REINFORCING OF SOFT COHESIVE SOILS WITH STONE COLUMNS

The history of their use and the empirical design is described of stone columns which consist of granular material compacted in long cylindrical holes which are utilized for improving the strength and consolidation characteristics of soft clays. A series of model experiments were conducted using radiographic techniques to determine the actual bahavior of a single column in a uniform normally consolidated clay. Simple results of plasticity theory were used to predict the limiting load. The good agreement between experiment and theory enabled the effectiveness of stone columns, used both singly and in groups. Stone columns significantly reinforce soft ground. Unlike pile foundations they make very efficient use of the soil near the surface. They are ideal for light loads. They are, however, less effective at supporting heavy loads because they cannot transfer the applied stresses to the deeper layers of soil. Details are given of the modern technique of forming stone columns which consists of first forming the hole with an instrument known as a vibroflot. This is essentially a long (10m) thin (0.5m dia) steel tube which vibrates at a high frequency of 50 or 60 Hertz. The French method is detailed of construction where the columns were placed in a large symmetric grid under the loaded area. They were constructed on a progressively finer grid until the wooden piles used to form the holes would not penetrate more than 0.005 m under their standard hammer blow. Details are given of the series of experiments, and some important conclusions are presented about the effectiveness of groups of stone columns under widespread loads.