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University of California, Davis
Civil & Environmental Engineering
ECI284 Term Project
Study of GroundImprovement Techniques
with an Emphasis on Deep
Soil Mixing
Payum Vossoughi
Instructor: Professor Boris Jeremic
March 2012
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Presentation Outline
Introduction/Motivation
Overview of available ground improvement techniques
Emphasis on Deep Soil Mixing (DM)
Laboratory testing
Numerical analysis
Statistical analysis
Conclusion
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Introduction/Motivation
How do we keep our civil infrastructure safe from liquefaction?
Liquefaction mitigation strategies:
1.) Modify designreduce damage to acceptable levels
2.) Do nothing alternativeaccept risk
3.) Abandon projectchoose another site
4.) Ground improvement
Increased density of underlying soil deposits
Reduced excess pore pressure build-up (maintain ru< 1.0)
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How does a project engineering choose a ground improvement
technique(s) to implement?
Case studies
Numerical & statistical analyses
Laboratory tests on soil specimens (QA/QC)
Benefit vs. cost analysis
Constructability issues + engineering judgment
Purpose: Increase confidencein ground improvement techniques to
better protect civil engineering infrastructure from future natural
disasters related to liquefaction
Introduction/Motivation cont
- NSPE Code of Ethics:
Engineers, in the fulfillment of
their professional duties, shall
hold paramount the safety,
health and welfare of the public
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Overview of AvailableGround Improvement
Techniques
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Ground Improvement techniques based on soil characterization
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Methods of Ground Improvement: Vibro Methods
Advantages Disadvantages
Densifies soil
Increases lateral stress
Reinforces soil mass
Provide drainage of excess pore Most effective technique for
liquefiable soils that fall within
the typical grain size range
Widely used
Economical
Ineffective for soils with high fines content
(>20%)
Inefficient for liquefiable soils with a limited
thickness at a significant depth Difficult to penetrate stiff strata (cemented,
cobbles)
Settlement of surrounding ground
Vertical conduit for environmental
contaminants
Construction of a vibro-replacement stone column
(Idriss & Boulanger 2008)
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Methods of Ground Improvement: Deep Dynamic Compaction
Advantages Disadvantages
Densifies soil
Increases lateral stress
Economicallarge areas
Simple; no insertion required
Effective for only upper 10 meters
Difficult to densify soil surrounding large
cobbles/boulders
Decreasing effectiveness with decreasing
permeability (>20% fines)damping effect
of generated dynamic shear stresses
Disturbing to local structures + occupants
Crawler cranes can drop tamper masses weighing up to 33 tons from heights of 30 meters
(Idriss & Boulanger 2008)
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Methods of Ground Improvement: Compaction Grouting
Advantages Disadvantages
Densifies soil
Increases lateral stress
Reinforces soil mass
Works well in low-overhead +
constricted spaces
Can target specific depth interval
Effective with high fines soil (>20%)
+ large particles (cobbles, boulders)
Ineffective for small depths
(
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Methods of Ground Improvement: Jet Grouting
Advantages Disadvantages
Construction of overlapping soil-cement columns
(in-ground shear walls)
Reduces earthquake-induced shear strains of
treated zones
Increases composite shear strength of treated
zones
Prevents migration of excess pore between
untreated and treated zones
Works well in low overhead
Can target specific depth interval
Works well in high fines soil (large applicability
range)
Hydraulic fracture
Overlapping soil-cement
columns are brittle =
tendency to crack with
earthquake shaking
High-pressure jets of air, water and/or groutoverlapping soil-cement columns
(Idriss & Boulanger 2008)
h d h h
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Other Ground Improvement Techniques Worth Mentioning Drainage systems
Coarse aggregates or geosynthetic composites
Design goal: control max. rulevels
