Australian Geomechanical Society Victoria chapter 18 th April Design of columnar-reinforced foundation

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1 Australian Geomechanical Society Victoria chapter 18 th April 2012 Design of columnar-reinforced foundation Prof. Mounir Bouassida University of Tunis El Manar, National Engineering School of Tunis, Tunisia Vice President of Tunisian Society of Soil Mechanics MB -CRF Melbourne

2 MB -CRF Melbourne

3 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 MB -CRF Melbourne

4 Column-Reinforced Foundation An improvement of in situ soils: weak and/or highly compressible: (coastal areas) * Soft clays : E s < 3 MPa and c u < 30 kpa * Loose sands 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 MB -CRF Melbourne

5 Soil improvement techniques Grain size of host (in situ) soil Sand compaction piles Deep mixing method MB -CRF Melbourne

6 Installation (1) A-B: Vibrocompaction.. C-D: Stone columns Stone columns: wet method Initial soil expanded! MB -CRF Melbourne

7 Installation (2) Deep mixing method (DMM) Initial soil: undisturbed/ stone columns MB -CRF Melbourne

8 Installation (3) Sand compaction pile (SCP) Lateral expansion of soft soil: a consequence of vertical compaction of sand MB -CRF Melbourne

9 Characteristics of CRF (1) Geometry Soil profile -Loaded area Columns (3D) Uniform settlement : δ Foundation (Area A) Columns cross section: A c Columns: End bearing: H = H c Floating: H > H c Improvement Area Ratio: η = A c A MB -CRF Melbourne

10 Mechanical characteristics of column material (experienced projects) Columns installation method Improvement Area Ratio (%) Columns diameter (m) Sand compaction piles 5 < η < Stone Columns & Vibrocompaction 10 < η < Lime-cement treated soil 15 < η < Material columns Friction angle Cohesion (kpa) Young modulus (kpa) Sand 35 < ϕ < E s to 10 E s Stone & Gravel ϕ > E s to 50 E s Lime-cement treated soil ϕ < C 200 C 50 E s to 200 E s Improvement area ratio (IAR)is the key parameter: Cost of treatment Targeted by the method of design MB -CRF Melbourne

11 1. Verifications: (Stability) Steps of design of CRF Bearing capacity: 1 st requirement Settlement : 2 nd requirement Optimized IAR? 2. Alternatives of columnar reinforcement: comparison 3. Assessment of predictions: trial in situ tests: Installation possible? Predicted performances suitable? 4. Study of the behavior of CRF * Experiments: laboratory (scaled test models), In situ (load tests) * Numerically: FE codes Recommendations MB -CRF Melbourne

12 Modelling of CRF (1) z Q z Q o y x x O y Isolated Column Loaded area = total reinforced section IAR = 100% Trench MB -CRF Melbourne

13 Modelling of CRF (2) Unit Cell Model (oedometer) a IAR = b 2 2 MB -CRF Melbourne

14 Review of methods Ultimate Bearing Capacity 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, > 1 in situ data French Standard (2005) Stone Columns Isolated column = 2 Limit analysis ( ) All Group of columns = 2 Settlement Methods of prediction Installation methods Modelling Balaam and Booker ( ) All Unit cell Terashi & Tanaka (1981) Deep mixing method Scaled test model Broms (1982) Lime-cement treated soil Group of columns Priebe ( ) Stone Columns Unit cell French Standard (2005) Stone Columns Unit cell Bouassida et al (2003) All Group of columns MB -CRF Melbourne

15 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 MB -CRF Melbourne

16 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 bearing stone columns illustrating the efficiency of novel methodology. 2.2 Study of optimized options of reinforcement by floating columns. MB -CRF Melbourne

17 Constituents of Column-reinforced foundation Homogeneous and isotropic Initial soil Columns material Bearing capacity C s ; ϕ s C c = k c C s ; ϕ s s Failure characteristics Settlement E s ; ν s E c > E s ; ν c Linear elastic MB -CRF Melbourne

18 1. Verification of UltimateBearing Capacity (Limit Analysis): lower and/or upper bounds results Bouassida et al,, ( ) Q A ult = ( 1 η )[ ] + η [ ] f s f c Known σ ult,rs ult,rs = (1 - η) σ ult,s + η σ ult,c ult,s Allowable Bearing Capacity Global Safety factor : F 1 <= F < 3 σ all,rs all,rs = ((1 - η) σ ult,s + η σ ult,c ) /F ult,s MB -CRF Melbourne

