Recent Advances in Column Technologies to Improve Soft Foundations

1. Recent Advances in Column Technologies to Improve Soft Foundations Jie Han, Ph.D., PE Professor The University of Kansas, USA

2. Outline of Presentation • Introduction • Innovations in Installation and Applications • Load Transfer Mechanisms • Settlement and Consolidation • Stability • Concluding Remarks

3. Introduction

4. Definition of Columns A vertical sub-structural element, installed in-situ by ground improvement techniques (replacement, displacement, and/or mixture with chemical agents), that carries the load of the super-structure or earth structure with surrounding soil and transmits it to geo-media around and/or below, through compression, shear, or rotation

5. Classification of Columns

6. Functions • Densification • Increase density, modulus, strength, and liquefaction • resistance of surrounding soil • Increase pre-consolidation stress of surrounding soil • Pile effect • Transfer loads to a deeper and competent geo-material • Stress concentration • Drainage • Accelerate consolidation • Increase liquefaction resistance • Reinforcement • Increase shear, tensile, and/or bending resistance

7. Design Considerations • Load transfer • Bearing capacity (e.g., Bouassida et al., 1995) • Settlement and consolidation • Slope stability • Liquefaction mitigation (e.g., Rollins et al.) • Earth retaining (e.g., Shao et al.)

8. Innovations in Column Installation and Applications

9. T-shape Deep Mixed Columns Mixing Mixing Mixing Mixing Mixing Mixing Mixing Courtesy of S.Y. Liu

10. T-shape Deep Mixing Courtesy of S.Y. Liu

11. Hollow Concrete Columns Referred to as Large Diameter Pipe Pile Using Cast-in-place Concrete (PCC) by Prof. Liu Courtesy of H.L. Liu

12. X-shape Concrete Columns Courtesy of H.L. Liu

13. Geosynthetic-encased Columns Alexiew et al. (2005)

14. Composite Columns Courtesy of G. Zheng

15. Composite Columns - Stiffened Deep Mixed Piles • Jet pressure =220 bar • Diameter =0.60 m • - L=7.00 m Courtesy of Bergado SDCM pile construction

16. Cement mix Spun pile Welding Composite Columns - Grouted Spun Pile Bhandari et al. (2009)

17. Pile-Column Combined Method Pile Column Huang and Li (2009) and Zheng et al. (2009)

18. DM-PVD Combined Method Liu et al (2008) DM column PVD Ye et al (2008)

19. Geosynthetic-reinforced fill platform Ds0 The Most Commonly Used Application – Column-supported Embankments Geosynthetics Embankment Ds0 Columns Firm soil or bedrock

20. Load Transfer Mechanisms

21. Equal Strain vs. Equal Stress c s Sc Ss Ss Ec Ec Es (a) Equal strain = rigid loading (b) Equal stress = flexible loading s c S Columns Sc Ss Ss Ec Ec Es How about a column-supported embankment?

22. Stress Concentration under Equal V. Strain c Stress Concentration Ratio, n = s c c s s Sc = Ss Sc = Ss Dc Ds Ec Es h 1-D unit cell Unit cell with lateral deformation z’ - (x’ - y’) z - (x - y) z = = = Es Ec Dc Ec s c z = n = n  Dc Ds Ds Es >

23. Stress Concentration Ratio vs. Strain Equal vertical strain condition Stress Stress concentration ratio, n Yielding c2 c3 Column c4 c1 Yielding Soil s4 s3 s2 s1 0 Strain Strain (a) Stress-strain relationship (b) Stress concentration ratio E.g., stone column: qcult = 15 to 25 cu, qsult = 5 to 6 cu n = qcult / qsult = 2 to 5

