HomeMy WebLinkAboutSubsoil Study for Foundation 03.10.16HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 11, PINYON 'MESA SUBDIVISION, FILING 1
SAGE MEADOW ROAD
GARFIELD COUNTY, COLORADO
JOB N0.116 046A
MARCH 10, 2016
PREPARED FOR:
VICTOR HERNANDEZ
P.O. BOX 3251
GLENWOOD SPRINGS, COLORADO 81602
victor830l@gmail.com
Parker 303 -841 -7119 • ColoradoSprings 719-633 -5562 • Silverthorne 970 -468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ -1 -
PROPOSED CONSTRUCTION .................................................................................... -l -
Sll'E CONDITIONS ....................................................................................................... -2 -
FIELD EXPLORATION ................................................................................................. -2-
SUBSURFACE CONDITTONS ...................................................................................... -2 -
SUBSIDENCE POTENTIAL ......................................................................................... -3 -
DESIGN RECOMMENDATIONS ................................................................................ -4-
FOUN'DATIONS ........................................................................................................ -4 -
FOUNDATION AND RETArn!NG W ALL.s ........................................................... -5 -
FI..OOR SLABS .......................................................................................................... -6 -
UNDERDRAIN SYSTEM .......................................................................................... - 7 -
SURFACE DRA.™AGE .............................................................................................. -7 -
L™ITATIONS ................................................................................................................ -8 -
AGURE 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
on Lot 11. Pinyon Mesa Subdivision. Filing 1. Sage Meadow Road. 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 geotechnical engineering services to Victor Hernandez
dated March 2, 2016. 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 on the lot to obtain information on the subsurface
conditions . Samples of the subsurface materials obtained during the field exploration
were tested in the laboratory to dctennine 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, desigo recommendations
and other geotechnical engineering considerations based on the proposed construction and
the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The residence will generally consist of a 2 story. wood frame structure over a basement
with an attached single story garage located as shown on Figure 1. Basement and garage
floors will be slab-on-grade. Grading for the residence is assumed to be relatively minor
with cut depths between about 3 to 10 feet. We assume relatively light foundation
loadings, typical of the proposed type of construction.
If building location, grading or loading information changes from that described above.
we should be notified to re-evaluate the recommendations presented in this report.
Job No. 114 l76A
-2-
SITE CONDITIONS
The Jot was vacant at the time of our field exploration. The Jot is localed on the north
(uphiU} side of Sage Meadow Road as shown on Figure I. The ground surface is
relatively flat with a gentle slope down to the south. There is about 2 to 3 feet of
elevation difference across the building envelope. Vegetation consists mainly of sage
brush with sparse grass and weeds.
FIELD EXPLORATION
The field exploration for the project was conducted on March 3. 2016. One exploratory
boring was drilled on the lot as shown on Figure 1 to evaluate the subsurface conditions.
The boring was advanced with 4-inch diameter continuous flight augers powered by a
truck-mounted CME-45B 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 subsurface materials 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-1586 . 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 profile encountered in the boring is shown on Figure 2.
The subsoils, below about 1/2 foot of organic topsoil. consist of about 4 feet of silty clayey
sand and gravel with rock fragments to possibly cobble size overlying stiff to very stiff.
sandy clayey silt. At a depth of about 12 feet. siltstone/claystone bedrock was
encountered down to the maximum explored depth of 41 feel The upper bedrock to a
depth of about 18 feet consisted mainly of hard gypsum.
Job No . 114 176A
-3-
Laboratory testing performed on samples obtained from the boring included natural
moisture content and density and percent finer than No. 200 sieve (silt and clay fraction)
gradation analysis . The results of swell-consolidation tests performed on relatively
undisturbed drive samples, presented on Figures 4 and 5, indicate the silt soils to be
moderately to highly compressible when loaded and wetted and the siltstone/claystone to
have minor expansion potential when wetted under light loading. The laboratory testing
is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils and
bedrock were slightly moist to moist with depth.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyan Mesa
subdivision. These rocks are a sequence of gyps if erous 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 VaJley Evaporite underlie the lot.
Dissolution of the gypsum under cenain conditions can cause sinkholes to develop and
can produce areas of localized subsidence. During previous work in the area, several
sinkholes were observed scattered throughout this part of Garfield County. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of
the Roaring Fork River valley.
Sinkholes were not observed in the immediate area of the subject loL 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 11 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 .