19 σ all,rs Q A app σ all, rs all,rs = ((1 - η) σ ult,s + η σ ult,c )/F ult,s η ( ) app /, F Q A σ σ σ ult, c ult, s ult s = η min min (1) η min Minimum Improvement Area Ratio: η min min : Needed reinforcement to increase the bearing capacity min = 0 : Reinforcement is not needed MB -CRF Melbourne

20 2. Verification of Settlement Linear elastic characteristics E s, ν s E c, ν c Principle of superposition : δ tot tot = δ rs + δ ur Reinforced soil (rs): Group of end bearing columns is assumed rs Variational method: Bouassida et al (2003) δ rs ( Q / ) ηe actual c A H + (1 η) E c s = + δ rs Apparent modulus δ + rs Upper bound = E E Unknown! hom rs Practical meaning! MB -CRF Melbourne

21 η min min > 0 Allowable settlement: = + δ δ rs δ ur Is η min enough? δ Agreed Yes: Possible for loose sands (Vibro compaction) ** No! High compressible soft soils ** No, minimum Improvement area ratio is not sufficient η η max δ δ + rs rs ( / )( / δ ) rs Q A H E app c s E c E s η (2) = max max : maximum Improvement Area Ratio MB -CRF Melbourne

22 Bounding the improvement area ratio (IAR) (1) & (2) η η η min opt max Well targeted IAR Completed almost at end of construction δ rs Don t forget settlement of unreinforced under layers! δ ur ur : especially for high compressible soils (evolution of settlement in time) MB -CRF Melbourne

23 Suggested Methodology: An optimized improvement area ratio is identified * Complies with bearing capacity and settlement verifications * Applicable for all types of columns installation * Incorporated in Columns 1.01 software (includes the acceleration of consolidation settlement for drained columns) MB -CRF Melbourne

24 Columns 1.01 software Elaborated by Simpro spinoff of Tunis El Manar University ( ) Initiated through Funded project on valorization of research results ( ) by Tunisian Ministry of High Education and Scientific Research. Incorporates results ( ) published by the Research Team of 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, 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 , Glasgow, 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. MB -CRF Melbourne

25 Tunisian case history (1980) Working load, q= 120 kpa, exceeds the allowable bearing capacity η min min = 13% does not comply with allowable settlement (6 cm) Columns 1.01 software predicts η opt = 30.64% opt Executed reinforcement: 35% 708 columns of diameter 1.2 m MB -CRF Melbourne

26 E a = Qactual / A δ / H r c Predictions by Columns 1.0 software Verification of settlement Zero horizontal displacement MB -CRF Melbourne

27 Interpretation of results (1) Allowable bearing capacity(kpa) [F = 1.3] Working load Limit analysis (Homogenisation lower bound) French standard (2005) Settlement (cm) : Centreline of tank Recorded Design Bouassida et al Balaam and Chow French standard Priebe (2003) Booker (1981) (1996) (2005) (1995) (n 2 ) 23 (n 0 ) Executed η =35% s = 1.9m ; N c = 708 «Columns» η=30.64% s = 2.06m ; N c = % saving of column material MB -CRF Melbourne

28 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, homogenized Young modulus of reinforced soil with IAR = 35% ; E c = 10 E s Back calculation: improved Young modulus of initial soil = 1.4 E s! Improvement of initial soil due to column installation: real fact, observed by comparing between pre and post treatment characteristics MB -CRF Melbourne

29 Performances of Columns 1.01 software Case histories, scaled test models, loading tests MB -CRF Melbourne

30 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. 14 th Pan-American and 64 th Canadian Geotechnical Conf. October , Toronto, Ontario, Canada. : q = 10 to 50 kpa Soft clay : S u = 12 kpa Numerical model (Plaxis 2D V8) Reinforcement options: 1. Cement stabilized clay (CSC); Full replacement by compacted sand over thickness of layer (1) 2. Floating columns with optimized IAR MB -CRF Melbourne

31 Geotechnical parameters Saadeldin et al (2011) Soft clay: Hardening Soil Model Parameter Undrained Drained Saturated unit weight (kn/m 3 ) Cohesion (kpa) 12 1 Friction angle (Degree) Angle of dilatancy 0 0 Stiffness (kpa) Tangent stiffness (kpa) Power (m) 1 1 Horizontal permeability (cm/sec) 1x10-6 1x10-6 Vertical permeability (cm/sec) 1x10-6 1x10-6 Initial void ratio Unloading / Reloading stiffness (kpa) Poisson s ratio Reference stress for stiffness s (kpa) Coefficient of lateral stress in NC Failure ratio Reinforced soil: Mohr Coulomb Parameter CSC Compacted Sand Fill Saturated Unit weight (kn/m 3 ) Cohesion (kpa) Dilatancy (degree) 0 41 Friction angle (degree) 0 14 Stiffness (kpa) Initial void ratio Poisson s ratio MB -CRF Melbourne

