HomeMy WebLinkAboutSubsoil Study 06.15.16HEPWORTH -PAWLAK GEOTECHNICAL
SUBSOIL STUDY
Hcpwurrh-Pawh1k Gc,,tcdmical, Jnc.
5020 County Rnml 154
Gknwotlll Spring~, ColoraJ,, 81601
Phone: 970-·9 45 -7988
Fa x: 970 -945-8454
email: h['gc·o@hrgcntcch.com
FOR EVALUATION OF DISTRESSED RESIDENCE
LOT 3, TELLER SPRINGS
1855 COUNTY ROAD 109
GARFIELD COUNTY, COLORADO
JOB NO. 116 121A
JUNE 15, 2016
PREPARED FOR:
TOM WILLIAMS
1855 COUNTY ROAD 109
GLENWOOD SPRINGS, COLORADO 81601
diatom @sopris .net
Parker 303-841-7119 • ColoradoSprings 719-633-5562 •Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ -1 -
EXISTING BUILDING CONDITIONS ......................................................................... -1 -
SUBSIDENCE POTENTIAL ......................................................................................... -2 -
FIELD EXPLORATION ................................................................................................. -2"
SUBSURFACE CONDITIONS ...................................................................................... -3 -
ENGINEERING ANALYSIS ......................................................................................... -4 -
DESIGN RECOMMENDATIONS ................................................................................ -4 -
UNDERDRAIN SYSTEM .......................................................................................... -6-
SURFACE DRAINAGE ............................................................................................. -6 -
LIMITATIONS ............................................................................................................... -7 -
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2 -LOGS OF EXPLORATORY BORINGS
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 TO 6-SWELL-CONSOLIDATION TEST RESULTS
FIGURE 7 -GRADATION TEST RESULTS
TABLE 1-SUMMARY OF LABORATORY TEST RESULTS
Job No. 116 121A ~tech
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for evaluation of distress to the
residence located on Lot 3, Teller Springs, 1855 County Road 109, Garfield County,
Colorado. The project site is shown on Figure 1. The purpose of the study was to
develop recommendations for mitigation design. The study was conducted in accordance
with our proposal for geotechnical engineering services to Tom Williams dated April 14,
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,
compressibility and other engineering characteristics. The results of the field exploration
and laboratory testing were analyzed to develop recommendations for underpinning the
existing 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 existing construction and the subsurface
conditions encountered.
EXISTING BUILDING CONDITIONS
The house consists of a one story log structure over a walkout basement. We understand
that the house distress has been evaluated by Bob Pattillo, structural engineer, and the
main floor has settled about 9 inches from the southwest, uphill corner to the downhill,
noutheast corner. About 15 years ago, seven "push piles" were installed around the
northeast building corner. The piles varied from 15 to 30 feet deep with the shallow piles
presumably encountering rocks large enough to stop their penetration into the overall
softer soils. We understand there was a wetting event, possibly from a broken pipe to the
septic system, which likely caused the previous settlement. The owner would like to stop
the settlement of the house and, if possible, return the house to a more level condition.
Job No. 116121A ~tech
-2-
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the residence and
crops out on the hillside to the west and uphill of the house. 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, several sinkholes were 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 borings
were for foundation mitigation 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 3 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.
We do not believe that the current settlement problem is due to sinkhole formation but are
pointing out that this is another hazard inherent to property underlain by Eagle Valley
Evaporite.
FIELD EXPLORATION
The field exploration for the project was conducted on April 25 and 26, 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-45B drill rig. The borings were logged by a
Job No. 116 121A ~tech
- 3 -
representative of Hepworth-Pawlak Geotechnical, Inc. Boring 1 was drilled adjacent to
the northeast corner of the house which has experienced the most settlement and the soils
are presumably more moist. Boring 2 was drilled about 20 feet away from the southeast
corner of the house which has experienced much less movement and the soils are
presumably drier. The borings were drilled to about 90 feet deep which was the limit of
the drilling equipment.
Samples of the subsoils were taken with 1 % inch and 2 inch I.D. spoon samplers. The
samplers were 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-1586. The penetration resistance values are an indication of the
relative density or consistency of the subsoils. Depths at which the samples were taken
and the penetration resistance values are 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 are shown on Figure 2.
