SQUEEZING GROUND | ROCK TUNNELS


steady-state conditions, where all visco-plastic deformations have developed in the case of creep and all excavation-induced excess pore pressures have dissipated in the case of consolidation. Time-development of displacements is qualitatively


similar in both cases. It can be directly inferred that the curves nearly overlap with appropriate scaling between the viscosity and the permeability, and selection of the material parameters. This demonstrates the difficulty of distinguishing creep from consolidation, as well as of back-analysing ground parameters from—or even simply interpreting—the observed time-dependent deformations. All strength and stiffness parameters being equal,


the instantaneous and steady-state displacements are consistently higher in the case of consolidation, even more so in the case of the higher pore pressure due to the more extensive ground plastification. The seepage forces do not depend on ground


characteristics and can be arbitrarily high depending on the in-situ pore pressure. It may then happen that equilibrium near the tunnel boundary is impossible and thus excessive cavity contraction or even complete cavity closure occurs. In this sense, seepage forces constitute a potential destabilising agent for the tunnel cross section and face. This is a distinguishing feature between creep and consolidation.


4 SHIELD LOADING DURING TBM ADVANCE AND STANDSTILLS The average rock pressure acting upon the shield depends, in general, on all independent problem parameters, i.e., in-situ vertical stress, tunnel radius, material constants, TBM parameters (Ks


, Kl , L, ΔR), the


advance rate, and the standstill time. In the case of creep it also depends on viscosity; in the case of consolidation on the permeability, in-situ pore pressure, unit weight of pore water, and size of the seepage flow domain.


4.1 Shield Loading During TBM Advance For the advancing tunnel face, plastic strain contours, displacements, convergences, and radial stresses could be graphed and evaluated for the cases of creep and consolidation, respectively, and for two values of in-situ pore pressures (1MPa and 4MPa), for two limit cases of rapid and slow excavation. The limit cases are considered via normalised


advance rates—how fast one tunnel radius is excavated in relation to the development rate of time-dependent deformations. During rapid excavation the ground response is purely elastic in the case of creep, and undrained elastoplastic in the case of consolidation; during slow excavation there is sufficient time for steady-state conditions to develop. Plastic strains and displacements are higher in the


case of consolidation, both during rapid and slow excavation. However, this is not necessarily reflected as higher


rock pressure on the shield as the mechanisms of the rock–shield interaction are complex.


December 2025 | 13


The pressure that ultimately develops on the shield


comes from the interplay of two counteracting effects of rock plastification: more plastic yielding leads to increased ground convergences, increased contact area and tentatively increased shield loading; and, more stress relief ahead of the face though tentatively lessens load transfer to the shield. Based on this interaction, the differences in rock pressure between creep and consolidation for the cases of rapid and slow excavation can be interpreted.


4.2 Counter-Intuitive Effect of Advance Rate The interplay of the two competing effects also produces a counter-intuitive result on the effect of advance rate. Figure 3 shows the average shield pressure as a function of the normalised advance rate for creep (black lines) and consolidation (red). This seemingly paradoxical behaviour is observed for both creep and consolidation, with the two distributions being qualitatively identical. Leone et al. (2023) showed that it is attributed


to delayed plastic deformation development with increasing viscosity, leading to smaller contact area between shield and ground, but limits stress relief ahead of the face thereby having a stiffening effect. Ramoni and Anagnostou (2011a) originally reported


and examined the same paradox for the case of consolidation, explaining it on the basis of the slower development of plastic deformations in the case of lower permeability. However, there is the additional effect of the negative pore pressures have a stabilising effect by increasing effective stresses and so reducing deformations and plastification.


4.3 Shield Loading During a Standstill Within the range of normalised advance rate, the conditions are also more unfavourable during a TBM standstill (see dashed lines in Figure 3). The rock pressure variation is qualitatively similar for both creep and consolidation, but its increase is in general more pronounced for the latter, since seepage flow progressively starts during standstill and seepage forces induce more extensive ground plastification.


Below, figure 4:


Qualitative interpretation of the destabilising effect of seepage forces at the unsupported tunnel boundary: development of tensile tangential stress at the tunnel face


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