Prediction of squeezing potential is one of the major problems while tunnelling through weak rock mass under high overburden. The present methods in vogue generally compare the rock mass strength with the in-situ stress to resolve this problem. It is suggested in this paper that comparison of strains may be a better approach as the deformations are easily measurable. Critical strain is defined as that strain level beyond which the squeezing problems will be likely to occur. It is shown that the critical strain depends on properties of rock material, configuration of joints and anisotropic nature of the rock mass. Expressions are suggested to assess the squeezing potential through a weakness coefficient called Joint Factor. Critical strain is also correlated with rock mass quality index Q.A classification is suggested to quantify the squeezing potential based on ratio of expected strain and critical strain for the tunnel section.
Assessment of squeezing is an important problem while excavating tunnels through poor quality rock masses under high rock cover. Due to excavation, redistribution of stresses around the tunnel takes place. If the tangential stress at the periphery exceeds the rock mass strength, the mass fails and the broken zone progresses in radial direction resulting in development of squeezing ground conditions. It is suggested in this paper that strain may be a better indicator of squeezing potential. If the tangential strain exceeds certain limit, the tunnel will develop squeezing conditions. This limiting strain is termed as critical strain. Methods are suggested in this paper to assess the critical strain. The expected strain likely to occur will depend on rock mass properties and in-situ stress conditions. It may be obtained numerically or through analysis of monitored deformations. The ratio of expected strain and the critical strain is used to define the squeezing potential of the tunnel.
Aydan et al. (1993) used analogy between the stress-strain response of rock in laboratory and tangential stress-strain around tunnels to define strain levels above which squeezing may occur.Various degrees of the squeezing were defined by Aydan et al. (1993). Hoek (2001) also used strain to define squeezing potential. He has taken critical strain as 1% irrespective of rock mass properties. Present work suggests that the critical strain will depend on the properties of the intact rock material and joints in the rock mass. The work is based on an experimental study performed by Singh (1997) on specimens of a jointed rock mass under uniaxial loading conditions. The study simulated strength and deformation behaviour of jointed rock mass with varying joint configurations. The details of the study may be obtained from elsewhere (Singh et al., 2002; Singh and Rao, 2005; Singh et al. 2007). A typical stress-strain curve for the jointed mass is shown in Fig. 1. Various strain levels used by Ayden et al. (1993) are explained in this figure.
