ROCK TUNNELS | SQUEEZING GROUND
The above becomes clearer for creep and
consolidation, respectively. Standstills are unfavourable in both cases. Therefore, models that disregard time dependency and assume that plastic deformations develop instantaneously are not conservative: they may overestimate shield loading during excavation but underestimate it during even short standstills.
5 ON THE DESTABILISING EFFECT OF THE SEEPAGE FORCES Permeability governs the rate of squeezing for consolidation, but an additional parameter to consider is in-situ pore pressure or its gradient, the seepage force. In homogeneous ground the steady-state pore
pressure field, and thus the magnitude of the seepage forces, depend solely on the hydraulic boundary conditions. To fulfil equilibrium, the seepage forces must
be resisted by stresses in the ground; however, the maximum resistance that the ground can provide depends on its mechanical characteristics. Under certain conditions the seepage forces may be sufficiently high to prevent equilibrium near the tunnel, resulting in excessive convergences and instability. As these effects, are induced by an external
agent they must be distinguished from familiar instability phenomena, e.g., those in rocks exhibiting strain softening that are relevant both in creep and consolidation; therefore, they may also occur in the absence of softening, in the perfectly plastic rocks examined in the present work.
5.1 Tunnel Cross Section Far Behind the Face The rotationally symmetric, plain strain problem of an unsupported tunnel cross section was considered. Where effective radial stress cannot increase with the
radius, and therefore cannot reach a state of equilibrium with the far-field stress, this according to Egger et al. (1982) means that an opening would be unstable. This points to a qualitative difference for ground behaviour in the case of creep, where a stable stress field always exists in the absence of softening. The difference is, however, only apparent; the instability would manifest itself by increasing convergences, which would result in an increasing curvature of the tunnel boundary, and equilibrium can always be reached if the curvature becomes sufficiently big. The instability as postulated cannot actually occur;
there is no difference between creep and consolidation in respect of equilibrium or lack thereof in the case of a circular opening. The reason is of a geometric nature: at the onset of instability the curvature increases and stabilises the system, even if this may happen at big convergences which, in practical terms, may fail to satisfy serviceability criteria. However, this finding points to a fundamental
difference between the behaviours of a tunnel’s cross section and face: the geometry of the former becomes increasingly favourable during cavity contraction,
14 | December 2025
whereas the geometry of the latter becomes more unfavourable during extrusion, as it becomes convex (Figure 4).
5.2 Tunnel Face The extrusion of the face was numerically investigated over the course of a sufficiently long TBM stand-still to reach steady state conditions. The model assumed no face support (open shield TBM), but in practice the ground establishes contact with the cutterhead after a certain amount of deformation. The face–cutterhead interaction was considered by introducing nonlinear springs in the model. From a practical viewpoint, the combined effect of
the extrusion and instability of the tunnel face, and the contraction of the tunnel cross section under seepage forces, may be particularly critical in respect of the thrust force and the torque of the cutterhead required for the TBM to restart its advance after a standstill. The studies also showed that, at restart, the skin
friction to be overcome increases monotonically with standstill duration. Conclusively, these results underscore the practical
significance of the seepage forces, which may result in significant loading of the cutterhead and have severe implications in respect of the design of the TBM, or even assessment of a TBM drive in consolidating ground— which does not exist for creep.
6 CONCLUSIONS The investigations highlighted several qualitative similarities between the mechanisms of time dependency of the ground behaviour, and underscored two prominent differences—resulting from the fundamentally different nature of the purely mechanical rheological creep processes and the coupled hydromechanical consolidation processes. The first difference is the consistently more
pronounced plastification in consolidating compared to creeping ground. The second difference can be traced to the seepage
forces, their magnitude unrelated to mechanical ground characteristics and depending only on hydraulic boundary conditions. The paper demonstrated that these phenomena are
particularly critical for shield jamming and cutterhead blocking in consolidating ground. Despite the above fundamental differences, these do
not allow for a distinction of creep from consolidation based upon the observed or observable ground behaviour in practice, particularly with uncertainty on strength and stiffness parameters. Concerning instability, there is no way of
distinguishing this based on the observed behaviour. In this sense, the only viable way of distinction is
advance exploration of the hydrogeological conditions of a project site and also appropriate laboratory tests to determine the rheological properties as well as the degree of saturation and permeability of the ground.
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