFACE CONDITIONS ...................................................................................... - 3 -
FOUNDATION BEARING CONDITIONS .................................................................. - 4 -
DESIGN RECOM.MENDATJONS ....................................................................................... -4 -
FOUNDATIONS ............................................................................................................... · 4 -
FOlJNDATION AND RE1'AINililG WALLS ........................................................... -6-
aOOR SLABS .......................................................................................................... - 7 -
UNDERDRAIN SYSTEM .......................................................................................... - 7 -
UNDERDRA.Il'l SYS1'EM .............................................................................................. -8 -
SURFACE DRAINAGE ............................................................................................... - 8 -
L™ITATIONS ........................................................................................................................... -9 -
FIGURE 1 -LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 and 5 -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 35, Pinyan Mesa. 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 agreement for gcotechnical engineering
services to you dated March 10, 2016 .
An exploratory boring was drilled to obtain infonnation on the subsurface conditions.
Samples of the subsoils obtained during the field exploration were tested in the laboratory
to detennine 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 one story wood frame construction above a crawlspace
with an attached garage. Garage floor will be slab-on -grade. Grading for the structure is
assumed to be relatively minor with cut depths between about 2 to 5 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. 116 060A
. .
-2-
SITE CONDITIONS
The site was vacant and free of snow at the time of our field exploration. Vegetation
consists of sagebrush, tall grasses and weeds. The ground surface slopes moderately
down to the west.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporitc underlies the Pinyan Mesa
development. 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 localized subsidence. During previous
work in the area, sinkholes have been observed scattered throughout the lower Roaring
Fork River Valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of
cavities was encountered in the subsurface materials ; however, the exploratory boring
was relatively shallow. for foundation design only. Based on our present knowledge of
the subsurface conditions at the site, it cannot be said for certain that sinkholes will not
develop. The risk of future ground subsidence on Lot 35 throughout the service life of
the proposed residence, in our opinion. is low; however, the owner should be made 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 March 18, 2016. One exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface
conditions. The boring was advanced with 4 inch diameter continuous flight augers
Job No.116 060A
-3-
powered by a truck-mounted CME-drill rig. The boring was logged by a representative
of Hepworth-Pawlak Geotechnical. Inc.
Samples of the subsoils were taken with a 2 inch l.D. 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 similar to the standard penetration test described by ASTM Method
D-l 586. The penetration resistance values are an indication of the relative density or
consistency of the subsoils and hardness of the bedrock. Depths at which the samples
were taken and the penetration resistance values 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 14!12 feet of stiff to
very stiff sandy clay and silt. Claystone/siltstone bedrock was encountered at a depth of
15 feet in the boring. Drilling in the claystone/siltstonc bedrock with auger equipment
was slow due to the hardness of the bedrock.
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 shallow undisturbed drive samples.
presented on Figure 4, indicate a low to moderate collapse potential {settlement under
constant load) when wetted and moderate to high compressibility under increased loading
after wetting. The sandy clay sample from JO feet deep, presented on Figure S, showed a
miner swell potential when wetted. The laboratory testing is summarized in Table l.
No free water was encountered in the boring at the time of drilling and the subsoils and
bedrock were slightly moist.
Job No. IJ6 060A
-4-
FOUNDATION BEARING CONDITIONS
The sandy silt and clay soils encountered at proposed foundation level tend to settle if
they become wet. The minor expansion pot~ntial measured on the sample from IO feet is
typical of sandy silty clay layers within the sandy clay and silt in this area and will not
impact the proposed shallow foundation. A shallow foundation placed on these soils will
have a risk of settlement if the soils become wet 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 term 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 be related to the depth and
extent of subsurface wetting. We expect that initial settlements will be less than 1 inch.
H wetting occurs, additional settlements of I to 2 inches could occur. Settlement in the
event of subsurface wetting will likely cause building distress and mitigation such as
structural fill below footing for a deep foundation, such as piles or piers extending down
below roughly 20 feet deep could be used to support the proposed house. If a deep
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 a minimum 5 feet of compacted structural fill with a risk of settlement,
particularly if the bearing soils become wet, is acceptable to the owner. Control of
surface and subsurface runoff will be critical to the long-term perfonnance of a shallow
spread footing foundation system.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
Job No. 116 060A
-5-
1) Footings placed on a minimum 5 feet of compacted structural fill 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 wm be about 1 inch or less.
Additional settlement of 1 to I~ inches could occur if the bearing soils
become wet. A VJ increase in the allowable bearing pressure can be taken
for toe pressure of eccentrically loaded footings.
2) The footings should have a minimum width of 20 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 also be designed to resist lateral earth pressures as
wscussed in the "Foundation and Retaining Walls" section of this report.
5) The topsoil and any loose or wsturbed soils should be removed below the
building area. The exposed soils in footing area should then be removed
down to 5 feet below design footing grade and the exposed subgrade
should be moistened and compacted. Structural fill should consist of low
permeable soil (such as the on site soils) compacted to at least 98%
standard Proctor density within 2% of optimum moisture content. The
structural fill should extend out from the edges of the fooling a distance of
at least ~ the depth of fill below the footing.
