australian geomechanical society victoria chapter
TRANSCRIPT
Design of columnar-reinforced foundation
Australian Geomechanical SocietyVictoria chapter
18th April 2012
Prof. Mounir Bouassida
University of Tunis El Manar,
National Engineering School of Tunis, Tunisiawww.enit.rnu.tn
Vice President of Tunisian Society of Soil [email protected]
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Outline
Introduction: What is a CRF? When it is used?
Benefits, methods of installation and associated
types of soil
Design of CRF: Review of existing methods
Suggested methodology: added value and implementation
Illustrations (study cases) & performances: Columns 1.01 software
Conclusions & recommendations
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Column-Reinforced Foundation
• An improvement of in situ soils:
weak and/or highly compressible: (coastal areas)* Soft clays : Es < 3 MPa and cu < 30 kPa
* Loose sands ϕ ϕ ϕ ϕ < 30° (N < 10).
Reinforcement:
* added material with enhanced stiffness and strength
** soil treatment by added binder
Benefits: increased bearing capacity, settlement reduction,
Accelerated consolidation, preventing liquefaction
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Soil improvement techniquesGrain size of host (in situ) soil
Sand compaction piles
Deep mixing
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Deep mixing method
Installation (1)
A-B : Vibrocompaction
…..
C-D: Stone columns
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Stone columns:wet method
Initial soil expanded!
Installation (2)Deep mixing method (DMM)
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Initial soil: undisturbed/ stone columns
Installation (3) Sand compaction pile (SCP)
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Lateral expansion of soft soil: a consequence of vertical compaction of sand
Characteristics of CRF (1)
• Geometry Soil profile - Loaded area – Columns (3D)
Foundation (Area A)
Uniform settlement : δδδδ
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Columns’ cross section: A cccc
End bearing: H = H ccccColumns:
Floating: H > H cccc
Improvement Area Ratio :
cA
Aη =
Mechanical characteristics of column material(experienced projects)
Columns installation method Improvement Area Ratio (%) Columns diameter (m)
Sand compaction piles 5 < ηηηη < 15 0.4 – 0.6Stone Columns & Vibrocompaction 10 < ηηηη < 35 0.8 – 1.2
Lime-cement treated soil 15 < ηηηη < 70 0.3 – 0.7
Material columns Friction angle Cohesion (kPa) Young modulus (kPa)
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Material columns Friction angle Cohesion (kPa) Young modulus (kPa)
Sand 35° < ϕ ϕ ϕ ϕ < 38° 0 5 Es to 10 Es
Stone & Gravel ϕϕϕϕ > 38° 5 - 15 15 Es to 50 Es
Lime-cement treated soil
ϕϕϕϕ < 20° 20 C – 200 C 50 Es to 200 Es
Improvement area ratio (IAR) is the key parameter: Cost of treatment
Targeted by the method of design
Steps of design of CRF1. Verifications: (Stability)
Bearing capacity: 1st requirementOptimized IAR ?
Settlement : 2nd requirement
2. Alternatives of columnar reinforcement: comparis on
3. Assessment of predictions: trial in situ tests:
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Installation possible? Predicted performances suita ble?
4. Study of the behavior of CRF
* Experiments: laboratory (scaled test models), In situ (load tests)
* Numerically: FE codes
Recommendations
Modelling of CRF (1)
y
x
z
o
Q
y
x
z Q
O
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Isolated Column TrenchLoaded area = total reinforced section
IAR = 100%
Modelling of CRF (2)
Unit Cell Model(oedometer)
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2
2
aIAR
b=
Review of methods
Methods of prediction Installation methods Modeling /.. Factor of safety
Aboshi et al (1979) Sand compaction pile Unit cell NA
Terashi and Tanaka (1981) Deep mixing method Scaled test model > 1
Broms (1982) Lime-cement treated soil Different models, in situ data
> 1
French Standard (2005) Stone Columns Isolated column = 2
Limit analysis (1995-2011) All Group of columns = 2
Ultimate Bearing Capacity
Settlement
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Methods of prediction Installation methods Modelling
Balaam and Booker (1981-1985) All Unit cell
Terashi & Tanaka (1981) Deep mixing method Scaled test model
Broms (1982) Lime-cement treated soil Group of columns
Priebe (1979-1995) Stone Columns Unit cell
French Standard (2005) Stone Columns Unit cell
Bouassida et al (2003) All Group of columns
Settlement
EXISTING DESIGN METHODS
1. Unique verification: bearing capacity or settlement
2. Unique column installation: stone columns (Priebe),
deep mixing (Broms), etc.