Job No . 114 176A
-4-
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, the proposed residence can be founded with spread
footings bearing on the natural soils or bedrock with a risk of differential foundation
settlement. The upper gravelly soils and silt soils encountered to a depth on the order of
12 feet are compressible when loaded and wetted and should be further evaluated for
settlement potential at the time of construction. Sub-excavation of the compressible soils.
especially in the shallow cut garage area, and replacing them with compacted fill to a
depth of about 5 feet cou ld be needed to reduce differential settlement potential .
The design and construction criteria presented below should be observed for n spread
footing foundation system.
1) Footings placed on the undisturbed natural soils or bedrock should be
designed for an allowable bearing pressure of 1 ,500 psf. Based on
experience, we expect initial settlement of footings designed and
constructed as discussed in this section will be abou t I inch or Jess. The
soils tend to be compressible after wetting under load and there could be
some additional post-construction differential foundation settlement of
about Y.i to I inch if the bearing soils are wetted. The bearing conditions
should be further evaluated at the time of construction. Precautions should
be taken to prevent post-construction wetting of the bearing soils .
2} 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 shou ld be reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14
feet. Foundatio n walls acting as retaining struc tures should also be
Job No 114 176A
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designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) The topsoil, silt soils to a depth of 5 feet below garage design footing level
and loose or disturbed soils should be removed down to undisturbed
natural soils or bedrock. The exposed soils in footing areas should then be
moistened and compacted prior. Structural fill placed below footing areas
should extend at least 2% feet beyond footing edges and be compacted to
at least 98% of standard Proctor density at near optimum moisture content.
Foundation concrete should contain sulfate resistant cement and be air
entrained.
6) A representative of the geotechnical engineer should observe aU footing
excavations for bearing conditions and evaluate compaction of structural
fill during its placement on a regular basis .
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 SS pcf
for backfill consisting of the on-site 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 back.fill consisting of
the on-site soils. Backfill should not contain topsoil, vegetation or oversized rock.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings. traffic, construction materials and
equipmenL The pressures recommended above assume drained conditions behind the
walls and a horizontal backftll 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 retaining walls.
Job No . 114 176A
-6-
Back.fill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed 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 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
calculated based on a coefficient of friction of 0.35. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 350 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 occu r 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 near
optimum moisture content.
R.OORSLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-
on-grade construction. The upper silt soils are compressible when wetted and could be
removed to a depth of al least 2 feet and replaced with structural fill to help limit the risk
of slab settlement and distress mainly if the subgrade soils are wetted. To reduce the
effeclS 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 joinlS 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
Job No. ll 4 176A
-1·
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 near optimum moisture content. Required fill can
consist of the on-site soils or imported granular soils (such as road base) 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 create a perched
condition. We recommend below-grade construction, such as retaining walls and
basement areas, be protected from wetting and hydrostatic pressure buildup by a
perimeter underdrain system. The drain system should not be constructed around garage
areas or other shallow footing areas of the building. Backfill should be adequately
compacted and the surface sloped to drain away from the residence.
Where provided, the drains should consist of drainpipe 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 or sump and pump. 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 Jeast 1 ~ feet deep. An impervious membrane such as a
30 mil PVC liner should be placed below the drain gravel in a trough shape and attached
to the foundation wall with mastic to prevent wetting of the bearing soils.
SURFACE DRAINAGE
Proper surface grading and drainage will be critical to keeping the bearing soils dry. The
following drainage precautions should be observed during construction and maintained at
all times after the residence bas been completed:
Job No. 114 176A
-8-
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 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.
Free-draining wall backfill should be capped with at least 2 feet of the on-
site soils to reduce surface water infilttation .
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill .
5) Sprinkler heads and landscaping which requires regular heavy irrigation,
such as sod, 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 geotechnica1
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 drilJcd at the location indicated
on Figure 1, the assumed 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 extrapolation of the subsurface conditions identified at
the exploratory boring and variations in the subsurface conditions may not become
evident until excavation is performed. H conditions encountered during construction
Job No. 114 l76A
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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 reconunendations 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.
Respectfully Submitted,
HEPWORTH -PAWLAK GEOTECHNICAL, INC.
Steven L. Pawlak, P.E.
Reviewed by:
Daniel E. Hardin, P.E.