6) A representative of the geotechnical engineer should observe all
subexcavated footing excavations prior to fill placement to evaluate
bearing conditions. We should monitor fill placement and test the
compaction of the structural fdl.
Job No. 116 060A
-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 full 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 co~sisting 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 lo 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 backfilJ should be expected, even if the material is
placed correctly, and could result in distress lo facilities constructed on the backfill.
The 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 of the footings can be
Job No. 116 060A
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calculated based on a coefficient of friction of 0.30. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 300 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. are suitable to support lightly loaded slab-
on-grade conslruction. 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 movemenL 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
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 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
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded stab-
on-grade construction with settlement risk similar to that described above in the event of
wetting of the sub grade soils. To reduce the effects of some differential movement. floor
Job No . 116 060A
-8-
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 the garage slab. 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 fill materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor densi _ty at a moisture content near optimum. Required fill can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
An underdrain should not be placed around shallow footing depth structures such as the
garage and shallow crawlspace areas.
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. W c
Job No. 116 060A
-9-
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 I 0 feet in paved areas .
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 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 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 driUed at the location indicated
on Figure I. 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
changes may require additional analysis or modifications to the recommendations
Job No . t 16 060A
-JO-
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:
-
Daniel E. Hardin, P.B.
LEE/ksw
Jab No. 116 060A
. .
APPROXIMATE SCALE
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116 060A G~cta
HMwortlt-Pawlllk Geolechnlcal
LOCATION OF EXPLORATORY BORING Figure 1
BORING 1
0 0
11/12 we -so
00 .. 93
-200 ~12
5 15/12 5
WC •61
00•93
10 24/12 10
wc .. s1
00•107
·200 •90
15 102/12 15
~ ~
I I
.c .c a a.
~ 20 20
Q)
50/1 0
25 50/1 25
30 50/1 30
35 35
NOTE: Explanallon of symbols Is shown on Figure 3.
116 060A LOG OF EXPLORATORY BORING Figure 2
LEGEND:
TOPSOIL; organic sandy silt and clay, firm, sUghtly moist, dark brown.
CLAY AND SILT (CL-ML); sllghtly sandy to sandy, sandy clay layers with depth, stiff to very stiff, slightly moist,
light brown, slightly calcareous.
CLAYSTONE/SILTSTONE; hard, slightly moist, gray. Eagle Valley Evaporite Formation.
Relatively undisturbed drive sample; 2-lnch l.D. California llner sample.
11 /12
Drive sample blow count; Indicates that 11 blows of a 140 pound hammer falllng 30 inches were
required to drive the California or SPT sampler 12 inches.
NOTES:
1. The exploratory boring was drilled on March 1 B, 2016 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 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 approxlmate boundaries between
material types and transitions may be gradual.
6. No free water was encountered f n the boring at the lime of drilling. Fluctuation In water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%}
DO = Dry Density (pcQ
-200 = Percent passing No. 200 sieve
116 060A LEGEND ANO NOTES Figure 3
Moisture Content 3 5 .0 percent
Ory Density ... 03 pcf
Sample of: Sandy Clay and Silt
From: Boring 1 at 2 ~ Feet
0 -i---""io-N)
2 Compression --[/ upon
'*' 4 1 ............... '""wetting
.§ v--IC:: ...... ""' ... :-
Ct.I
(I)
~ 6 a.
E
0
(..)
8
10 ,1,
\ 12
\
14 r'\
\
16 '\
18
l\D
0 .1 1.0 10 100
APPLIED PRESSURE • ksf
116 060A ~ HeDWCirth-Po-.tak Geoltdlnlcol
SWELL-CONSOLIDATION TEST RESULTS Figure 4
Moislure Conlent = 6.1 percenl
Ory Oel'\Slty ... 93 pcf
0 Sample of: Sandy Clay and Sill
n1 From: Boring 1al5 Feet
1
~ r---I' < [::::
2
\ r-.... r-Compression -r--,.. .. ,_upon
'ii-3 wetting
6 ~ .iii
~ 4 c. ~ 8 \ (...)
5
'
6 \
' i>
7
0.1 1.0 10 100
APPLIED PRESSURE -ksf
Moisture Content • 6.1 percent
Dry Density • 107 pct
Sample of: Sandy Clay
From: Boring 1al10 Feel
~
-~ 0 .... ~ ~ UI
"" c ~~ m c.
in 1
• ' 1>
. § \ (I)
:3 2 a. Expansion
~ upon
welling
0.1 1.0 10 100
APPLIED PRESSURE • ksf
116060A omec11 SWELL-CONSOLIDATION TEST RESULTS Figure 5
tw.Warih-Pawtak Geatedlnlcd
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116 060A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATIJRAL NATURAL GRADATION PERCENT ATIERBERG LIMITS UHCONfINED
MOJsnJRE DRY GRAVEL SANO PASSING UQUJO Pl.ASTJC COMPRESSlVE SOJLOR BORING DEPTH CONTENT DENStlY N0.200 UMIT INDEX STRENG1lt BEDROCK TYPE <"'> (1111) SIEVE
fft:l fll&l CDCfl Cll&l Cci&l CPSFl
l 2112 5.0 83 72 Sandy Clay and Silt
5 6.1 93 Sandy Clay and Silt
10 6.1 107 90 Sandy Clay