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CASE HISTORY:
Design, Construction, and Performanceof Stone Column Ground Improvementbeneath an MSE Wall
Karen Dawson, P.E. & Sean Shin, Ph.D, P.E., CH2M HILL, Bellevue, WA, USA
Suthan Pooranampillai, Ph.D., AMEC E&I, Edmonton, AB, Canada
Dominic Parmantier, P.E., Condon Johnson & Associates, Inc., Kent, WA, USA
37thAnnual Conference on Deep Foundations
Houston, TX October 16-19, 2012
Presentation Overview
Description
Project
Subsurface Conditions
Design Considerations andMethods
Construction
Layout
Equipment
Quality
Observations
Conclusions
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Project Location
Subsurface Conditions
ML with occasional layers of MH,
CL, SM, and OL. Some gravel in
lower interbeds
Properties
PI = generally 0 to 15,
occasionally >20
Zones with organics (LOI up to 7%)
Cc = 0.14
c = 1.6 ft2/day
C = 0.005
pH = 4.5 and 5.9
Resistivity = 225 to 8200 -cm
Liquefiable under design 0.27 g
PGA
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Roadway Features
New bridge abutment and approach embankment up to 50high
Walls ~700 long required because of space limitations
Time available for preloading
Need for Ground Improvement
Liquefiable soils
Maintain global stability during seismic event
Secondary benefit - Limit consolidation settlement of wall
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Global Stability Cases
1. Static long-term
2. Static construction (included preload)
3. Start of shaking
4. During shaking
5. Post shaking
Design Steps
1. Defined allowable post-earthquake deformations
< 6 at bridge abutment
< 12 wall away from abutment
2. Determined yield accelerations that would result in allowable
deformations (Bray and Travasarou, 2007)
3. Determine composite strength from global stability analyses
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Design Steps Continued
4. Determine AR to provide composite strength
Used = 40 degrees for stone
Used equations by Priebe (1995) to develop composite
Increased stiffness from columns resulted in reduction in CSR
(Baez and Martin, 1993; Priebe, 1998) so that native soil
between columns was no longer liquefiable.
5. Use ground improvement factor (Priebe, 1995) to estimate
primary consolidation
Final Design
AR = 15%
10 month preload with 20% surcharge (additional criteria forpavement: limit secondary compression to 2 in 15 years)
Instrumentation for settlement verification
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Construction Contract Requirements
AR = 15%
Minimum column diameter = 30
qt > 110 tsf or N60 > 24 for clean or already dense layers
Yield plots to verify stone volume
Additional explorations to define bearing layer
Test sections Verify diameter and volume of stone in layered stratigraphy
Verify equipment response indicates bearing layer reached
Verify continuous stone by sonic coring
Means and Methods
Bearing LayerVerification
14 CPT
6 SPT
Total counting
owners
explorations ~
1 per 1,700 SF
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Means and Methods
Layout Triangular with 8 spacing and 3.25 diameter columns
Triangular with 11 spacing and 3.5 diameter through
existing embankment
D-54
D-56
E-55
F-54
F-56
O-169
O-171
P-170
R-169
R-171
TestArea1TestArea2
AREAA
AREAB
AREAC
CSWALL
RVWALL
8ft 8
ft
Means and Methods
Equipment Manitowoc 4000 and 4100
crawler cranes
Dry bottom feed electric
probe (V23) 11.7 x 13.8,
2.4 tons, 34 tons dynamic
force
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Means and Methods
After first test section, predrilled most holes (SR-60) 16 kelly
Means and Methods
Automated Data Acquisition System
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Data Acquisition System Printout
High amperage at tip
Observations
Vibrations: Mostly < 0.5 ips
Peak vector sum = 1.1 ips
with probe working 7 from
gas main.
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Observations
Fines migration intocore was variable
Clean core
Relatively clean core
Dirty core
Observations
Fines migration continued Correlation with
stratigraphy? Not that we
could tell.
Revised technique to
improve stone percentage
(e.g. shorter pulls, more
care in fully inserting probe
between pulls, changed air
pressure)
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Observations
Fines migration continued
Backcalculated c ignoring drainage to stone column
matches lab testing values and typical values in literature.
Laboratory average
cz = 1.6 ft2/day
Backcalc no radial drainage
cz = 1.5 to 2.0 ft2/day
Backcalc with drainage cr~ 0.5 ft
2/day
cz ~ 0.05 ft2/day
Conclusions
1. Abundance of subsurface explorations is important for
planning and pay, especially in layered stratigraphy (1 per
2,000 SF+)
2. Automated data acquisition system is and excellent tool for
quality control, especially in silty soils with AR-based
performance
3. Rapid drainage through stone columns should not be
assumed.
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Items for Improvement?
Methods for verifying insitu strength of stone column inAR-based design.
Questions?