Behaviour of soft clay subjected to capsule expansion–electro-osmosis

Long-term settlement of shield tunnels in soft soil is difficult to control by grouting reinforcement owing to uncontrolled grout diffusion and significant soil disturbance. This study proposes a capsule expansion–electro-osmosis method (CE-EM) for ground improvement and reports a field test in soft clay. In this method, pre-installed capsules wrapped in carbon fibre cloth serve as the anodes, and metal tubes as the cathodes. The soil between the electrodes is reinforced through capsule-expansion-induced compaction and electro-osmotic drainage. In the test, piezometers and layered settlement instruments were used to monitor changes in excess pore water pressure (EPWP) and vertical soil displacement, and in situ piezocone penetration tests were used to assess changes in undrained shear strength before and after reinforcement. Results indicate that capsule expansion induces heave of overlying ground and raises EPWP, whereas electro-osmosis accelerates EPWP dissipation. Combining the two methods reduces peak heave and EPWP, diminishes post-grouting re-settlement, and mitigates soil disturbance caused by capsule expansion. After reinforcement, the undrained shear strength increased from 20·9 to 40·6 kPa. This study shows that CE-EM effectively controls post-grouting ground settlement in soft soil and improves the reinforcement effect, providing an efficient, novel approach for soft ground reinforcement and overlying tunnel settlement control.