1www.HaywardBaker.com

Shear Reinforcement Effects of Discrete Columns and Soilcrete Grids for Liquefaction Mitigation

DFI Soil Mixing SeminarSan Francisco, CAOct 24~25, 2013

Lisheng Shao

Ph.D, PE, GE

Outline

Liquefaction Evaluation & Liquefaction Mitigation Methods

Verification of the Mitigation Effectiveness Densification Reinforcement

Research of Shear Reinforcement Effects Discrete Columns Soilcrete Grid

2Liquefaction Ingredients

Saturated ground.Loose granular and other non-plastic soils.Strong ground motion. Shear strains transfer forces to pore water. Excess pore water pressure cannot dissipate fast

enough. Effective stress is reduced to zero After shaking stops pore water pressures dissipate

Liquefaction Evaluation

NCEER 97 California SP 117 Robertson & Wang 2004 Boulanger and Idriss 2004, 2007 Baez and Martin 1993, 1995 Boulanger et al 2012

3SPT Based Approaches

CPT Based Approaches

Densification

Reinforcem

ent

41. Remove and replace with nonliquefiable soil2. Densify loose granular soil3. Modify cohesive properties of soil4. Deep Foundations piles or piers5. Reinforced Shallow Foundations grade beams,

combined footings, rigid raft foundations,

6. Design to accommodate settlement, loss of strength

Approach to Mitigate Liquefaction

Methods of Liquefaction Mitigation

Densification Methods Deep Dynamic

Compaction (DDC) Vibro Displacement

(STONE COLUMNS) Compaction Grouting

5Methods of Liquefaction Mitigation

Improvement of Cohesive Properties Deep Mixing Jet Grouting Permeation Grouting

Verification of LQ Mitigation Effectiveness

Densification Verification (in sands, below water table)SPT CPTShear Wave Velocity Modulus/Plate Load Test?Void Reduction vs Volume Intake?

ReinforcementCSR Reduction

6Verification of Reinforcement Effect

Discrete Columns Aggregate/Sand Columns Soil Mixing/Jet Grouting Columns Auger Displacement Piles Compaction Grouting Columns Rigid Inclusion Columns

Cellular Structures Soil Mixing/Jet Grouting Panels

Failure Modes

Aggregate does not have tensile strengthSoilcrete is a brittle material

Failure strain

7Liquefaction Mitigation-Reinforcement

Reduce cyclic shear stress applied to liquefiable soil by installing stiffer elements within soil matrix that attract stress.

Can be used in non-densifiablesoils (silts, silty sands).

Not verifiable Post-installation CPT or SPT

results will not differ from pre-installation.

Vertical load testing of elements is not applicable.

soil soilcol

8Reinforcement Analysis

Reinforcement Analysis

9Liquefaction Mitigation-Reinforcement

Design Methodology Shear stress reduction factor (KG) (Baez and Martin, 1993):

GINC=Inclusion shear modulus GSoil=Soil shear modulus ARR=Ainclusion/Atotal

Strain compatibility and force equilibrium CSRapplied to soil = KG * CSRearthquake

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1

Soil

INCG

GGARR

K

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10

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Stiffness Values

Can a column be too stiff?Strain Compatibility?Failure mechanism of column

BendingShear

Shear Reinforcement for Liquefaction Mitigation Research Team

PI: Dr. Ross Boulenger, UC Davis Thang V. Nguyen

Dr. Ahmed Elgamal, UCSD Dr. Jinchi Lu

Dr. Scott A. Ashford, OSU Deepak Rayamajhi

Dr. Lisheng Shao, Hayward Baker, Inc22

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Scope of research

Illustration analyses Literature research Run 3-D unit cell by FEM(Opensees) in linear elastic

and checking the column strength limits Run 30+ earthquake time histories Shear modulus ratio = 3, 5, 10, 45, 150,