Limited to high k soils
Prevent void redistribution or water film generation
at interfaces
Permeation Grouting
Reduces the potential for contraction during cyclic
shearing
Optimization and QC through computer automation
Minimal disturbance; clean sands
Generally expensive (special circumstances)
Explosive Compaction
Propagation of dynamic shear stresses
Deep deposits; clean sands
Non-uniform densification Permitting requirement (large disturbance)
Removal and Replacement
Excavate and replace with compacted fill
High degree of confidence
Practical for shallow liquefiable layers;
groundwater needs to be controlled
Low khigh k interface drain system
Exposed soil bulbs resulting from permeation grouting
Sediment boils resulting from explosive compaction
(Idriss & Boulanger 2008)
f
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Methods of Ground Improvement: Deep Soil Mixing
Advantages Disadvantages
Construction of overlapping soil-cement
columns (in-ground shear walls)
Reduces earthquake-induced shear strains
of treated zones
Increases composite shear strength of
treated zones Prevents migration of excess pore between
untreated and treated zones
Works well in high fines soil (large
applicability range)
Requires overhead clearance
Stiff strata can impede augers
Inefficient for liquefiable soils
with a limited thickness at a
significant depth
Overlapping soil-cementcolumns are brittle = tendency
to crack with earthquake
shaking
Cementitious materials/grout are injected through auger stems and mixed with native soils
(Idriss & Boulanger 2008)
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Additional Information Regarding Deep Soil Mixing DM was put into practice inJapanin the middle of 1970sto improve soft marine deposits, and then spread into China, South
East Asia, and recently to the other parts of the world.
The maximum improvement depth on land is typically 30 meters in comparison to 50 meters for off-shore applications
The 2 principal hardening agents used in DM are cement and lime
The curing process for cement-treated soils is much longer for cohesive soils (approx. 56 90 days) compared to non-cohesive
soils (approx. 28 days). The strengths achieved in non-cohesive soils are larger.
Cement powderscan achieve higher strengths than cement slurries when used
to treat clayey soils. Cement powders have lower water contents
Typically SPT N-values of soils to be treated:
Cohesive: N < 4 blows (heavier equipment --- N < 10 blows)
Granular: N < 10 blows (heavier equipment --- N < 30 blows)
Alternative approach: dryjet mixing
Minimizing of hardening agent (cement or lime) Minimizing waste slurry and spoil
Reducing resources consumed/project expenses + increased sustainability
Markets for DM:
1.) Roads and Railways 4.) Foundation support -
2.) Water/sewage treatment works 5.) Temporary works
3.) Tunneling 6.) Seismic engineering
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Laboratory Testing on
DM Stabilized Soil
Emphasis on Deep Soil Mixing
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QA/QC
Kaolin clay stabilized with
ordinary Portland cement.
4 moulding techniques ,4binder contents -- 160 total
specimens obtained
28 days curing - unconfinedcompression test (axial strain
rate =1%/min);
Wet density also measured
Laboratory Testing on Stabilized Soil
(Kitazume 2012)
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Laboratory Testing on Stabilized Soil cont
Tapping technique
Most uniform
Densest
Highest shear strength Most correlated
Minimal pockets/cavities
Static Compaction
Least uniform
Loosest Lowest shear strength
Least correlated
Maximum pockets/cavities
Strength increases approx. linearly
with increasing wet density(regardless of moulding technique)
Future Research: Perform similarstudies using
Different soils
Different laboratories
Wet Density
Parameter
Unconfined Compressive
Strength Parameter
(Kitazume 2012)
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Numerical Analysis of
Lattice Shaped GroundImprovement in Japan
Emphasis on Deep Soil Mixing
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Oreintal Hotel in Kobe, Japan
Hyogo-Ken Nanbu earthquake1995
Dimensions: 134 m x 59m (plan) & 60m tall
Liquefaction potential in the upper loose fill
Lattice shaped ground improvement (16 m)
Treatment area ratio* = 20%
Unconfined compressive strength after
curing (6 weeks) = 46 MPa.
