3D numerical modelling of brittle failure in mechanized excavation of deep tunnels

For long and deep tunnels as currently under construction through the Alps, mechanised excavation by using Tunnel Boring Machines (TBMs) may contribute significantly to savings in construction times and costs. Questions are however posed due to the severe ground conditions which in cases are anticipated or encountered along the main tunnel alignment. One of these geological hazards is the stress-induced brittle failure of hard rocks, leading to spalling (formation of rock slabs) and/or rockbursting (violent ejection of rock fragments). This thesis initially deals with brittle failure, both in laboratory tests (uniaxial and/or triaxial) and in tunnelling, with a special interest in TBM excavation experience. In fact, this failure mode can create a significant hazard for workers immediately behind the shield, as well as gripper problems and machine operation delays. Rock bursting can even lead to TBM jamming. The description of the many models proposed in the current literature for predicting brittle failure around underground openings is given. The rock mass can be considered a continuum, a discontinuum or a hybrid continuum/discontinuum. The most practical and widely used approach is the so-called phenomenological/empirical approach (continuum assumption). Then, the interest is moved to the current state-of-the-art in numerical modelling of the TBM excavation process. It is noted that only axisymmetric or fully threedimensional (3D) models are capable to study in detail the complex interaction between the rock mass, the tunnel machine and its system components, and the tunnel support. The most advanced and accurate models are the 3D models. It is seen however that very few fully 3D models for mechanised deep tunnel excavation have been developed so far and no one refers to spalling conditions. The main scope of this thesis is therefore to present a novel and completely 3D FEM simulator for mechanized tunnel excavation of deep tunnels in brittle conditions. The main machine and support components are taken into account. The TBM of reference is the technologically advanced Double Shield TBM. The model is conceived in particular for modelling the spalling behaviour of hard rock masses. An elasto-brittle-plastic constitutive law based on the criterion developed by Diederichs has been proposed and successfully validated through the comparison with field data and the results of other codes and criteria. The considered case study is related to the Brenner Base Tunnel excavation on the Italian side, in hard brittle granitic rocks. Two simulations have been carried out. The first one is a design analysis with reference to the spalling hazard along the main tunnel. The second one reproduces an instability problem which occurred during the excavation of the Aicha exploratory tunnel at chainage 6+151, due to the presence of a thin granite diaphragm between a subvertical fault and the tunnel wall. The results obtained show that the 3D model is highly effective in reproducing both the rock mass response (in terms of extent, shape and depth of the failure zones) and its interaction with the TBM system components (confinement effects producing a reduction of the failure zones).