Ground Modification for Liquefaction Mitigation

Report
Ground Modification
for Liquefaction
Mitigation
January 11, 2013
Kansas City, MO
Tanner Blackburn, Ph.D., P.E.
Assistant Chief Engineer
www.HaywardBaker.com
Presentation Summary
 Determining liquefaction susceptibility
 NCEER guidelines
 Mitigation methods
 Densification
 Reinforcement
 Drainage
Geotechnical Seismic Hazards
 Liquefaction
 Bearing capacity
 Excessive settlement
 Lateral spreading
 Slope Stability
 Cyclic shear strength
 Kinematic loading of slopes/earth
Liquefaction
 Function of:
 Earthquake magnitude
 Distance from site
 Groundwater conditions (current or ‘high
water’?)
 Depth to ‘liquefiable’ strata (svo , rd)
 Common Input Parameters:
 Peak Ground Acceleration (PGA)
 Magnitude (M)
Liquefaction
 National Center for Earthquake Engineering
Research (NCEER) Summary Report (1997
Meeting, published in JGGE, 2001).
 Seed and Idriss (1971):
 cyclic  0 . 65
a max
s vo rd
g
 Normalized by vertical effective stress:
CSR eq  0 . 65
a eq s vo
g s
'
vo
rd
Liquefaction
 Resistance to liquefaction
 Referred to as Cyclic Resistance Ratio
(CRR) or CSRfield
 Function of:
 Geologic history (deposit type, age, OCR)
 Soil structure (relative density, clay content)
 Groundwater conditions
 Factor of Safety = CRR/CSR
Liquefaction
 Evaluation of CRR (NCEER, 1997):

SPT blow count (N)



Corrected blow count
Need fines content
Corrected clean sand blow count – N1(60)CS

CPT tip resistance (qc) and sleeve friction
(fs)
 Shear wave velocity (Vs)
 Corrections for magnitude (M)

Scaling factor (MSF) – apply to F.S.
Liquefaction – SPT Analysis
Liquefaction – CPT Analysis
 To address FC:

(qc1N)cs instead of qc1N

(qc1N)cs = Kc*qc1N

Kc = f(qc, fs, svo, s’vo)
 This eliminates need
for sampling to
determine FC.
Liquefaction – Shear Wave
Liquefaction - MSF
Example
 Loose Sand



(N1)60 at 15’ depth = 10
Fines Content < 5% (SW/SP)
Water table during earthquake @ 5’ depth
 Soil Parameters:





svo’=1176 psf
svo= 1800 psf
rd = 0.97
PGA=0.15g
M=5.8
Example (cont’d)
CSR eq  0 . 65
a eq s vo
g s
'
vo
rd
 CSR =
(0.65)(0.15)(1800/1176)(0.97)
 CSR = 0.15
 Using NCEER figure for (N1)60=
10: CRR=0.11
 MSF ≈2
 FS = MSF*(CRR/CSR) =
2*(0.11/0.15) = 1.47
 Note the influence of MSF!
Liquefaction - FS
0
qt [tsf]
40 80 120 160 200
Rf [%]
0 0.4 0.8 1.2 1.6 2
0
CSR and CRR
0.5
1
0
0
0
0
10
10
10
10
20
20
20
20
0
Depth [ft]
30
Depth [ft]
30
Depth [ft]
Depth [ft]
CPT-9
30
40
40
40
50
50
50
60
60
60
30
40
CSR
CRR
50
60
Factor of Safety
0.5 1 1.5
2
Liquefaction – Cohesive Materials
 Strength loss – not technically liquefaction
 ‘Seismic softening’
 ‘Chinese’ Criteria (Seed et al. 1983)
 Function of wc, LL, clay content
 Not well accepted anymore...
 Bray and Sancio (2006)
 No defined criteria, but good overview.
 Boulanger and Idriss (2006, 2007)
 Chris Baxter at URI - Silts
Liquefaction – Lateral Spreading
 Lateral spreading can occur in gradual slopes
(<2°)
 Must design for static and dynamic driving
forces with residual undrained shear strengths
 Even for cohesionless materials
Liquefaction-induced Settlement
Tokimatsu and
Seed, 1987
Ishihara and
Yoshimine, 1992
Zhang et al., 2002
Liquefaction Mitigation
 Increase strength ( CRR)
 Ground improvement (densification or grouting)
 Decrease driving stress ( CSR)
 Shear reinforcement with ‘stiffer’ elements within
soil mass
 Decrease excess pore pressure quickly
 Reduce drainage path distance with tightly
spaced drains
Mitigation - Densification
 Increase cyclic shear strength (CRR) by

increasing relative density of cohesionless
materials
Advantages:
 Field Verifiable!
 Conduct field testing before and after treatment
 Employed for over 50 years, through several large magnitude
earthquakes.
 Several peer-reviewed documents describing the methods,
efficiency, and mechanics of densification.
 Approved by CA Office of Statewide Health Planning and
Development (OSHPD) for hospital and school construction.
Mitigation - Densification
 Methods:





Dynamic compaction
Vibro-compaction
Vibro-replacement
Blast densification
Compaction grouting
Liquefaction Mitigation-Densification

Loose sand
zone
0

Hospital site
2

Vibroreplacement
to 45 ft.
0
qt [MPa]
10
20
0
0
CSR and CRR
0.5
1
0
1.5
Factor of Safety
1
Seismic Settlement [mm]
0
20
40
2
0
0
2
2
4
4
6
6
8
8
10
10
12
12
10
4
Depth [m]
6
20
8
30
10
12
40
Post Treatment
Pre-Treatment
Treatment Depth
14
14
50
14
Liquefaction Mitigation-Densification
qt [tsf]
0 40 80 120 160 200
 Sandy site
CSR and CRR
0.5
1
0
0
0
10
10
10
10
20
20
20
20
30
Depth [ft]
30
Depth [ft]
Depth [ft]
30
Depth [ft]
CPT-9
grouting for
liquefaction
mitigation
vibrations
0
0
0
 Compaction
 Urban site, no
Rf [%]
0 0.4 0.8 1.2 1.6 2
30
40
40
40
40
50
50
50
50
CSR
CRR Pre
CRR Post
60
60
60
60
Factor of Safety
0.5 1 1.5
2
Liquefaction Mitigation
 Increase strength ( CRR)
 Ground improvement (densification or grouting)
 Decrease driving stress ( CSR)
 Shear reinforcement with ‘stiffer’ elements within
soil mass
 Decrease excess pore pressure quickly
 Reduce drainage path distance with tightly
spaced drains
Mitigation - Reinforcement
 Reduce cyclic shear stress



applied to liquefiable soil by
installing ‘stiffer’ elements
within soil matrix that attract
stress.
Can be used in nondensifiable soils (silts, silty
sands).
Large magnitude EQs
Not verifiable
 Post-installation CPT or SPT
results will not differ from
pre-installation.
 Vertical load testing of
elements is not applicable.
soil
inc
soil
GI for Large Earthquakes
 Large magnitude
earthquakes:
 PGA ~0.3-1.0g
 M >7
 Typical CSR values
~ 0.3-0.6
 High liquefaction
potential for all soils
N<30
 Densification has
limited application
Reinforcement
 Original Design Methodology
 Shear stress reduction factor (KG) (Baez
and Martin, 1993):
KG 
1
G

1  ARR  INC  1 
 G Soil

GINC=Inclusion shear modulus
GSoil=Soil shear modulus
ARR=Ainclusion/Atotal
 Strain compatibility and force
equilibrium
 Assumes linear elastic soil and INC
behavior
 CSRapplied to soil = KG * CSRearthquake
Mitigation - Reinforcement
 10% Area
0
qt [tsf]
40 80 120 160 200
0
0.5
0
0
0
0
10
10
10
10
20
20
20
20
0
Replacement
CSR and CRR
Rf [%]
0 0.4 0.8 1.2 1.6 2
CPT-9
 GINC/GSOIL=5
30
30
Depth [ft]
G

1  ARR  INC  1 
 G Soil

30
Depth [ft]
1
Depth [ft]
KG 
Depth [ft]
 KG=0.7
30
40
40
40
40
50
50
50
50
CSR Pre
CSR Post
CRR Pre
60
60
60
60
Factor of Safety
0.5 1 1.5
2
Reinforcement
 Methods:
 Deep soil mixing
 Stone Columns
(aggregate piers)
– New research
indicates this
reinforcement
effect is limited
 Jet Grouting
Mitigation - Reinforcement
 Requires engineering judgment regarding
input parameters
 Is there a limit to the ‘inclusion’ stiffness?
 What is the deformation mechanism (bending or shear)?
 Is there a maximum spacing that should be used?
 If the soil liquefies around a stone column, what is the
strength of the stone column?
 Few peer-reviewed publications or
references regarding use and efficiency
 Vendor/contractor ‘white-papers’ do not qualify as design
standards or peer-reviewed methods
 State-of-the-practice is developing
Liquefaction Mitigation-Reinforcement
 Example of required judgment:
KG 
1
 G INC

1  ARR 
 1 
 G Soil

 Say we need KG=0.8, what ARR do we
need?
 Stone columns?
 Typical GSC/Gsoil ~ 5 (Baez/Martin,
Mitchell, FHWA)
 ARR = 6% (11’ grid spacing-36”
columns)
Liquefaction Mitigation-Reinforcement
 Example of required judgment:
KG 
1
 G INC