24. Influence of Column Lateral Deformation and Yielding Stress concentration ratio, n Castro and Sagaseta (2011)

25. Influence of Modulus Ratio and Column Yielding Rigid column Semi-rigid Flexible Jiang et al. (2010)

26. Stress Concentration vs. Consolidation 20 kPa 40 kPa Yin and Fang (2008)

27. n vs. Ec/Es Cutoff ratio for stone columns

28. Stress Transfer under Unequal Vertical Strain Modified from Schlosser and Simon (2008)

29. Stress Transfer in Geosynthetic-reinforced Column-supported Embankment W   H Hcr ps T d Ec ss Es sc Effects: (1) modulus ratio effect, (2) soil arching, (3) tensioned membrane/slab stiffening Modified from Han (1998)

30. Field Stress Concentration Ratio Findings: (1) n increases with stress level (2) n increases with rigidity of loading Han and Wayne (2000)

31. DEM Modeling of Dynamic Behavior Loading Findings: (1) geosynthetic increases rigidity of loading (2) n decreases with soil arching

32. Settlement and Consolidation

33. Methods of Settlement Calculation 1. Stress reduction factor (e.g., Aboshi et al, 1978) 2. Improvement factor method (e.g., Priebe, 1995) 3. Elastic-plastic solution (e.g., Pulko and Majes, 2005; Castro and Sagaseta, 2009) 4. Column penetration method (e.g., Chai et al., 2010) 5. Pier-raft method (e.g., Han et al., 2009) 5. Numerical method

34. Stress Reduction Factor Method Settlement of untreated ground Settlement of treated ground Settlement ratio If assume mv,s = mv,s’ Stress reduction factor Aboshi et al. (1978)

35. Stress Reduction Factor Methodvs. Numerical Method Ec/Es Jiang et al. (2013)

36. Improvement Factor Method Basic Method Assume incompressible columns with bulging over column length Improvement factor Settlement of stone column foundation Modified Method In addition to column bulging, column compressibility and overburden stress are considered Priebe (1995)

37. Basic Improvement Factor Method Priebe (1995)

38. Elastic-Plastic Solution for Stone Columns • Assume soft soil is linearly elastic • Assume stone columns are linearly elastic-perfectly • plastic with Mohr-Coulomb failure criterion with a • constant dilantancy angle • Plasticity starts with the upper portion of the column and • can extend deeper to the whole length of column with • applied load Pulko and Majes (2005) Castro and Sagaseta (2009)

39. Column Penetration Method Equivalent unimproved zone thickness due to column penetration Area replacement ratio Improvement depth ratio Hc = HL f() g() h() Pressure strength ratio Chai et al. (2010) and Chai (2012)

40. Pier-raft Approach for Settlement of Soil-cement or Concrete Columns Raft Es deq Horikoshi and Randolph (1999) Eeq Ag Randolph (1984) Han et al. (2009)

41. Calculated Settlements by Pier-raft Aproach 10m Settlement (cm) 0.8m Method 10m (a) Plan view Group Equivalent pier 7.4m Analytical 15.9 (16.9*) 15MN Raft Numerical 0.5m 15.6 16.9 Lp =10m DM columns (Ep=100MPa) h = 30m * Without considering finite depth effect (b) Cross section Es=5MPa Han et al. (2009)

42. kv kh Consolidation of Stone Columns(Han and Ye, 2001; 2002) Rate of consolidation due to radial flow: de p Drainage surface rs rc z Modified time factor in radial flow H Ec Es Stone column 2H ks kc r Drainage surface re

43. Han & Ye (2001) Khine (2004) Degree of Consolidation Free-draining stone column

44. Dissipation of Excess Pore Pressure Han and Ye (2001)

45. Well Resistance Effect Han (2010)

46. Consolidation of Column-improved Soft Foundation over Soft Soil Zhu and Yin’s (1999) closed-form solution for consolidation of two-layered soils can be used for calculation of consolidation rate Chai and Pongsivasathit (2009)

47. Consolidation of Soil-cement Column-improved Foundations kc = ks Jiang et al. (2013)

48. Stability

49. Column Failure Modes under Embankment Loading Modified from Kitazume (2008) and Broms (1999)

50. Factor of Safety under Undrained Condition for Stone Columns FS (individual) = 0.9 FS (equivalent) Abusharar and Han (2010)