SQUEEZING GROUND | ROCK TUNNELS
BASIC TIME EFFECTS PART A:
1 INTRODUCTION Squeezing, the phenomenon of rock deformations or pressures in tunnelling, may occur rapidly or develop slowly, depending on the rheological properties of the ground (creep), by stress redistributions associated with transient seepage flow in the case of low-permeability, saturated ground (consolidation), or both. Rheological processes are of a purely mechanical
nature and in general independent of the presence of pore water. It has been shown to be more pronounced when the ground is overstressed, particularly approaching failure state; hence, it is especially evident under squeezing conditions. Consolidation is relevant in water-bearing ground
of low permeability and is associated with the transient seepage flow triggered by the tunnel excavation. The flow enables progressive dissipation of the excess pore pressures, inducing variations in the effective stresses and ground deformations. Pore pressure has been known to intensify squeezing phenomena. In tunnelling practice, it is not always directly
distinguishable whether the source of time dependency of the ground behaviour is creep, consolidation, or the superposition of both. This is partially due to the different perception concerning the presence and influence of the pore water depending on the rock. The questions arise: what are the fundamental
differences in the phenomenological ground behaviour? Can these help to distinguish creep from consolidation in practical situations? Although several existing works have separately examined the effects of creep, this question has not been addressed in the literature. This paper evaluates comparatively some fundamental aspects of creep and consolidation in tunnelling. Emphasis is placed on the time development of ground deformations and the problems of shield or cutterhead jamming in mechanised tunnelling.
2 COMPUTATIONAL ASSUMPTIONS Numerical models were formulated in Abaqus® (Dassault Systèmes 2018) for a 12m-diameter cylindrical tunnel (R0
= 6m; tunnel radius, undeformed config) (see Figure 1).
2.1 Assumptions Common to Both Models In the transient mechanical analyses considering creep, a traction equal to the in-situ stress σ0
prevailing at
the depth of the tunnel is prescribed at the far-field boundary of the computational domain, 400m (ca. 67 R0
) away from the tunnel axis. December 2025 | 11 A linear elastic-viscous perfectly plastic constitutive
model is adopted for the rock. Also employed are Mohr–Coulomb yield condition and a non-associated visco-plastic flow rule. The model uses five mechanical parameters - Young’s Modulus E, Poisson’s ratio v, uniaxial compressive strength fc, angle of internal friction ϕ, and angle of dilation ψ, as well as a single rheological parameter - viscosity η, which determines the rate of visco-plastic deformation development due to creep. In the coupled hydromechanical consolidation
analyses, the uniform traction σ0 is also prescribed at the
far-field boundary, and a uniform pore pressure equal to the in-situ value po
prevailing at the depth of the tunnel. On account of the potential development of negative
pore pressures during ground excavation under undrained conditions, a mixed hydraulic boundary condition is prescribed. The rock is modelled as a
Below, figure 1:
Computational models for (a) the plane strain problem, and (b) the axisymmetric problem
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