ROCK TUNNELS | SQUEEZING GROUND
SQUEEZING GROUND
CREEP VERSUS CONSOLIDATION IN TUNNELLING THROUGH
The effects of creep and consolidation on shield tunnelling in rock are discussed in two summarised, complementary papers (Parts A and B) which consider Basic Time Effects and Transferability of Experience, respectively
Thomas Leone1, Alexandros N. Nordas1 1
Although squeezing ground may undergo rapid convergences following tunnel excavation, its behaviour is often markedly time dependent due to creep or consolidation. The effects of creep (a purely mechanical rheological process) and consolidation (a coupled hydromechanical process) on shield tunnelling are discussed, with the aim of demonstrating their qualitative similarities and distinctive features.
Part A: Basic Time Effects The first paper investigates the basic time effects, looking at the time development of ground deformations and the complex interaction between ground, the TBM and tunnel support during both excavation and construction standstills. Numerical simulations indicate several qualitative
similarities between the two mechanisms of time dependency, such as: time development of rock deformation and shield loading during advance and increased shield loading with increasing advance rate under certain conditions in creep and consolidation. However, the investigation herein also underscores
two prominent differences and these result from the fundamentally different nature of creep and consolidation: ● First, consistently more extensive plastic yielding in consolidating ground, which is partially associated with the seepage forces exerted by the pore water on the solid rock constituents;
● Second, the role of seepage forces as a potential destabilising agent, particularly for the tunnel face, which does not happen in the case of creep and may be critical for shield and cutterhead jamming.
10 | December 2025 , Georgios Anagnostou1 ETH ZURICH, SWITZERLAND
Part B: Transferability of Experience The second paper builds upon the comparison of creep and consolidation on shield tunnelling, considering the practical transferability of experiences from existing tunnels as a reference for the required thrust force at tunnels of different diameters or to adjacent tunnels. First, the effect of the tunnel diameter on the risk of
shield jamming is examined. The paper demonstrates that larger-diameter tunnels are more favourable in poor-quality ground, while the opposite holds in better- quality ground, as well as where there is pronounced time-dependent ground behaviour. Second, the effect of a tunnel on the required
thrust force in a neighbouring tunnel built later is examined. The paper shows that this interaction effect is particularly important in water-bearing ground of low permeability: the drainage action of the first tunnel induces pore pressure relief and ground consolidation in an extensive area, leading to a substantial reduction of the thrust force in the second tunnel. Conversely, in the case of creep the interaction is
negligible even under extremely squeezing conditions, even for extreme conditions and large overcuts, due to the fundamentally different nature of the purely mechanical rheological processes. The investigations into transferability are valuable for
tunnelling practice, in cases of twin tunnels as well as where a smaller-diameter tunnel is built first (e.g., a pilot tunnel, advance drainage or ground improvement), or also the opposite (e.g., upgrade of a road tunnel by adding a safety tunnel with a smaller diameter). The summarised papers, Parts A and B, are presented
below.
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