Stone Columns Settlement Prediction with Hyperbolic Method.pdf
- 1. Stone Columns Settlement Prediction with Hyperbolic Method Dr. Ng Kok Shien MARA University of Technology, Malaysia 5th International Conference on Civil Engineering and Urban Planning (CEUP 2016) 23-26 Aug 2016, Xi’an, China
- 2. Introduction Shallow foundation - failed in bearing capacity or settlements. Ground improvement - small group of stone columns (SC).
- 4. Performance of SC Settlement Bearing capacity Stability Consolidation rate
- 5. Problem Statement Most design for SC – assume infinite columns grid (large group of columns) Ultimate bearing capacity and settlement of small column groups in drained condition: not well understood Wood (2000) – multiple failure modes i.e. wedge shearing, bulging, and punching Non-linear response of improved ground
- 6. Methodology Kondner (1963), Duncan & Chang (1970) & Jeon & Kulhawy (2001) used hyperbolic relationship to describe load-displacement curve for a foundation: (1) where q = applied bearing pressure Δ = footing displacement a & b = curve fitting parameters
- 7. For improved ground: Modified a1 & a2 : M = reduction factor
- 8. Reduction Factor, M d =the thickness of soft soil below footing Lopt = optimum length of stone column = area replacement ratio D= diameter of footing t = thickness of granular mat over stone columns
- 9. Methodology Obtain plain footing load- displacement curve Use hyperbolic method to obtain a & b Apply reduction factor, M to a & b to obtain SC load- displacement curve q e or s/B e/ q e a b 1 a1 = a/M a1 = b/M q e With SC
- 10. FEM Validation
- 11. Result – Plain footing Breath of column, B = 2.2 m Embedment depth = 0.8 m Econc = 30 Gpa, v = 0.15 , linear elastic material 0 20 40 60 80 100 120 140 160 0 50 100 150 200 Applied pressure (kPa) Settlement (mm) Jardine et al. (1995) PLAXIS 3D 0.1s s = 0.259 m 4.05 m = 1.84 B
- 12. FEM Validation
- 16. Results
- 18. Model test validation Wood et al (2000) L/ro = 3.4 L = 1.7D (Longer than optimum) = 24%, rc = 8:75 mm
- 19. Results – Model test 0 2 4 6 8 10 12 0.0 0.1 0.2 0.3 Footing load/initial undrained strength Settlement/diameter (mm) No improvement with SC Prediction y = 0.129596x + 0.010176 0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0 0.1 0.2 0.3 s/D/q s/D
- 20. Field load test validation Stuedlein (2008) Square footing (2.7 m x 2.7 m) 5 floating columns = 0.3 Diameter of SC = 0.76 m L = 3.05 < Lopt = 4.34 m Silty soils, over-consolidated Duration of test = 8.5 – 10 hrs
- 21. Results - Field load test 0 100 200 300 400 0 0.01 0.02 0.03 0.04 0.05 q s/B G3 G4 Predicted G3 = Plain footing G4 = Footing with SC y = 0.002186x + 0.00002556 0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0 0.01 0.02 0.03 0.04 0.05 s/B/q s/B qult = 602 kN/m2
- 22. Conclusion This paper presents a validation of a simplified design approach against a numerical model test , lab model test and a field load test where the results proved the method to be sufficiently accurate in estimating the drained settlement of stone columns supported foundation. The load-deformation response of the foundation can be characterized with hyperbolic function as shown in the above validation exercise. Albeit simple and easy in the approach, the method is indeed a rational method which has taken into account many intrinsic behaviors of stone columns such as the load sharing mechanism, nonlinearity and length ratio.