Rock mass creep in tunnelling through weak rock masses under high overburden may lead to the potential development of significant convergence. Should this convergence be restrained by a rigid support shell, tunnel loading tends to increase. Although, such increase could be significant, it is often not taken into account in design, due to uncertainties in the calculation procedure. This paper proposes a simplified approach for estimating the additional creep loads on the lining of circular tunnels excavated in one phase. This is achieved by performing a parametric set of 2D numerical analyses with the finite element code ABAQUS. The rock mass is modeled as an elasto-plastic material yielding according to the Drucker-Prager criterion, with time-dependent characteristics described by the Singh-Mitchell creep model. The time-dependent behaviour is initially formulated upon the assumption of a new set of parameters. Typical ranges of these parameters are estimated using creep experiments from the literature.
In tunnel design and construction through weak rock masses under high overburden the role of rock mass creep may prove important for the final wall convergence and ground loads exerted on temporary support or final lining. However, this time-dependent behaviour is usually not considered in design, due to the large uncertainties of creep and the need for complex laboratory tests and constitutive models.
In tunneling, the time-dependent behaviour of the rock mass is not typical creep, since deformation does not freely develop under constant state of stress. The construction of the support shell restrains the potential creep deformation, leading to increased tunnel loads and rock mass creep evolves with varying stresses. Moreover, the rock mass creep in tunnelling can be separated in two categories (this separation is only in relation to the specific project which determines the time scale):
Short-term creep. Creep deformation influences the excavation procedure and the temporary support.
Long-termcreep. Creep deformation develops during the tunnel service life leading to increased ground loads on final lining.
There are several case studies in literature, of tunnels that experienced large deformation and in some cases severe failures due to the short-term creep behaviour of the surrounding rock mass (e.g. the Saint Martin La Porte access adit, along the Torino – Lyon Base Tunnel and the Lötschberg Tunnel). Debernardi (2008) describes the case of the Saint Martin La Porte, where up to 2.0m convergence developed during the excavation. Using back analysis the dominant role of the creep of the surrounding rock mass was revealed. Moreover, Barla et al. (2010) presented stress measurements in the final lining, clarifying that the increase of the stresses due to the rock mass creep varied from70% to 100%. Sandrone et al. (2006) employed three different models to simulate the very large time-dependent convergence (up to 80 cm) developed when the tunnel crossed the base sediments of sandstone and siltstone and up to 1.0m thick beds of carbon and anthracite.