3. Optimization of the quantity of column material not
discussed, improvement are ratio is a given data from
experienced projects
Bearing capacity and settlement are not tackled jointly
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SUGGESTED METHODOLOGY
1. Steps of design
1.1 Ultimate bearing capacity
1.2 Settlement estimation
1.3 Added value
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2. Validation of software predictions
2.1 Studied case histories : Reinforcement by end b earing stone columns illustrating the efficiency of novel method ology.
2.2 Study of optimized options of reinforcement by floating columns.
Constituents of Column-reinforced foundation
Bearing capacity
Homogeneous and isotropic
Initial soil Columns material
Cs ; ϕϕϕϕsCc = kc Cs ; ϕϕϕϕ
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s s
SettlementSettlement
Linear elastic
Ec > Es ; ννννccccEs ; ννννssss
Failure characteristics
1. Verification of Ultimate Bearing Capacity
(Limit Analysis): lower and/or upper bounds results
Bouassida et al, …, (1995-2011)
( )[ ] [ ]cs ffA
Q
ult
ηη +−=
−
1
σσσσult,rsult,rsult,rsult,rs = (1 = (1 = (1 = (1 ---- ηηηη) ) ) ) σσσσult,sult,sult,sult,s + + + + ηηηη σσσσult,c
Known
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AllowableAllowable Bearing CapacityBearing Capacity
Global Safety factor : F
σσσσall,rsall,rsall,rsall,rs = ((1 = ((1 = ((1 = ((1 ---- ηηηη) ) ) ) σσσσult,sult,sult,sult,s + + + + ηηηη σσσσult,c ) /F
σσσσult,rsult,rsult,rsult,rs = (1 = (1 = (1 = (1 ---- ηηηη) ) ) ) σσσσult,sult,sult,sult,s + + + + ηηηη σσσσult,c
1 <= F < 3
,app
all rs
Q
Aσ
≤
σσσσall,rsall,rsall,rsall,rs = ((1 = ((1 = ((1 = ((1 ---- ηηηη) ) ) ) σσσσult,sult,sult,sult,s + + + + ηηηη σσσσult,c )/F
( ) ,/app ult sF Q A ση
σ σ−
≥−
= η= η= η= ηminminminmin (1)
Minimum Improvement Area Ratio: ηηηηminminminmin
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: Needed reinforcement to increase the bearing capacity
, ,ult c ult sσ σ−
ηηηηminminminmin= 0= 0= 0= 0 : Reinforcement is not needed
2. Verification of Settlement
Linear elastic characteristics
Principle of superposition : δδδδtottottottot = = = = δδδδrsrsrsrs + + + + δδδδurReinforced soil (rs): Group of end bearing columns is assumed
Variational method: Bouassida et al (2003)
Es , ννννssss Ec , ννννcccc
( )
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+rsδ Upper bound
( )/
(1 )actual c
rsc s
Q A H
E Eδ
η η≤
+ −+rsδ=
Practical meaning!
rsEE ≤= homUnknown !Apparent modulus
Allowable settlement:
Yes: Possible for loose sands (Vibro compaction)
** No, minimum Improvement area ratio is not sufficient
Is ηηηηmin enough ?
** No ! High compressible soft soils
Agreed
ηηηηminminminmin> 0> 0> 0> 0
δ
rs urδ δ δ= +
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** No, minimum Improvement area ratio is not sufficient
( )( )/ / rsapp c s
c s
Q A H E
E E
δη
−≤
−maxη= (2)
ηηηηmaxmaxmaxmax: maximum Improvement Area Ratio
rs rsδ δ +≤
Bounding the improvement area ratio (IAR)
δδδδ
(1) & (2)
min maxoptη η η≤ ≤
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Don’t forget settlement of unreinforced under layer s!