Aftermath: minimal damage to the cast-in-
place pile foundation and superstructure
due to implemented DM walls
Numerical Analysis:
Case Study
* Treatment area ratio is defined as the ratio of the improved soil area to the whole site area (Namikawa 2007)
i l l i
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3-D FE Analysis
Mesh = 8 node isoparametric elements
Two-phase u-U formulation (excess pore)
Boundary conditions: cyclic, viscous, undrained
Elastic and Elasto-Plastic Model
Accounts for partial damage
Can notsimulate cyclic mobility after the
initial liquefaction
Rayleigh Damping (solid phase)
Saturated sand densification model (Mohr Coulomb)
Distribution in stress
(Tensile --- cracking)
Distribution in the
number of Gauss points
that reach failure
Numerical Analysis:
Important Aspects to the Model
(Namikawa 2007)
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Numerical Analysis:
Cases/Results
2 varying parameters in the
analysis of the improved soil
Without ground improvement Representative of case
study
(Namikawa 2007)
N i l A l i C l di R k
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Improved soil in Case 1 satisfies both the liquefaction potential & internal stability criteria
Partial Failures (as witnessed in cases 2 & 3) do not greatly affect potential of liquefactionmitigation. Note, however, that internal stabilities are not satisfied for cases 2 & 3.
Primary factors affecting the potential of liquefaction mitigation:
A.) Treatment area ratio
B.) Elastic modulus of the improved soil
What if the site experiences another earthquake (of equivalent energy/shaking)?
If the induced failure region from the 1st earthquake is small (represented by a smalldamage volume ratio), the potential of liquefaction mitigation remains approximatelythe same for the 2ndearthquake.
Future Research Topics:1.) Investigate the influence of input wave characteristics
2.)Expand the numerical model to account for plastic deformations (nonlinear behavior)
Numerical Analysis: Concluding Remarks
(Namikawa 2007)
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Statistical analysis of
DM implementation inSingapore
Emphasis on Deep Soil Mixing
St ti ti l A l i
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Marina Bay Financial Centre (MBFC)
Soil deposition:primarily marine clay & some sand
with seashell fragment. The design UCS is 0.8MPa.
Sampling conducted after 2 months of DM
installationsQA/QC
Nicoll Highway MRT Station (NCHS)
Soil deposition: thick layer of marine clay, thinner
layers of estuarine clay, fluvial sand cohesionless soil
& layer of land reclamation fill (ground surface).
The design UCS is 0.9MPa.
Sampling conducted after 2 months of the DM
installationsQA/QC
Site layoutMBFC project
Site layoutNCHS project
(Chen 2011)
Statistical Analysis:
Case Studies
St ti ti l A l i
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Statistical Analysis:
Results from MBFC Project
97 Samples
156 Samples
73 Samples
75 Samples
84 Samples
Total = 485 Improvement in the uniformity
of the mixing as the project
proceeds. Phase 4 has the
smallest COV, corresponding to
the greatest uniformity
Operating parameters play a
significant role in DM statistical
analysis
Small pockets of high strength
can significantly skew mean
UCS upward
COV values fit within the
expected range: 0.20
0.60(Chen 2011)
Statistical Anal sis
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Statistical Analysis:
Results from NCHS Project
112 Samples
28 Samples
16 Samples
17 Samples
51 Samples
COV range is lowerfor NCHS
compared to MBFC project
Histograms are left-skewed
with high mean values---
modified DM approach =
Jet Mechanical Mixing (JMM)
Ignoring the fill layer, the
naturally-occurring soil deposit
with the lowest mean UCS and
poorest mixing quality is the M
layer (or high plasticity clay)
The F1 layer (a sandy deposit)
shows the best improvement
(Chen 2011)
Statistical Analysis: Concluding Remarks
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MBFC strength < NCHS strength
Increasing cement content increases strength (operating parameter)
MBFC COV > NCHS COV
Increasing presence of marine clay deposit increases COV (geologic condition)
MBFC blade rotation number < NCHS blade rotation number
Increasing penetration and withdrawal rates decreases mixing quality, and UCS (operating
parameter)
Constructed (actual) strength > design (target) strength for all phases/layers; both projects
High conservatism observed in construction
Lack of understanding regarding strength variability in DM columns
Future Research:
Improvements to DM construction method to effectively treat Singapore marine clay
Acquire better understanding of strength variability in DM columns
Statistical Analysis: Concluding Remarks
MBFC - DM NCHSJMM (mod. DM)(Chen 2011)
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