Lessons Learned from a Stone Column Test Program in Glacial Deposits

A stone column test program was conducted for ground improvement below an earth embankment project in western Washington. Stone columns were initially designed to mitigate soil liquefaction, improve shear strength, and reduce subgrade settlements to provide a stable foundation for the embankment. The test program includes four test areas of stone columns installed with varying control parameters such as diameter, spacing, and area replacement ratio. Each test area consists of 32 to 41 vibro-replacement stone columns. Stone columns were installed through the post-glacial, loose to medium dense, silty sand and soft to medium stiff silt and embedded into glacially overridden, dense, silty sand and stiff to hard, sandy silt. Standard Penetration Tests (SPT) and Cone Penetration Tests (CPT) were used as verification tests in each of the four test areas before and after the installation of stone columns. Results indicated densification of the matrix in soil zones containing less than approximately 15 percent fines and little to no improvement in soil zones containing over approximately 15 percent fines. In some deeper zones where native soils were over-consolidated by glacial actions, disturbance in the stiff to hard silt actually caused a decrease in penetration resistance.