32 Stability of embankment on unreinforced soft clay 1. Ultimate bearing capacity q = x 1 2 = k P a u lt q F ( q ) F = 1 q u lt k P a 2. Estimation of settlement at centre line of embankment Plaxis: consolidation Columns 1.01: linear elastic For q = 10 to 20 kpa the elastic settlement is 85% the long term one, same evolution MB -CRF Melbourne

33 Normalized settlement at ground surface settlement: (q = cte) settlement of reinforced soil/settlement of soft clay 1 st Reinforcement options: Cement stabilized clay (CSC); Full replacement by compacted sand over thickness of layer (1) Plaxis 2D-V8 predictions Columns 1.01: Improvement area ratio = 100% Predictions of settlement reduction are almost similar by Plaxis and Columns software Two reinforcement options seem equivalent MB -CRF Melbourne

34 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% MB -CRF Melbourne

35 2 nd 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 settlement. Settlement of reinforced soil completed at the end of construction: Allowable settlement = that of unreinforced layers 10 cm (long term). Applied Load (kpa) Columns reinforcement by Cement stabilized Clay Column s depth(m) Optimized improvement area ratio η opt (%) (%) of saving over 100 m 3 of substitution material Vs Full substitution over 5 m depth Floating columns of length 8 m provides 53% saving of treated soil MB -CRF Melbourne

36 Reinforcement by Compacted sand Columns Applied Load (kpa) Column s depth(m) Optimized improvement area ratio η opt (%) (%) of saving over 100 m 3 of substitution material Vs Full substitution over 5 m depth Floating columns of length 7.5 m provides 75% saving of substituted soil MB -CRF Melbourne

37 Performance of embankment on reinforced soft clay (2) Saga Japan (Chai and Carter, 2012) Compression index = 2 Floating columns H c = 8.5 m - In situ executed IAR = 30% (experience) MB -CRF Melbourne

38 Settlement: predictions, evolution Embankment 6 m height on reinforced soil by floating DMM columns Saga Japan (Chai and Carter, 2011) 35 Software Columns 1.01 Observations Allowable settlement (cm) Settlement of reinforced soil (cm) δ ur η min IAR < 30% OK! Optimized IAR Settlement of unreinforced soil: Predicted Columns 1.01 = 12.6 cm Observed (Total) = 19 cm Reasonable! Need of rigid blanket layer at surface of reinforced soil MB -CRF Melbourne

39 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 soilin lagon environmenttill 12 m depth of 30 kpa undrained cohesion. Unallowable bearing capacity Reinforcement by Stone columns has been executed to increase bearing capacity MB -CRF Melbourne

40 Single floating stone column under main pier, confined by: - 2 or 3 neighboured columns (corner piers) - 4 neighboured columns (current piers) 0.46 m 1.8 m 0.5 m 0.64 m 8 m Column 0.92 m Column Layer n 1 4 m Layer n 2 Rigid Stratum MB -CRF Melbourne

41 Benin: National Deputy House MB -CRF Melbourne

42 Incorporation of stone material (1) MB -CRF Melbourne

43 Incorporation of stone material (2) MB -CRF Melbourne

44 Load plate test on isolated stone column MB -CRF Melbourne

45 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. 3. Installed confining columns (8 m length): very conservative design & waste of very good selected material (lack of experienced stone columns projects). MB -CRF Melbourne

46 Construction in progress (1) MB -CRF Melbourne

47 Construction in progress (2) MB -CRF Melbourne

48 Performances of Columns 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 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 MB -CRF Melbourne

49 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 reinforcement Predictions validated: test models, recorded data form case histories, numerical predictions. Needs further options: consolidation settlement, improved initial soil characteristics Work in progress: Study of behaviour of CRF by numerical codes based on identified improvement area ratio. MB -CRF Melbourne

50 Achievements Acknowledgments to collaborators : 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, 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; 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), citations Professors P. De Buhan & L. Dormieux (ENPC, Paris) JM Debats (Vibroflotation Group, France) Drs Z. Guetif, B. Jellali, W. Frikha & S. Ellouze Members of Geotechnical Engineering Research Team (ENIT) MB -CRF Melbourne

51 3 rd International Conference on Geotechnical Engineering Hammamet (Tunisia) rd February (2013) Deadline abstract submission: April 30, 2012 Thanks for your attention MB -CRF Melbourne