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The construction of large diameter tunnels can be challenging, especially in soft soil conditions. Stability intervention by means of ground improvement has been identified as a possible method to allow for the safe construction of such tunnels. A 1-g physical model study of the stability of a tunnel with a cement-treated soil layer surrounding it, along with related failure loads and mechanisms, is reported here. The failure mechanism of such tunnels is found to be very different from conventional tunnels with no ground improvement as cement-treated soil is very brittle. Failure occurs locally, with cracks developing in the tensile region in the cement treated layer. These cracks consistently occur at the crown, springline and invert, over a range of improved soil layer thickness-to-tunnel diameter (t/D) ratio. These cracks are also distinctly flexural tensile in nature. Prior to the onset of cracking, the deformation of the tunnel is minimal. This in turn results in a negligible settlement trough. This implies that caution is needed on the use of ground movement as a measure of the strength mobilization, owing to the lack of warning signs. The stability of such tunnels is found to be related to the t/D ratio and strength of the improved soil layer, while the significance of the unimproved surrounding soil is found to be minimal. This leads to the formulation of a new stability equation.

1-g model tests of tunnels with a surrounding cement-treated soil ring