Sudip Basack

Numerical Solution of Stone Column–Improved Soft Soil Considering Arching, Clogging, and Smear Effects

AbstractImprovement of soft clay deposits by the installation of stone columns is one of the most popular techniques followed worldwide. The stone columns not only act as reinforcing material increasing the overall strength and stiffness of the compressible soft soil, but they also promote consolidation through effective drainage. The analytical and numerical solutions available for ascertaining the response of column-reinforced soil have been developed on the basis of the equal strain hypothesis. For typical surcharge (embankment) loading, the free strain analysis appears to give more realistic results comparable to field data. The paper presents a novel numerical model (finite-difference method) to analyze the response of stone column–reinforced soft soil under embankment loading, adopting the free strain approach and considering both arching and clogging effects. Apart from predicting the dissipation of excess pore water pressure and the resulting consolidation settlement with time, the load transfer m...

Settlement prediction and back analysis of Young's modulus and dilation angle of stone columns

Ground improvement using stone columns is one of the most suitable methods for deformation control of soft soils. The use of stone columns can improve the bearing capacity, reduce settlement, accelerate consolidation process and enhance slope stability as well as resistance to liquefaction. Settlement prediction of the ground influenced by the large stiffness difference between the columns and the surrounding soil and the induced arching is discussed in this paper. Plate load test is one of the common methods to be used for quality control of stone column reinforced ground. The test results can be used to back calculate some of the design parameters of the stone column. A finite element based procedure to estimate the Youngs modulus and dilation angle of stone column material using plate load test results is proposed. The employed finite element model is axisymmetric and suitable for a cylindrical stone column. The field measurements after calibration have been in good agreement with numerical results for predicting deformations. The procedure developed in this study can be adopted by practicing engineers to estimate the Youngs modulus and dilation angle of stone columns by conducting a plate load test.

Influence of relative pile-soil stiffness and load eccentricity on single pile response in sand under lateral cyclic loading

The environment prevalent in ocean necessitates the piles supporting offshore structures to be designed against lateral cyclic loading initiated by wave action, which induces deterioration in the strength and stiffness of the pile-soil system introducing progressive reduction in the bearing capacity associated with increased settlement of the pile foundation. A thorough and detailed review of literature indicates that significant works have already been carried out in the relevant field of investigation. It is a well established phenomenon that the variation of relative pile-soil stiffness (Krs) and load eccentricity (e/D) significantly affect the response of piles subjected to lateral static load. However, the influence of lateral cyclic load on axial response of single pile in sand, more specifically the effect of Krs and e/D on the cyclic behavior, is yet to be investigated. The present work has aimed to bridge up this gap. To carry out numerical analysis (boundary element method), the conventional elastic approach has been used as a guideline with relevant modifications. The model developed has been validated by comparing with available experimental (laboratory model and field tests) results, which indicate the accuracy of the solutions formulated. Thereafter, the methodology is applied successfully to selected parametric studies for understanding the magnitude and pattern of degradation of axial pile capacity induced due to lateral cyclic loading, as well as the influence of Krs and e/D on such degradation.

Modeling the Performance of Stone Column–Reinforced Soft Ground under Static and Cyclic Loads

AbstractInstalling stone columns is a convenient method of soft ground improvement. Although several analytical and numerical solutions are available to predict the performance of stone column–improved soft ground, these models are incapable of capturing the influence of cyclic loading on transport corridors, such as highways and railways. The authors developed a novel finite-difference model adopting modified Cam clay theory to analyze the response of stone column–reinforced soft soil under static and cyclic loadings. Apart from predicting excess pore water pressure dissipation and the resulting settlement, the model also captures the effect of cyclic loading by considering yield surface contraction. The solutions developed were validated using available field observations and laboratory test results. The model was successfully applied to a selected case study at the Australian National Field Testing Facility at Ballina, New South Wales, Australia. The limitations of the proposed model are highlighted as...

Numerical Solution of Single Piles Subjected to Pure Torsion

AbstractTorsional load on pile foundations is induced by the action of eccentric horizontal forces on the supporting structures. Such loading not only initiates twist at the pile head, but also reduces its axial capacity significantly associated with increased settlement of the foundation. Inadequate design of piles under torsion may lead to disastrous consequences. Although several theoretical and experimental investigations have already been carried out on piles under torsional load as evidenced from the available literature, there is ample scope for further contributions on the influence of torsional load on pile foundation. This paper presents a novel numerical model (boundary element method) to analyze the response of a single, vertical, floating pile subjected to pure torsional load. The nonlinear stress-strain response of soil has been incorporated in the model by hyperbolic correlation, whereas the pile material has been idealized as elastic–perfectly plastic. The effect of progressive pile-soil s...