δδδδurururur : especially for high compressible soils : especially for high compressible soils : especially for high compressible soils : especially for high compressible soils
(evolution of settlement in time)(evolution of settlement in time)(evolution of settlement in time)(evolution of settlement in time)
Well targeted IAR δδδδrsrsrsrsCompleted almost at end of construction
An optimized improvement area ratio is identified
* Complies with bearing capacity and settlement verifications
Suggested Methodology:
* Applicable for all types of columns installation
* Incorporated in Columns 1.01 software (includes the acceleration of consolidation settlement for drained columns)
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Columns 1.01 software
www.simpro-tn.com
• Elaborated by Simpro spinoff of Tunis El Manar University (2005-2009)
• Initiated through Funded project on valorization of research results
(2007-2009) by Tunisian Ministry of High Education and Scientific
Research.
• Incorporates results (1995-2007) published by the Research Team of
Geotechnical Engineering (National Engineering School of Tunis).Geotechnical Engineering (National Engineering School of Tunis).
• Related publications:• Bouassida M. & Hazzar L. (2012). Novel tool for optimised design of reinforced soils by columns.
Ground Improvement: Proc. ICE 165, Issue 1, 31 –40.
• Bouassida M., Hazzar L. & de Buhan P. (2009). A software for the design of reinforced soils by
columns. Proc. 2nd Int. Workshop on Geotechnics of Soft Soils- Focus on Ground Improvement-
Karstunen & Leoni (Editors), September 03-05 2008, Glasgow, 327-332.
• Bouassida M., Hazzar L. & Mejri A. (2012). Assessment of software for the design of columnar
reinforced soil. Accepted in International Symposium on Ground Improvement IS-GI Brussels 31
May & 1 June.
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Tunisian case history (1980)
Working load, q= 120 kPa, exceeds the allowable bea ring capacity
ηηηηminminminmin = 13% does not comply with
allowable settlement (6 cm)
Columns 1.01 software predicts
ηηηη = 30.64%
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ηηηηoptoptoptopt = 30.64%
Executed reinforcement: 35%
708 columns of diameter 1.2 m
Predictions by Columns 1.0 softwareVerification of settlement
Zero horizontal displacement
/
/actual
ar c
Q AE
Hδ=
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Interpretation of results (1)� Allowable bearing c apacity (kPa) [F = 1.3]
WorkingWorking loadload LimitLimit analysisanalysis ((HomogenisationHomogenisationlowerlower boundbound))
French standardFrench standard(2005)(2005)
120120 160160 534534
� Settlement (cm) : Centreline of tank
RecordedRecorded Bouassida et alBouassida et al(2003)(2003)
BalaamBalaam and and BookerBooker (1981)(1981)
Chow Chow (1996)(1996)
French standardFrench standard(2005)(2005)
Priebe Priebe (1995)(1995)
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30.64%η=
(2003)(2003) BookerBooker (1981)(1981) (1996)(1996) (2005)(2005) (1995)(1995)
4.04.0 5.85.8 5.15.1 4.24.2 5.55.5 6.1 (n6.1 (n22))23 (n23 (n00))
� Design
« Columns »
35%η =Executed s = 1.9m ; N c = 708
s = 2.06m ; N c = 620
10 % saving of column material
Interpretation of results (2)All predictions are conservative/recorded data
• Regardless column material characteristics, those of host soil were
underestimated with respect to in situ conditions and the (more or
less) adopted oedometric condition.
• Improvement of host soil characteristics was not taken into account
Consider recorded settlement = 4 cm ,
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Consider recorded settlement = 4 cm ,homogenized Young modulus of reinforced soil with IAR = 35% ; E c c c c = 10 E s s s s
Back calculation: improved Young modulus of initial soil = 1.4 E ssss !
Improvement of initial soil due to column installat ion: real fact, observed by comparing between pre and post treatmen t characteristics…
Performances
of Columns 1.01 softwareCase histories, scaled test models, loading tests
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Performance of embankment on reinforced soft clay (1)R. Saadeldin, M. A. Salem & H.A. Lotfi (2011). Performance of road embankment on cement stabilized soft clay. Proc.
14th Pan-American and 64th Canadian Geotechnical Conf. October 2-6 2011, Toronto, Ontario, Canada.