The subsoils consist of about 5 feet of fill in Boring 1 overlying interlayered, mostly silty
to clayey sand with varying amounts of gravel and cobbles down to 90 feet. Drilling in
the layers with more gravel and cobbles with auger equipment was difficult due to the
rock content.
Laboratory testing performed on samples obtained from the borings included natural
moisture content, density and gradation analyses. Results of swell-consolidation testing
performed on relatively undisturbed drive samples, presented on Figures 4 to 6, indicate
low compressibility under light loading at existing moisture content and moderate to high
collapse potential (settlement under constant load) when wetted. The samples were
moderately to highly compressible under increased loading after wetting. Results of
gradation analyses performed on small diameter drive samples (minus 1 Y2 inch fraction)
Job No. 11612\A ~tech
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of the coarser granular subsoils from between 22 and 25 feet in Boring 1 are shown on
Figure 7. The l aboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
generally moist to slightly moist with depth at Boring 1 and slightly moist at Boring 2.
ENGINEERING ANALYSIS
The soils were evaluated for collapse potential using the CGS Collapse Susceptibility
Graph which compares in place moisture content and dry density. In general, soils with
higher dry densities and higher moisture contents tend to be less settlement prone than
drier, less dense soils.
The soils in Boring 1 generally had about 4 % more moisture than the soils in Boring 2
and the deeper soils in both borings tended to be drier than the shallower soils. The
deeper soils in both borings below about 30 feet plot at low to no collapse potential and
the shallower soils plot in the low to moderate collapse range with a few samples in the
moderate to high collapse range.
We also evaluated the soils for collapse potential using the relationship between dry
density and percent passing the No. 200 sieve (silt and clay fraction) for soils in the
Glenwood Springs area (Mock-Pawlak, 1983). This analysis also showed that samples of
soils below 30 feet in both borings plot in the low to no collapse potential range and soils
from shallower depths plot in the moderate to high collapse potential range .
DESIGN RECOMMENDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
evaluation of the collapse potential of the existing soils, we recommend the residence be
underpinned with a deep pile or pier system which extends at least 50 feet and _Rossibly 90
feet below the existing foundation. This will put the bottom of the piles/piers at least 20
Job No. 116 121A ~tech
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feet into the less settlement-prone soils. Typically, underpinning should go to a dense
gravel soil or to bedrock. This would require drilling to 100 feet or more which may not
be economically feasible. Keep in mind that piles or piers that do not go down to a high
strength relatively incompressible bearing material will have a risk of future settlement
which may have happened to the previous push piles at the northeast building corner.
As the house is currently partially supported on "push piles" (small diameter steel casing
pushed into the soil with hydraulic jacks using the weight of the house as a reaction
weight), this system could be used for the rest of the house except that piles that refuse
short of the 50 to 90 foot depth would need to have additional piles placed adjacent to the
"short" piles and jackerl rlown to 50 to 90 feet until the structural engineer is satisfied that
the house is adequately supported.
Helical piers (which are screwed into the ground until they reach penetration or torque
refusal) could also be used but they would potentially have the same problem with
shallow refusal that would need to be overcome by adding helixes and/or using a heavier
pile section and installation system.
A drilled micropile underpinning system could also be used. Micropiles consist of a
grouted high strength steel bar placed in a drilled hole to the specified depth. Micropiles
tend to have higher capacities provided they reach high strength and relatively
incompressible soils or rock, and can be advanced through rocks within the soil.
Micropiles, helical piers and push piles are attached to the existing foundation with
brackets which transfer the weight of the house to the pier or pile. The brackets require
pits to be dug around the foundation erimeter and interior pads to allow for the
attachment of the brackets to the existing foundatiom. Many installers have pump
manifolds which allow loading several piers or piles at once which can allow for
relatively uniform upward lift with less distress to the structure than just jacking on one
pile at a time. It is likely that some lift can be accom lished although getting the floor
com letely level is not likely.
JobNo.116121A ~tech
-6 -
Another underpinning alternative that has been used in this area is compaction grouting,
which injects no-slump grout with relatively high pressure into the ground under the
existing foundation. The advantage of compaction grouting is that it can be done from
the ground smface with limited digging of pits since there is no direct attachment of the
grout column to the foundation. A disadvantage is that lifting of the structure is limited.
Compaction grouting is more effective in more moist soils of limited depth and getting
compaction of the deeper drier soils may be ineffective. At this particular site,
compaction grouting would probably not be suitable due to the required grout depths of
greater than 50 feet and is not recommended.
