Australian Geomechanical Society Victoria chapter

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]

1MB - CRF Melbourne 18 4 12

MB - CRF Melbourne 18 4 12 2

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

3MB - CRF Melbourne 18 4 12

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


Soil improvement techniquesGrain size of host (in situ) soil

Sand compaction piles

Deep mixing


Deep mixing method

Installation (1)

A-B : Vibrocompaction

…..

C-D: Stone columns


Stone columns:wet method

Initial soil expanded!

Installation (2)Deep mixing method (DMM)


Initial soil: undisturbed/ stone columns

Installation (3) Sand compaction pile (SCP)


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


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)


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:


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


Isolated Column TrenchLoaded area = total reinforced section

IAR = 100%

Modelling of CRF (2)

Unit Cell Model(oedometer)


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


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


SUGGESTED METHODOLOGY

1. Steps of design

1.1 Ultimate bearing capacity

1.2 Settlement estimation

1.3 Added value


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


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


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


: 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

( )


+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= +


** 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≤ ≤


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)


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.


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%


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=


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)


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 ,


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


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


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



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


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


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


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)


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


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


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


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


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


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)


4 m

8 m

0.92 m

Rigid Stratum

Layer n°2

Layer n°1

ColumnColumn

Benin: National Deputy House


Incorporation of stone material (1)


Incorporation of stone material (2)


Load plate test on isolated stone column


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)….


Construction in progress (1)


Construction in progress (2)


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


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


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


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


Documents

Australian Geomechanical Society Victoria chapter