Numerical model (Plaxis 2D – V8)
: q = 10 to 50 kPa
Soft clay : Suuuu= 12 kPa
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Reinforcement options:1. Cement stabilized clay (CSC); Full replacement by compacted sand over thickness of
layer (1)
Soft clay : Suuuu= 12 kPa
2. Floating columns with optimized IAR
Geotechnical parameters Saadeldin et al (2011)
Parameter Undrained Drained
Saturated unit weight (kN/m3) 15.8 15.8
Cohesion (kPa) 12 1
Friction angle (Degree) 0 25.6
Angle of dilatancy 0 0
Stiffness (kPa) 430 430
Tangent stiffness (kPa) 500 500
Power (m) 1 1
Horizontal permeability (cm/sec) 1x10-6 1x10-6
Vertical permeability (cm/sec) 1x10-6 1x10-6
Initial void ratio 1.81 1.81
Unloading / Reloading stiffness (kPa)1300 1300
Poisson’s ratio 0.45 0.2
Soft clay:Hardening Soil Model
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Poisson’s ratio 0.45 0.2
Reference stress for stiffness’s (kPa)62 62
Coefficient of lateral stress in NC 1 0.568
Failure ratio 0.9 0.9
Parameter CSC Compacted Sand Fill
Saturated Unit weight (kN/m3) 18.5 20
Cohesion (kPa) 121 1
Dilatancy (degree) 0 41
Friction angle (degree) 0 14
Stiffness (kPa) 5000 37000
Initial void ratio 0.9 1
Poisson’s ratio 0.2 0.3
Reinforced soil:Mohr Coulomb
Stability of embankment on unreinforced soft clay
1. Ultimate bearing capacity 5 .1 4 1 2 6 1 .7u ltq x k P a= =
( 4 0 )u ltq F q+≻ 1F = 2 1 .7q k P a≺
2. Estimation of settlement at centre line of emban kment
Plaxis: consolidation Columns 1.01: linear elastic
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For q = 10 to 20 kPa the elastic settlement is 85% the long term one, same evolution
Normalized settlement at ground surface settlement: (q = cte)
settlement of reinforced soil/settlement of soft clay
1st Reinforcement options: Cement stabilized clay (CSC); Full replacement by compacted sand over thickness of layer (1)
Columns 1.01: Improvement area ratio = 100%
Plaxis 2D-V8 predictions
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Predictions of settlement reduction are almost similar by Plaxis and Columns softwareTwo reinforcement options seem equivalent
Cement stabilized clay (CSC); Full replacement by
compacted sand over thickness of layer (1)
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One meter increase in depth of substituted soil provides settlement reduction :by Plaxis by Columns 1.01
For CSC: 15% 5.8%For Compacted sand 17% 6.6%
2nd Reinforcement option:
Floating columns with optimized IAR (Columns software)
IAR < 100%: Length of columns is increased ( > 5 m)
Optimized IAR depends on loading and allowable settl ement.
Settlement of reinforced soil completed at the end of construction: Allowable settlement = that of unreinforced layers 10 cm (long term).
(%) of saving over 100 m3 of
Columns reinforcement by Cement stabilized Clay
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Applied Load (kPa) Column’s depth(m)Optimized improvement area
ratio ηopt (%)
(%) of saving over 100 m3 of substitution material
10 7.5 47 29
20 7.5 56 15.5
30 7.5 60 10
40 8 31 53
50 8 31 50
Vs Full substitution over 5 m depth
Floating columns of length 8 m provides 53% saving of treated soil
Reinforcement by Compacted sand Columns
Applied Load (kPa) Column’s depth(m)Optimized improvement area
ratio ηopt(%)
(%) of saving over 100 m3 of substitution material
10 7 32 55
20 7.5 17 75
30 7.5 31 54
40 7.5 44.5 33
Vs Full substitution over 5 m depth
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40 7.5 44.5 33
50 7.5 58 12.5
Floating columns of length 7.5 m provides 75% savin g of substituted soil
Performance of embankment on reinforced soft clay (2)
Saga Japan (Chai and Carter, 2012)
Compression index = 2
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Floating columns H cccc = 8.5 m - In situ executed IAR = 30% (experience)
Settlement: predictions, evolutionEmbankment 6 m height on reinforced soil by floating DMM columns
Saga Japan (Chai and Carter, 2011)
20
25
30
35
Allowable settlement (cm)
ObservationsSoftware Columns 1.01
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5
10
15
10 20 30 40 50 60 70 80
Settlement of reinforced soil (cm)
Optimized IAR
Settlement of unreinforced soil:Predicted “Columns 1.01” = 12.6 cmObserved (Total) = 19 cm
Need of rigid blanket layer at surface of reinforced soil
Reasonable!