Generalization analysis and design charts Run more cases (parametric study) over item 1 Using non-linear soil and column model, may also

include soil liquefaction model Develop design charts for aggregate and soilcrete

column reinforcement factor, find out design boundary

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Discrete Column

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Discrete Column

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, max,,0.65' '

s U U vU d U

v v

aCSR r

g

, max,

,0.65' 's I I v

I d Iv v

aCSR r

g

max, ,

a maxmax, ,

I d IICSR rd

U U d U

a rCSRR R RCSR a r

Ramax : ratio of peak ground accelerations, Rrd :ratio of shear stress reduction coefficient for improved &unimproved caser: ratio of shear strains in the discrete column relative to the surrounding soil

pseudo-static analysis0.2g acceleration, Gr=10, Ar=20%

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pseudo-static analysis0.2g acceleration, Gr=10, Ar=20%

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EARTHQUAKE TIME HISTORY ANALYSIS Cape Mendocino Earthquake (1992) at CDMG STATION 89324

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Spatial distribution Rrd and r from earthquake time history analysis with Ar=20% and Gr=10

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Rrd was developed that includes adjustment factors for the effects of discrete column flexure and shear strain incompatibility

CG : equivalent shear factor of the discrete column CG = 1.0 for circular discrete columns

r is dependent on Gr and independent of Ar. KG from Baez (1995) is equivalent to RCSR=(Rrd)(Ramax)

pseudo-static analyses, Ramax =1 & RCSR = Rrd 30

1

1 1rd

r r r G rr

RG A C A

G

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Comparison of Rrd

Page 31

Typically, a 10%-15% reduction

Formoreindepthdiscussion:Deepak Rayamajhi, Thang V. Nguyen, Scott A. Ashford, Ross W. Boulanger, Jinchi Lu, Ahmed Elgamal, and Lisheng

Shao. (2012). "Effect of discrete columns on shear stress distribution in liquefiable soil." Geo-Congress 2012: State of the Art and Practice in Geotechnical Engineering

Conclusions Discrete Columns Current design practice

assumes discrete columns deforming in pure shear shear strains compatible between columns & soil

3D FEM analyses discrete columns deformed in both flexure & shear flexural & rotational greatly diminished their ability to reduce

dynamic shear stresses in the surrounding soils.

Current design methods overestimate reduction in dynamic shear stresses in soil

Revised design equation accounts for column flexure & difference in shear strains between

column & surrounding soil more reasonable estimates shear stress reduction provided by

discrete circular columns. 32

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LINEAR ELASTIC ANALYSIS OF DEEP SOIL STABILIZATION GRIDS WITH OPENSEES PLATFORM

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Linear Elastic Soil Profile DSM Half Unit CellHalf DSM Unit Cell Mesh in

OpenSeesPL

Linear Elastic FE DSM Model

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Standard DSM Half Unit Cell Under Earthquake

Spatial Variation

Great similarity between Pseudo Static and Earthquake case was observed which lead to the following proposed design equation.

EQ

Pseudo Static

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Tensile Stress in DSM Wall

The upper 3 m of the DSM wall regardless whether it is the side wall of back wall show the most tension ratio.

Proposed Design Relationships

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ProposedEquation StrainCompatibilityEquationTypically, a 60%-70% reduction in CSR

Formoreindepthdiscussion:TV.Nguyen,D.Rayamajhi,R.W.Boulanger,S.A.Ashford,J.Lu,A.Elgamal,andL.Shao,

DesignofDSMGridsforLiquefactionMitigation.JournalofGeotechnicalandGeoenvironmental Engineering,November,2013

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Conclusion Soilcrete Grid DSM grids affect both:

seismic site response (e.g., amax) seismic shear stress distributions (e.g. spatially averaged Rrd)

DSM grids on seismic site response can be significant and may require site-specific FEM analyses

The reduction in seismic shear stresses by DSM grids can be significantly over-estimated by current design methods that assume shear strain compatibility.

A modified equation is proposed for estimating seismic shear stress reduction effects. The modified equations account for non-compatible shear strains and flexure in some wall panels.

The top 2m-3m of DSM wall could potentially be the critical wall section in term of tension development.

Design of DSM Grids for Liquefaction Remediation

T V. Nguyen, D. Rayamajhi, R. W. Boulanger, S. A. Ashford, J. Lu, A. Elgamal, and L. Shao

November , 2013

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Commentary on the Selection, Design and Specification of Ground Improvement for Mitigation of Earthquake-Induced Liquefaction

Ground Improvement Committee of DFI

Ground Improvement Committee of DFI

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Thank You !