Stability and Collapse Mechanisms of Unreinforced and Forepole-Reinforced Tunnel Headings

Abstract

The stabilities of shallow tunnel headings with and without forepole reinforcements were studied by the use of small scale centrifugal physical models and upper bound plasticity models. The physical models were tested at 100 g in the NUS geotechnical centrifuge which modelled a prototype tunnel that has 6.5 m of diameter and under the same depth of ground cover. Reconstituted kaolin clay, which was made stiff by overconsolidation, was used for modelling the ground. Zinc chloride solution, which was produced to have the same bulk density as the stiff clay, was used for modelling the tunnel support pressure. Several modes of collapses were identified from the model tests which were dependant on the length of tunnel heading protrusion and length of forepoles. A collapsing tunnel heading with long unlined length was shown to display large roof movements and relatively lesser movements at the face. The presence of forepoles was found to modify the collapse mechanisms. Moderate forepole lengths promote the formation of the face wedge. Whereas a tunnel heading reinforced with very long forepoles was found to be susceptible to void formation and migration. The mechanistic studies guided the formulation of upper bound plasticity models. The stabilising effects of forepoles were also included, which were modelled as beams that undergo plastic collapses and their interaction with the ground was through complete lateral shearing of beam sections. Predicted stability conditions and collapse mechanisms of tunnel headings according to these theoretical models were in reasonable agreements with observations from the physical models.