Vacuum preloading is a ground improvement technique in which effective stress in soil is increased by reducing pore-water pressure. To understand the pore-water pressure, effective stress changes and deformation mechanism of soft soil subjected to underwater vacuum preloading, theoretical analysis, centrifuge modelling and numerical modelling were carried out....[ Read more ]

Vacuum preloading is a ground improvement technique in which effective stress in soil is increased by reducing pore-water pressure. To understand the pore-water pressure, effective stress changes and deformation mechanism of soft soil subjected to underwater vacuum preloading, theoretical analysis, centrifuge modelling and numerical modelling were carried out.

Analytical solutions of average degree of radial consolidation of soft clay subjected to vacuum preloading were developed. Well resistance, smear effect, vacuum loss, large lateral deformation and variation of soil stiffness and permeability are considered. Centrifuge tests of clay with a single vertical sand drain and a vertical sand drain group at a spacing of 8D (D is drain diameter) were carried out. Subsurface displacement was measured by particle image velocimetry (PIV) and photogrammetry. Coupled axis-symmetric and coupled three-dimensional finite-element analyses were carried out to model consolidation behaviour of soft clay subjected to underwater vacuum preloading with a vertical drain group. Numerical experiments were carried out to investigate the behaviour of soft clay subjected to underwater vacuum preloading, on-land vacuum preloading and underwater surcharge preloading under idealized boundary conditions.

Both measured and computed rates of consolidation with a vertical drain group were greater than those with a single vertical drain. Measured surface settlement near a vertical drain was 80% of that away from the vertical drain. Measured maximum inward lateral displacement between vertical drains in centrifuge tests was about 5% surface settlement. At the centre of a vertical drain group, final lateral earth pressure coefficient at the end of underwater and on-land vacuum preloading is greater than lateral earth pressure coefficient at rest, while that in underwater surcharge preloading is less than lateral earth pressure coefficient at rest. Computed inward lateral displacement at the edge of a treated zone is about 27% of surface settlement by underwater and on-land vacuum preloading. The magnitude of computed lateral displacement by underwater or on-land vacuum preloading is 16% of that by underwater surcharge preloading. At a distance of 18.5 m from the treated zone, the magnitude of computed lateral displacement by underwater and on-land vacuum preloading is 22% of maximum surface settlement, while that by underwater surcharge preloading is 1% of maximum surface settlement.