Treatment of Soft Clays by Combined Encased Stone Columns and Vacuum Consolidation-Paper

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The study experimentally evaluated combining geosynthetic-encased stone columns with vacuum consolidation to improve extremely soft clay. Stone columns provide reinforcement and drainage, while geosynthetic encasement confines the aggregate and limits intrusion of clay. Vacuum applied through the columns creates negative pore pressure, increasing soil effective stress and accelerating consolidation without increasing total stress.<br /><br />Tests used a large tank containing reconstituted silty clay initially consolidated under a 6 kPa surcharge. Fifty-millimetre-diameter encased columns were installed in a group, connected to a vacuum system, and treated at approximately 65 kPa. Vacuum readings confirmed effective pressure transfer into the soil.<br /><br />Primary consolidation produced about 100 mm of settlement in roughly 10 days. After treatment, the clay’s measured shear strength increased from 2.5 kPa to 18 kPa. Column load tests also showed substantial improvement: at 10 mm settlement, load capacity was about 2.7 times higher, and ultimate capacity increased about 2.1 times compared with pretreatment. Column stiffness improved as well.<br /><br />The results indicate that vacuum applied through encased stone columns can strengthen both the surrounding clay and the columns while shortening consolidation time. The method may therefore speed construction and improve project economics. Increased soil strength and column capacity may also allow wider column spacing. These conclusions are based on laboratory-scale testing, and further work would be needed to establish performance under field conditions.

Keywords

geosynthetic-encased stone columns

vacuum consolidation

soft clay improvement

negative pore pressure

soil consolidation

shear strength increase

column load capacity

drainage and reinforcement

laboratory testing

ground improvement