Recent Advances in Column Technologies to Improve Soft Foundations Jie Han, Ph.D., PE Professor The University of Kansas, USA Outline of Presentation Introduction Innovations in Installation and Applications Load Transfer Mechanisms Settlement and Consolidation Stability Concluding Remarks Introduction 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 Classification of Columns Method Type Technology Examples Installation Replacement Stone columns Displacement Sand compaction piles, stone columns Mixture DM columns, grouted columns Combination Rammed aggregate piers Material Granular Chemically-stabilized Concrete Composite Rigidity Flexible Sand compaction piles, stone columns, rammed aggregate piers DM columns and grouted columns Concrete columns, cement-flyash-gravel (CFG) columns Geosynthetic-encased soil columns, stiffened DM columns, and composite spun piles Sand compaction piles, stone columns, rammed aggregate piers Semi-rigid DM columns, grouted columns, composite columns Rigid Concrete columns 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 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.) Innovations in Column Installation and Applications T-shape Deep Mixed Columns Rotation direction 1 Grouting 2 Grouting 3 Mixing Mixing Mixing 4 Grouting 5 Mixing Grouting 6 Mixing Mixing 7 Mixing 8 Courtesy of S.Y. Liu T-shape Deep Mixing Courtesy of S.Y. Liu Hollow Concrete Columns Referred to as Large Diameter Pipe Pile Using Cast-in-place Concrete (PCC) by Prof. Liu Courtesy of H.L. Liu X-shape Concrete Columns Courtesy of H.L. Liu Geosynthetic-encased Columns Alexiew et al. (2005) Composite Columns Courtesy of G. Zheng Composite Columns - Stiffened Deep Mixed Piles - Jet pressure =220 bar - Diameter =0.60 m - L=7.00 m Courtesy of Bergado SDCM pile construction Composite Columns - Grouted Spun Pile Cement mix Spun pile Welding Bhandari et al. (2009) Pile-Column Combined Method Pile Column Huang and Li (2009) and Zheng et al. (2009) DM-PVD Combined Method Embankment DJM column PVD Liu et al (2008) Inclinometer Settlement plate Earth pressure cell Piezometer Not to scale DM column Ye et al (2008) PVD The Most Commonly Used Application – Column-supported Embankments Ds0 Geosynthetic-reinforced Geosynthetics fill platform Embankment Ds0 Columns Firm soil or bedrock Load Transfer Mechanisms Equal Strain vs. Equal Stress c s Ec Ss Es Sc Ec Ss Ec s c Ss Es Sc Ec Ss (a) Equal strain = rigid loading (b) Equal stress = flexible loading DS Columns How about a column-supported embankment? Stress Concentration under Equal V. Strain Stress Concentration Ratio, n = c Ds Sc = Ss 1-D unit cell z = c Dc n= = Dc Ds s Ds s c s Dc c Ec s Es h Sc = Ss Unit cell with lateral deformation z’ - (x’ - y’) z - (x - y) z = = Es Ec Ec > n Es Stress Concentration Ratio vs. Strain Equal vertical strain condition Stress c2 c3 c4 c1 Yielding Stress concentration ratio, n c n s Column Yielding Soil s4 s3 s2 s1 Strain (a) Stress-strain relationship 0 Strain (b) Stress concentration ratio E.g., stone column: qcult = 15 to 25 cu, qsult = 5 to 6 cu n = qcult / qsult = 2 to 5 Stress concentration ratio, n Influence of Column Lateral Deformation and Yielding Castro and Sagaseta (2011) Influence of Modulus Ratio and Column Yielding Stress concentration ratios 70 Rigid column L/d e=4 as =0.1 kc/kv=1 60 50 40 Ec/E 10 30 50 20 100 Semi-rigid Flexible 10 0 0.1 1 10 1000 100 Time (days) 10000 100000 Jiang et al. (2010) Stress Concentration vs. Consolidation kPa 2040kPa Yin and Fang (2008) n vs. Ec/Es 10 Stress Concentration Ratio, n 9 8 7 6 Cutoff ratio for stone columns n = 1 + 0.217 (Ec /Es - 1) 5 4 Barksdale and Bachus (1983) 3 2 1 0 0 10 20 Modulus Ratio, Ec /Es 30 40 Stress Transfer under Unequal Vertical Strain Settlement, S(z) Shear stress, t(z) Average vertical stress, (z) Equal settlement (upper plane) Fill hc Soft soil re Sc Column t<0 Ss 0 s f rc t>0 Equal settlement (lower plane) Bearing layer z Sc at r < rc Ss at r = r e c z t at r = rc z c at r < rc s at r = r e Modified from Schlosser and Simon (2008) Stress Transfer in Geosynthetic-reinforced Column-supported Embankment t Hcr W t ps T c s H Es Ec Effects: (1) modulus ratio effect, (2) soil arching, (3) tensioned membrane/slab stiffening Modified from Han (1998) Stress Concentration Ratio, n Field Stress Concentration Ratio 70 All plate loading test data from Han and Ye (1991) 60 50 40 30 20 10 0 0 100 Flexible column PLT/lime columns 200 300 400 Applied pressure, p (kPa) Semi-rigid column PLT/DM columns PLT/stone columns GCSE/DM columns PLT = Plate loading test CSE = Column-supported embankment GCSE = Geosynthetic-reinforced column-supported embankment Findings: 500 600 Rigid column PLT/VCC PLT/concrete columns GCSE/VCC GCSE/concrete columns CSE/concrete columns (1) n increases with stress level (2) n increases with rigidity of loading Han and Wayne (2000) 09 DEM Modeling of Dynamic Behavior Loading 19 20 21 22 