STABILITY OF RECTANGULAR TUNNELS IN CEMENT-TREATED SOFT SOILS

Abstract

Tunnel construction in soft soils is a one of the challenging tasks. In this study the stability of rectangular tunnels in cement treated soft soil has been investigated. Firstly, finite element model is developed in finite element software Plaxis 2D and validated with available centrifuge test results, thereafter further analysis is carried out by conducting the parametric study on rectangular tunnel of given width, height and cover ratio using plain strain condition. Three parameters considered for the study are cover ratio, thickness of improves soil surround, and strength of improved soil surround. The results of the study shows the failure behaviour of tunnel in improves soil surround at different parametric conditions. For improved soil surround of low strength and thickness, shearing is dominant failure mode and for high strength and thickness tension becomes tension the dominant failure mode while for the intermediate cases rupture is observed. Also, stability is found to increase with strength and thickness of improved soil surround. The analysis of effect of cover ratio showed the similar trend of increasing stability with cover ratio at low cover ratios. But at high cover ratio values, stability is decreases due to overburden pressure effect. The failure pattern is found to be dependent on the strength ratio (qui/qus) value at different cover ratio values. Stability chart and a failure map is also presented in the study which can serve as design chart for such rectangular tunnels in improved soil surround.