Installation effects of vibro replacement stone columns in soft clay

Abstract

The installation of vibro replacement stone columns in any soil causes significant changes to the stress regime around each probe location. In soft clays, the principal effects are caused by a cavity expan- sion mechanism as the probe is introduced into the soil and the stone column constructed. This paper presents observations of changes in soil volume and pore water pressure during stone column installation. The implica- tions for the most suitable column construction techniques and the soil strength limiting column construction arising from the observations are discussed. The data presented are also of interest to those validating numerical models of stone column installation, and issues relevant to the practical application of numerical modeling are considered. This paper considers the observed effects of both sin- gle and multiple stone column installation in soft clay and demonstrates that successful ground improvement can be achieved with this technique. A detailed expla- nation of the various Vibro Replacement and Compac- tion techniques available is provided by Sonderman & Wehr (2004) and is not repeated here. This paper con- centrates on stone columns installed using the dry bottom feed method, which is the most widely used technique in the UK for routine projects on soft clay sites, often preferred nowadays to the wet method. A significant advantage of the dry bottom feed method over the wet method is that a supply of water flush is not required with the attendant requirement of handling and disposal of the wet spoil. Numerous case studies illustrating the use of the dry bottom feed method for a wide range of projects have been published, for exam- ple, Cooper & Rose (1999), Wehr (2006), McNeill (2007), Castro (2007) and Egan (2008). Three aspects of the installation process are con- sidered in this paper: (a) the ground response during the installation of a dry bottom feed stone column, (b) changes to the soil stress field and pore water pressure during the equalization period following stone column construction and (c) the interaction effects resulting from the construction of both small groups and large grids of columns.