Effect of jet grouting and dewatering on an adjacent tunnel in a Shanghai deep excavation

In general, jet grouting is usually adopted in deep excavation projects to improve the soils inside and outside the pit to protect surrounding tunnels or building foundations. A cost-effective way is to keep the deformations caused by pit excavation within limitations with the minimum jet-grouting. In this study, different types of jet-grouting soils around tunnels or inside pits were compared and analysed in detail using three-dimensional finite-element modelling. Tunnels on one side of the pit or underneath the formation level were considered. A dewatering method was also introduced in some finite-element models to explore the effect of pore-water pressure (PWP) relief wells in soft clay with very low permeability. The finite-element modelling results indicated that enhancing the soils around tunnels is more profitable than only improving the soils between tunnels and retaining walls. The positive effect of jet-grouting soils above the tunnel may exert only when the excess PWP dissipates. This study may assist engineers who are seeking more cost-effective ways to protect surrounding tunnels or building foundations during deep excavation in soft clay.