Behaviour of Geogrid-Encased Group of Stone Columns Under Monotonic and Cyclic Loading

Although the efficacy of stone columns as a ground improvement technique for soft soils is well-established, their effectiveness diminishes in very soft soils (q_u < 25 kPa) due to insufficient lateral support. In such situations, encasement with geosynthetics may be beneficial. This paper presents the results of model tests on various types of stone columns (floating and end-bearing) with different diameters (40 mm and 60 mm), both ordinary and geogrid-encased, in very soft clay with varying undrained shear strengths. The tests were conducted under monotonic and cyclic loading conditions in a plane-strain configuration. The study evaluates the impact of key parameters, including column length and diameter, base support conditions, undrained shear strength of clay, and geogrid encasement length, on the performance of improved ground through a total of 28 model tests. The results show that regardless of the soil's undrained shear strength, the encasement of stone columns with geogrids significantly enhances ground performance. Under monotonic loading, this improvement ranges from 22 to 140% depending on the length of geosynthetics encasement and base support conditions. Under incremental cyclic loads, the improvement varies from 25 to 50%. It is also observed that the geogrid encasement's effectiveness significantly increases when it encompasses the entire length of the stone columns, as it extends the lateral bulging zone below the encasement length.