1  ARR 
 1 
 G Soil

 Say we need KG=0.8, what ARR do we
need?
 Piles?
 Typical GSteel/Gsoil ~ 2500
 W14x120 – A=0.23 ft2
 ARR = 0.01%
 50’ Spacing!!
Current research by Boulanger,
Elgamal, et al.
34
Spatial distribution Rrd
35
Reinforcement – Panels and Grids
Figure : Basic Treatment Patterns (Bruce 2003)
Linear Elastic FE DSM Model
Boulanger, Elgamal, et al.
Linear Elastic Soil Profile
DSM Half Unit Cell
Shear reduction - panels
Ratio of shear stress reduction coefficients; (a) Gr = 13.5, (b) Gr = 50
Conclusion – Soilcrete Grid
per Boulanger, Elgamal et. al
 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 reinforcement 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 noncompatible 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.
Thanks to Masaki Kitazume, Tokyo Institute of Technology
Provided images to HBI.
Thanks to Masaki Kitazume, Tokyo Institute of Technology
Provided images to HBI.
Thanks to Masaki Kitazume, Tokyo Institute of Technology
Provided images to HBI.
Brunswick Nuclear Plant
Southport, NC
Spoil Deposit
Batch Plant
Intake Canal
N
Ventura Cancer Center, CA
Liquefaction Mitigation
 Increase strength ( CRR)
 Ground improvement (densification or grouting)
 Decrease driving stress ( CSR)
 Shear reinforcement with ‘stiffer’ elements within
soil mass
 Decrease excess pore pressure quickly
 Reduce drainage path distance with tightly
spaced drains
Mitigation - Drainage
 Limit excess pore pressure increase and duration of
increased pore pressure during cyclic shearing by
providing short drainage paths in cohesionless
materials.
 Not verifiable with in situ testing
 Limited peer-reviewed publications or design standards.
 Methods:
 EQ Drains – perforated pipe installed on tight grid
 Stone columns – additional feature, but not relied on for
design
 Permeability of stone column material
 Contamination with outside material.
EQ Drain Theory
 Reduce the excess pore pressure
1
1
0.8
0.8
Pore pressure ratio
Pore pressure ratio
accumulation during earthquake
0.6
0.4
0.2
0.6
0.4
0.2
0
0
0
5
10
15
Shear stress cycles
20
25
0
5
10
15
Shear stress cycles
20
25
EQ Drain Details




Typically 75-150 mm diameter
Slotted PVC pipe with filter fabric
Typical spacing 1-2 m triangular
Installed with large steel probe with wings (densification also
intended)
EQ Drain Installation
EQ Drain Design Concept

Based on radial dissipation theory (just like vertical consolidation, but
radial geometry)
  k h 1 u


 r   w r  r
ch 
ch

 u u g
  mv 
 t  t



t
N
t
• Change in PP per cycle
depends on PP of
previous cycle
kh
m v w
  1 u   u u g



 r  r  r    t
t
u g










• NL based on CSR of soil,
SPT, Fines
u g N
N
t
N eq
Assume periodic wave form
td
1
ug
so
'
 N  2

 arcsin 


 Nl 
2
DeAlba et al., 1975
 ~ 0 .7
N l  Number of uniform
stress cycles causing
 r 
tan   u 
N


s
eq
 2 





r 
N
  N l t d 
2 
sin   u 
 2 
u g
'
o
liquefacti
on in undrained
• Neq, td are functions of
earthquake, but there are
correlations to magnitude
test
Derivations
 Factor of safety is inverse of Ru
 Settlement
 layer  T layer
m
'




s
v  New
v
m
'



R
s
v  New
u
vo
s v  u
'
 u  R u s vo
'
 layer  T layer
EQ Drain Design
 Graphical solutions to diff equation (JGS):

Address drain size, well resistance

Provides Ru, but no settlement calculations
 FEQDrain – Finite Element software program

Provides Ru and settlement calculations
 Both methods need the following:

Soil permeability, kh

Soil compressibility, mv,

Earthquake duration, td

Number of earthquake cycles, Neq

Drain spacing (trial values)
EQ Drain with Stone Column Installation
Stone Column Installation with EQ Drains
Liquefaction Mitigation
 Increase strength ( CRR)
 Ground improvement (densification or grouting)
 Decrease driving stress ( CSR)
 Shear reinforcement with ‘stiffer’ elements within
soil mass
 Decrease excess pore pressure quickly
 Reduce drainage path distance with tightly
spaced drains
Questions

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