Investigation into the Stabilizing Effect of Pipe Umbrellas
One of the main hazard scenarios in tunnelling is ground instability in the excavation area, i.e. a collapse of the tunnel face and/or of the tunnel crown and walls over the unsupported span immediately after or during the excavation round. Pipe umbrellas are often applied to mitigate these risks of instability and thus to increase safety during construction. However, the increase in face stability by forepoling is controversial and has so far primarily been indicated by experimental studies. This paper investigates the above-mentioned stabilisation effects by means of limit analyses and three-dimensional numerical calculations considering the interaction among pipes, surrounding ground, temporary support and face reinforcement. The results demonstrate that the contribution of pipe umbrellas to face stability and thus to mitigation of risk of damage on the existing infrastructure is rather negligible, at least for typical pipe umbrella layouts. The effectiveness of pipe umbrellas presupposes face stability and not vice versa. However, the risk of instability at the unsupported span can be greatly reduced and is found to be primarily associated with insufficient quantity of the pipes or oversized pipe spacings, leading to lateral instability at the tunnel bench or local instability between adjacent pipes respectively. The numerically determined degree of structural utilization of commonly applied pipe umbrella layouts is rather low, indicating that pipe failure is very unlikely.