ηηηηmin min min min IAR < 30% OK!
urδ
National Deputy House of Benin, June 2009
• Buildings 3 to 5 stories: isolated square footings,
1.8 m width, assembled by connecting strings; applied
load 200 kN
• Very soft soil in lagon environment till 12 m depth
of 30 kPa undrained cohesion.of 30 kPa undrained cohesion.
Unallowable bearing capacity
Reinforcement by Stone columns has been
executed to increase bearing capacity
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0.46 m0.64 m0.5 m
1.8 m
Single floating stone column under main pier, confined by: - 2 or 3 neighboured columns (corner piers)- 4 neighboured columns (current piers)
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4 m
8 m
0.92 m
Rigid Stratum
Layer n°2
Layer n°1
ColumnColumn
Benin: National Deputy House
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Incorporation of stone material (1)
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Incorporation of stone material (2)
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Load plate test on isolated stone column
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Main piers: IAR= 0.2 : Floating columns
1. Increase of bearing capacity (conservative): 50%,
2. Settlement reduction:
(Columns modulus = 25 times host soil modulus): 100%!
Validation: Load plate field test on isolated column:
No observed settlement under applied 250 kN load.No observed settlement under applied 250 kN load.
3. Installed confining columns (8 m length): very conservative
design & waste of very good selected material (lack of
experienced stone columns projects)….
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Construction in progress (1)
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Construction in progress (2)
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Performances of Columns 1.01
1. Recent tool of design of CRF
2. Based on comprehensive methodology
3. Predicts and optimized IAR, cost effective design:
overestimation by other methods evidenced
4. Validation made for various case histories: performance 4. Validation made for various case histories: performance
of floating DMM columns
5. Settlement prediction: end of construction, the
prediction of consolidation settlement: to be
incorporated
6. Optimized IAR only related to reinforced soil settlement:
more it is allowed, more cost effective design
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Conclusions & Recommendations
Novel methodology for the design of CRF, valid for all installation methods
Optimized IAR is identified that makes possible cost effective solution
Methodology implemented in Columns 1.01 software
Efficient tool, offering several alternatives of re inforcement
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Efficient tool, offering several alternatives of re inforcement
Predictions validated: test models, recorded data f orm case histories, numerical predictions.
Needs further options: consolidation settlement, im proved initial soil characteristics
Work in progress: Study of behaviour of CRF by nume rical codes based on identified improvement area ratio.
Achievements
Acknowledgments to collaborators
1995-2012 : 14 articles & 02 discussions int. Journals
02 invited papers, special publication and 40 papers in Int. Conf.
04 PhDs and 13 MSc defended
Elaborated software on sale & set up of consulting geotechnical bureau
M. Bouassida; P. de Buhan; L. Dormieux (1995). Bearing capacity of a foundation resting on a soil reinforced by a group of columns. Géotechnique, Vol. 45, n° 1, 25-34. 27 citations
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Professors P. De Buhan & L. Dormieux (ENPC, Paris)JM Debats (Vibroflotation Group, France)Drs Z. Guetif, B. Jellali, W. Frikha & S. EllouzeMembers of Geotechnical Engineering Research Team ( ENIT)
reinforced by a group of columns. Géotechnique, Vol. 45, n° 1, 25-34. 27 citations
Z. Guetif; M. Bouassida ; J. M. Debats (2007). Improved Soft Clay Characteristics Due to Stone Column Installation. Computers and Geotechnics. Vol 34 n°2; 104-111. 22 citations
B. Jellali; M. Bouassida ; P. de Buhan (2005). A Homogenisation method for estimating the bearing capacity of soils reinforced by columns. Int. Journal of Num & Analyt. Meth. in Geomechanics. Vol. 29 (10), 989-1004. 11 citations
3rd International Conference on Geotechnical Engineerin g
Hammamet (Tunisia) 21-23 rd February (2013)
www.icge13.com
Deadline abstract submission: April 30, 2012
Thanks for your attention
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