SLP/ksw
Job No. 114 176A
LOT10
116 046A
I BUILDING SETBACK LINE I
LOT 11
PROPOSED
RESIDENCE
GARAGE
BORING 1 •
SAGE MEADOW ROAD
I
APPROXIMATE SCALE
1· -20·
LOT12
~ LOCATION OF EXPLORATORY BORING Figure 1
Heoworth-Pollfok Geotedlnlcal
0
5
10
15
~ 20
I
.r: a
Q)
0
25
30
35
40
116 046A
BORING 1
17/12
14/12
WC -64
00 •98
20/12
WC -56
00:.103
·200 -75
50/J
31/12
WC r ll .O
00 •122
46/12
46/12
52/12
42/12
NOTE: Explanation of symbols is shown on Figure 3.
LOG OF EXPLORATORY BORING
0
5
10
15
20 Qj
QJ u.
I
.e. a Q)
0
25
30
35
40
Figure 2
LE GENO:
TOPSOIL: organic sandy si lt wilh gravel , brown. root zone.
SAND AND GRAVEL (SM-GM); silty, clayey, medium dense, sl ightly moist, brown.
SILT (ML): sandy, clayey, sliff to very stiff, slightly moi st , lighl brown, some gypsum.
WEATHERED SILTSTONE/CLAYSTONE BEDROCK; highly gypsiferous and hard to 1 a feet then slightly
gypsiferous and medium hard below. slightly moist to moist with depth, white to grey-brown with depth.
Relative ly undisturbed drive sample; 2·inch 1.0 . California liner sample.
17112 Drive sample blow count ; indicates that 17 blows of a 140 pound hammer falling 30 inches were
requi red to drive the California sampler 12 inches.
NOTES :
1. The exploratory boring was drilled on March 3 , 2016 with 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 me asu1ed and the log of exptoratory bori ng is drawn to depth .
4. The exploratory boring location and elevat ion should be considered accura te only to the degree implied by the
melhod 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 waler was encountered in the boring at the time of drilling Fluctua tion in water level may occur wi lh ti me
7. laboratory Testi ng Resulls ·
WC -Waler Content (%)
DD -Dry Densily (pc~
-200 Percent passing No. 200 sieve
116 046A ~
HEPWORTH-PAWLAK Gl!01'ECHNICAI..
LEGEND AND NOTES Figure 3
..
Moisture Content = 6.4 percent
Ory Density -96 pcf 0 Sample of : Sandy Clayey Silt
~ From: Boring 1 at 5 Feet
1
~ ~ ~ r--.... I'.
2 I\ r-.. ...
\ " ~ "'i-. " No movement
upon
-oe. 3 wetting
-I
c: ~ 0
'iii
gi 4 ....
' c.
~ 5 \
\
6
7
I~
0.1 1.0 10 100
APPLIED PRESSURE -ksf
Moisture Content :.: 5 .6 percent
Dry Density -103 pcl
Sample of : Sandy Clayey Silt
From : Boring 1at10 Feet
0
ri
~ 1 Ii)......._ e
c: "-.. ---... ~ ... Compression .2 "' ~\ upon UI a> wetting ii 2
§ " u ~' 3 -
0.1 1.0 10 100
APPLIED PAESSURE -ksf
116 046A oa-'1ech SWELL-CONSOLIDATION TEST RESULTS Figure 4
H.oworth..Pawlak Geotedlnlcal
Moisture Content = 11 .0 percent
Ory Density = 122 pcf
i Sample of: Weathered Sillstone/Claystone
I From: Boring 1 al 20 Feel I
1 I
fl.
c:
0 "iii
c: 0 co '"--Q.
di ~" r--c ~ I
c 1 r--..' .Q
\ (/)
Cl)
Q) ... c.
E 2 0 Ex ' . u pansson
upon
wettina
I
0.1 l .O 10 100
APPLIED PRESSURE· ksf
116 046A ~
H1111warth-Pawlok Geotedlnli:al
SWELL-CONSOLIDATION TEST RESULTS Figure 5
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116 046A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION N,_lUllAL GRADATION AlTERBERG LIMITS UNCONFINED
MOIST\JRE
N,_lURAl PERCENT COMPRESSIVE GRAVEL SANO PLASTIC SOil OR
SORING DEPTH CONTENT ORY DENSITY PASSING NO. UQUIDUMff STRENGni
l"I (%) 200SlEVE INDEX BEDROCK TYPE
flt) '"' (pd) l"I '"' (PSF)
1 5 6.4 98 Sandy Clayey Silt
10 5.6 103 75 Sandy Clayey Silt
20 11.0 122 Weathered
Siltstone/Claystone