# Failure in the Tension Zone around a Circular Tunnel Excavated in Saturated Porous Rock
## 1 Introduction  
Failure caused by the excavation of tunnels in rocks has been largely studied in many civil and mining engineering projects. The main mechanical factors affecting the stability are the far-field stresses, groundwater, strength, and deformability of the rock. The failure process that controls the stability of openings excavated in hard rocks at large depths is ruled by brittle crack initiation and propagation, which leads to spalling, slabbing, and, in extreme cases, rock-bursts [1]. The prediction of crack initiation and propagation at the boundary of tunnels and the identification of the appropriate parameters to model this occurrence were largely investigated over the past decades [2, 3, 4, 5, 6, 7, 8]. Tunnels excavated in very poor quality rock masses have received wide attention. An extensive analysis of squeezing problems in tunnel excavation in weak heterogeneous rock masses and rock masses of very poor quality is reported in the literature [1, 9, 10, 11, 12, 13, 14, 15, 16]. These studies take into account the effect of weak rock strata, such as joints and bedding planes, which can trigger rock mass disintegration at large depths [17]. Furthermore, the analysis of the orientation of thinly spaced weakness planes on the boundary of excavations has received wide attention, because of the occurrence of unexpected slip failure [18, 19, 20, 21, 22]. On the other hand, when rocks such as marks, claystones, siltstones, and weak sandstones have experienced geological stable conditions or a post tectonic environment, they have few discontinuities and can be treated as continuous intact rock material [1]. These studies outlined that the influence of groundwater on the behavior of the rock mass surrounding a tunnel is very important because it can cause severe tunneling problems, such as due to either physical deterioration of the components of the rock mass or the reduction of the effective stress confinement due to pore water pressure [10, 14, 16, 23]. Rock mass classification schemes aim at classifying and characterizing the rock masses and provide a basis for estimating deformation and strength properties for the design ofunderground excavation and support [1; 23; 24; 25]. In some classification schemes, attempts are made to account for the influence of groundwater pressure or flow on the stability of underground excavations [1; 23; 24; 25; 26]. In these schemes, the rock masses are of reasonable quality, and the effect of water is taken into account along the discontinuities. In this way, the resulting rock parameters are affected by the presence of water and the role of water pressure is not directly accounted for. [PERSON] [10] and [PERSON] et al. [1] outlined that the design of tunnels in rock masses affected by water pressure should include a full effective stress numerical analysis. Accordingly, the GSI rock mass classification scheme, developed for the Hoek and Brown criterion (H&B) [27], does not take into account the presence of groundwater. Coupled hydro-mechanical analyses are frequently carried out in saturated rocks with low strength and high deformability that are prone to ground squeezing [28; 29; 30]. Consequently, the influence of pore water pressure on tunnel excavation is mainly investigated in \"soil-like\" clay formations [31; 32; 33; 34]. The excavation induces the development of a wide plastic zone where pore pressure may significantly decrease up to reach negative values, in a wide area surrounding the tunnel. This results in a long-term critical condition with the occurrence of swelling. [31; 32; 33; 34]. The analysis of squeezing occurrence during TBM excavation in a low-permeability ground indicates that the water content remains constant, and the negative excess pore pressure generation determines a more favorable condition in the short-term [34]. The occurrence of undrained conditions during the tunnel excavation in brittle \"moderately weak\" porous rocks has been less investigated [35].