UNDERDRAIN SYSTEM
We recommend that the perimeter drain system behind the uphill basement wall be
replaced as part of the underpinning project.
The new drain should consist of drainpipe placed at the bottom of the wall backfill
surrounded above the invert level with free-draining granular material. The drain should
be placed at each level of excavation and at least 1 foot below lowest adjacent finish
grade and sloped at a minimum 1 % to a suitable gravity outlet. Free-draining granular
material used in the underdrain system should contain less than 2% passing the No. 200
sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The
drain gravel backfill should be at least 1 Y2 feet deep. An impervious membrane such as
30 mil PVC 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
After underpinning is complete we recommend the following surface drainage
improvements be implemented and maintained at all times after construction has been
completed:
Job No. 116 121A ~tech
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1) 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.
2) The ground surface surrounding the exterior of the building should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in paved areas.
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.
Surface swales uphill and around the building should have a minimum
slope of 4%.
3) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
4) 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 borings drilled at the locations
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
Job No. ll6 l2lA ~tech
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conditions identified at the exploratory borings 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 mitigation 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 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,
Reviewed by:
Steven L. Pawlak, P.E.
DEH/ksw
cc: Pattillo Associated Engineers -Bob Pattillo (bob @pa nginee rs .c om )
REFERENCE
R. G. Mock and S. L. Pawlak, Alluvial Fan Hazards at Glenwood Springs, Geological
Environment and Soil Properties, ASCE, October, 1983
Job No. 116 121A ~tech
LO T 2
116 121 A ~
He worth-Pawlak Geotechnlcal
COUNTY ROAD 109
EXISTING
RESIDENCE
1855 109 RD .
LOT3
BORING 2
APPROXIMATE SCALE
1" = 100'
LOCATION OF EXPLORATORY BORINGS Figure 1
0 BORING 1
WC=10.3
00=109
-200=29
10
20
30 18/12 WC=8.3
00=124
-200=31
40
Q)
Q)
LL
_c
a_
Q)
0
50
60
70
80
90
116121A ~
He worth-Pawlak Geatechnlcol
WC=11.6
00=108
-200=34
WC=9.1
00=98
-200=37
WC=10.4
00=99
BORING 2
10 12
11/12
15/12
32/12
8/12
14/12
12/12
17/12
20/12
11/12
WC=5.3
00=106
50/3
24/12
34/12
37/12
WC='.3.1
00=112
-200=28
24/12
WC=3.5
00=120
28/12
24/12
64/12
43/12
WC=3 .6
00=125
-200=29
59/12
WC =6.3
00 =94
-200=28
WC=6.0
00=97
-200=35
WC =3.7
00=129
-200=35
Note: Explanation of symbols
is shown on Figure 3.
64/12
0
10
20
30
40
05
(!)
LL
L o_
(!)
0
50
60
70
80
90
LOGS OF EXPLORATORY BORINGS Figure 2
LEGEND:
FILL; silty clayey sand with rock fragments, loose, moist, brown .
SAND (SC-SM); silty to clayey with gravel and possible cobbles, loose to medium dense, moist to slightly moist
with depth in Boring 1, slightly moist in Boring 2, brown.
GRAVEL (GM-GC); sandy, silty to clayey, medium dense to dense, moist to slightly moist with depth, brown .
9/12
NOTES:
Relatively undisturbed drive sample; 2-inch l.D. California liner sample .
Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-1586.