Numerical Solution of Single Pile Subjected to Simultaneous Torsional and Axial Loads

AbstractTorsional loads on pile foundations are induced by the action of eccentric horizontal forces on the supporting structures. Such loading not only initiates twisting at the pile head but also reduces its axial capacity significantly with increased settlement of the foundation. Several theoretical and experimental investigations have already been carried out on piles under torsional load, as evidenced by the available literature on the subject, although contributions on the influence of combined torsional and axial loads on pile foundations are rather limited. This paper presents a novel numerical model (boundary-element method) to analyze the response of a single vertical floating pile subjected to simultaneous torsional and axial loads. The nonlinear stress-strain response of soil has been incorporated in the model by means of the hyperbolic model, whereas the pile material has been idealized as elastic–perfectly plastic. The effect of progressive pile-soil slippage at the interface is considered. ...

Active earth pressure on translating rigid retaining structures considering soil arching effect

Accurate determination of active earth pressure distribution on rigid retaining wall including the magnitude and height of application of its resultant is of immense importance for designing the earth retaining structures. In this paper, an analytical solution based on the soil arching effect is presented. According to Mohr’s stress circle, a new relationship between active earth pressure and normal stress on failure surface at any depth from backfill surface is obtained. By analysing parabolic arch of minor principal stresses, a new coefficient of active lateral earth pressure is proposed. The problems that the vertical static equilibrium equation of differential flat element considering soil arching effect is not reasonable are analysed. Then, based on the limit equilibrium of differential flat element, new formulas relevant to the distribution of active earth pressure, the magnitude and height of application of its resultant are derived. The effects of backfill internal friction angle, wall–soil friction angle, wall height, surcharge load and backfill unit weight on those formulas are investigated in detail. In addition, the comparisons of the predictions by proposed equations with available test data as well as existing solutions are carried out, which implies adequate accuracy of the proposed solutions for all ratios of wall–soil friction angle to internal friction angle.

Modeling the Stone Column Behavior in Soft Ground with Special Emphasis on Lateral Deformation

AbstractAmong various ground-improvement techniques, soft-soil reinforcement by stone columns is one of the most common and convenient methods with numerous benefits including increased bearing cap...

Theoretical and numerical perspectives on performance of stone-column-improved soft ground with reference to transport infrastructure

Abstract In this chapter, an axisymmetric solution is presented to study the performance of soft ground reinforced with stone columns with special emphasis on transport infrastructure. For typical surcharge (embankment) loading, significant column-to-soil stiffness ratio and the flexible nature of the embankment initiate the fill weight to “arch over” the stone columns, imposing uneven load distribution on the surface. In addition to the arching effect, the numerical model captured the influence of smear and clogging on the consolidation characteristics of soft clay. To simulate the field condition relevant to transport corridors, a free strain hypothesis was adopted. The entire numerical analysis was performed by means of finite difference technique with composite central and forward difference methods applied on axisymmetric soft soil elements under K0-condition. The model was validated by comparing with existing solutions and available field test results. Using the solutions developed, a design recommendation with relevant curves was proposed. The model and the design curves are successfully applied to selected case studies with several important conclusions.

Significance of Hydro-geological and Hydro-chemical Analysis in the Evaluation of Groundwater Resources: A Case Study from the East Coast of India

The work described in this paper is aimed towards conducting a thorough and in-depth field based study on the groundwater development and water quality variation in the district of Purba Medinipur, West Bengal, India. 20 nos. locations were selected along the coastal belt of the study area to investigate the groundwater quality. Based on the field test results,contour line were plotted for Chloride concentration. The 3- Dimensional views of piezometric surface pre monsoon ( 2006 ), pre-monsoon and post-monsoon average piezometric surfaces (2004) had also been developed. The subsurface characterization and the groundwater quality analysis were thereafter utilized to obtain the probable path of saline water ingress into the aquifers of the study area. Owing to heterogeneity of the aquifer in the selected locations, the spatial irregularity in salinity was observed to be significant. The present study was carried out to understand the quality of ground water and delineate the subsurface formations in an area of Purba Midnapur district. The physico chemical studies involve pH value, hydraulic conductivity, total hardness, anions (HCO, Cl, CO) and Fe.