23 9 Unreinforced Reinforced 14 15 16 17 18 Embankment 1.3m 9 Optional geogrid 4 10 5 1 Pile cap 11 6 2 12 7 13 8 0.3 m 3 Stress concentration ratio 8 7 6 5 4 3 2 1 0 0 5 10 15 20 25 Cycle 0.3 m Findings: 0.9 m 0.3 m (1) geosynthetic increases rigidity of loading (2) n decreases with soil arching 30 Settlement and Consolidation 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 Stress Reduction Factor Method Settlement of untreated ground ss mv,s Dz H Settlement of treated ground ssc m'v,s D'z H m'v,ss Dz H Settlement ratio m 'v ,s s sc s s s m v ,s If assume mv,s = mv,s’ ssc 1 s ss 1 a s (n 1) Stress reduction factor Aboshi et al. (1978) Stress Reduction Factor Method vs. Numerical Method Consolidation settlement (mm) 300 H/d e = 4 as = 0.1 k c/k v = 1 250 200 Numerical Simplified 150 100 50 0 0 20 40 60 80 100 /Es EEcc/E Jiang et al. (2013) Improvement Factor Method Basic Method Assume incompressible columns with bulging over column length Improvement factor 5 as If 1 a s 1 2 o 41 a s tan 45 c / 2 Settlement of stone column foundation ss s sc If Modified Method In addition to column bulging, column compressibility and overburden stress are considered Priebe (1995) Basic Improvement Factor Method 8 Friction angle of column (deg.) 7 Improvement Factor 35 37.5 6 40 5 42.5 45 4 3 2 1 0 0.1 0.2 0.3 0.4 0.5 Area Replacement Ratio Priebe (1995) 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) Column Penetration Method Equivalent unimproved zone thickness due to column penetration Area replacement Improvement depth ratio ratio Hc = HL f() g() h() Pressure strength ratio Chai et al. (2010) and Chai (2012) Pier-raft Approach for Settlement of Soilcement or Concrete Columns Eeq Es E p Es Raft Es deq Atp Ag Horikoshi and Randolph (1999) Eeq Ag K pr Pp Pr Spr K p K r 1 2cp 2 1 K r / K p cp Randolph (1984) Han et al. (2009) Calculated Settlements by Pier-raft Aproach 10m 0.8m 10m (a) Plan view Settlement (cm) Method Group 7.4m Analytical 15MN Raft 0.5m Numerical Equivalent pier 15.9 (16.9*) 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) Consolidation of Stone Columns (Han and Ye, 2001; 2002) Rate of consolidation due to radial flow: de p Ur 1 e Drainage surface rs Stone column Ec rc z Es kv kc ks 8 Tr' ' Fm ( N ) Modified time factor in radial flow H 2H c'r t T 2 de ' r kh Drainage surface r re 1 c c r 1 n s 2 N 1 ' r Degree of Consolidation 0 0.2 Barron (1947) 0.4 U n=10 0.6 n=1 Free-draining stone column Balaam and Booker (1981) Han and Ye (2001) 0.8 1 0.0001 0.001 0.01 0.1 Tr Han & Ye (2001) Khine (2004) Dissipation of Excess Pore Pressure Dissipation of Average Excess Pore Water Pressure, D u/p 1.0 N=3, ns=5 0.9 0.8 Due to stress reduction 0.7 0.6 0.5 0.4 0.3 Due to drainage 0.2 0.1 0.0 0 0.02 0.04 0.06 0.08 0.1 0.12 Time Factor, Tr Han and Ye (2001) Well Resistance Effect Time (day) 0 20 40 60 80 100 120 0 10 Settlement (mm) 20 30 40 Field data (Tan et al., 2008) No well resistance (Han and Ye, 2002) Well resistance (Han and Ye, 2002) 50 60 70 80 90 Han (2010) 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) Consolidation of Soil-cement Column-improved Foundations 0 Average degree of consolidation (%) . 10 20 30 40 50 kc = ks 60 70 80 90 100 0.0001 Ec/Es 5 10 50 100 0.001 0.01 2 Time factor Tv =cv t/H 0.1 1 Jiang et al. (2013) Stability Column Failure Modes under Embankment Loading Soft soil Columns Embankment Embankment Embankment Columns Soft soil Columns Soft soil Stiff layer Stiff layer Stiff layer Sliding direction (a) Sliding (b) Collapse (rotational) (c) bending o Embankment Embankment Berm Embankment S Columns Soft soil Columns Stiff layer (d) Circular shear Soft soil Stiff layer (e) Horizontal shear Columns Tensile failure Bending failure (f) Combined Modified from Kitazume (2008) and Broms (1999) Factor of Safety under Undrained Condition for Stone Columns b a Backfill water level Backfill water level Stone columns Equivalent area Clay Clay Sand Sand FS (individual) = 0.9 FS (equivalent) Abusharar and Han (2010) Numerical Modeling with DM Columns 6 Numerical Bishop Factor of Safety 5 Han et al. (2005; 2010) 4 3 Shear Bending Rotation 2 1 0 0 100 200 300 400 Cohesion of DM Walls (kPa) 500 600 Centrifuge Tests with Rigid Columns Single column Column group Zheng et al. (2011) Concluding Remarks A variety of column technologies have been developed and successfully adopted for different applications Composite columns or combined technologies with columns have been increasingly used to combine their advantages Stress concentration ratio depends on rigidity of loading, modulus ratio, lateral deformation, yielding of columns, stress level, and dynamic loading Columns can accelerate the rate of consolidation through drainage and/or stress transfer Columns under embankment loading can fail under shear, tension, bending, rotation, or a combination. Bending and rotation failure are dominant for semirigid and rigid columns Thank You!