ABSTRACT: The stone column technique of ground improvement is extensively used to improve the strength of weak soil layers. Stone columns essentially increase the bearing capacity of cohesive soils. In cohesive soils, the drainage path provided by stone columns accelerates the rate of residual settlement due to consolidation. The paper gives the design and construction methods of stone columns and highlights the errors that can affect stone columns. Two case studies of stone column failure are presented in detail. These studies reveal the errors that occurred in the design and construction methods. The case studies also highlight the probable field conditions that could create excessive settlements. Based on these observations, conclusions are derived indicating the specific care that should be taken in the design and execution of stone columns.
INTRODUCTION Stone column technique for ground improvement is being extensively used to undertake constructions in weak soils. The stone columns essentially increase the bearing capacity of loose cohesionless soils. In cohesive soils, along with the increase of bearing capacity, the consolidation settlement of the ground under loading is also considerably reduced. In addition, in cohesive soils stone columns act as drainage paths to accelerate the rate of consolidation of the residual settlement. Even though stone columns are very useful for these purposes, designs made without proper concept of the behaviour of the stone column and execution of work not understanding the stone column and behaviour pattern of the non-treated ground leads to complications. This paper attempts to highlight these factors. A case study where the foundation failure of a structure constructed on a soil improved by stone columns highlights the various aspects discussed.
CONCEPT OF DESIGN Stone columns are essentially designed to take load when the columns bulge under loading and the surrounding soil offers passive resistance.