Drive sample blow count; indicates that 9 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
1. Exploratory borings were drilled on April 25 and 26, 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. Elevations 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 exploratory 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 time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pct)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
116121A ~
Hepworth-Pawlak Geotechnlcal
LEGEND AND NOTES Figure 3
Moisture Content = 10.3 percent
Dry Density = 109 pcf
Sample of : Silty Clayey Sand with Gravel (Fill)
From: Boring 1 at 2 Feet
0
r-r-. r--;--.... ~ ...-v
1
-i--. .-.---/
""'
{ No movement _,__
upon
'#.. 2 wetting
c \ 0
"Ui r\ (f)
3 Q)
Q_ ~ E \ 0 u
4
\
5 ~~
0.1 1 .0 10 100
APPLIED PRE SS URE -ksf
116121A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4
Hepworth-Pawlak Geotechnical
Moisture Content = 9 .8 percent
Dry Dens ity = 104 pct
Samp le of: Silty Clayey Sand with Rock
Fragments
0 From : Bori ng 1 at 1 O Feet
1
<ft. 2
c Compression
0 ~ ~ v upon .iii L---/ i-1-..... wetting (f) i--(j) 3 ~
Q_
E
0 u
4
5
6
7
( l
8 \
9 \
\
10
11 \
\
12 \
(~ \
13 \
\
1D
14
0 .1 1.0 10 100
APPUEDPRESSURE-k~
116121A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 5
Heoworth-Pawlak Geotechnlcal
Moisture Content = 5.3 percent
Dry Density = 106 pcf
Sample of: Silty Clayey Sand with Gravel
From: Boring 2 at 20 Feet
0 -r--r--i-~
1 i--.o
Compression
upon
* 2 -----i7 ,,,,,....-"""'wetting
~ v / c (_ y
0 ~ (f)
(f)
3 (!) o_
E
0 u
4 I l
5 \
\
6 \
\
7 \ u\
8 I\
1)
9
0 .1 1.0 10 100
APP UE D PRE SS URE-k ~
116121A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 6
Hepworth-Pawlak Geotechnical
HYDROMETER ANALYSIS S IEVE ANALYSIS I
I TIME READINGS
24 HR 7 HR
O 45 MIN. 15 MIN . 60MIN19MIN.4 MIN . 1 MIN . #200 #100 #50 #30
I U.S. STANDARD SERIES
#16 #8
I CLEAR SQUARE OPENINGS
3/8" 3/4" 1 1/2" 3" 5" 6" #4 8' 100
:
10 90
20 80
30
70
0 . 60 (.!)
w 40 z z Vi ::;;: (/)
f-<( w 0... a::: :
50 50 f-f-z z w w (.)
(.) -a::: a::: -w w -0... -40 0... 60
70 30
80 20
90 10
100 0
.001 .002 .005 .009 .019 037 .074 150 300 .600 1.18 2 36 4.75 9 .5 19.0 37.5 76 .2 152 203
12 5 127
DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT I
I CONG' : CO BBLES I I CQA -I
GRAVEL 54 % SAND 24 % SILT AND CLAY 22 %
LIQUID LIMIT % PLASTICITY INDE X %
SAMPLE OF: Silty Sandy Gravel FROM: Boring 1 at 22 and 24 Feet Combined
116121A ~ GRADATION TEST RESULTS Figure 7
Heoworth-Powlak Geotechnical
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116121A
SUMMARY OF LABORATORY TEST RESULTS
p 1 f 2 age 0
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT COLLAPSE MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%)
SIEVE
(ft) (%) (pcf) (%) (%) (PSF) %
1 2 10 .3 109 29 0.1 Silty Clayey Sand with
Gravel (F ill)
4 11.6 108 34 Silty Clayey Sand with
Grav el (Fill)
6 12.6 105 39 Silty Clayey Gravelly
Sand
8 9.1 98 37 Silty Clayey Sand with
Rock Fragments
10 9.8 104 38 7.0 Silty Clayey Sand with
Rock Fragments
12 10.4 99 Silty Sand with Rock
Fragments
16 & 18 8.3 30 Silty Sand with Rock
Fragments
20 9.6 95 Silty Clayey Sand
22 &24 4.3 54 24 22 Silty Sandy Gravel
30 8.3 124 31 Silty Clayey Gravelly
Sand
35 8.1 126 Silty Clayey Gravelly
Sand
40 6.8 121 45 Very Silty Clayey
Sand
60 8.6 117 Silty Clayey Sand
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 116121A
SUMMARY OF LABORATORY TEST RESULTS
p 2 f 2 aqe 0
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT COLLAPSE MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%)
SIEVE
(ft) (%) (pcf) 1%l 1%) (PSF) %
2 2 6.3 94 28 Silty Clayey Sand with
Gravel
12 6.0 97 35 Silty Clayey Sand with
Gravel
20 5.3 106 3.1 Silty Clayey Sand with
Gravel
30 3.7 129 35 Silty Clayey Sand with
Gravel
35 3.1 112 28 Silty Clayey Sand with
Gravel
40 3.5 120 Silty Clayey Sand with
Gravel
70 3.6 125 29 Clayey Sand with
Rock Fragments