The Impact of Earthquakes on Tunnels in different Rock Mass Quality Q- : a numerical analysis

The impact of earthquakes, due to ground shaking, which affects the whole tunnel length from ovaling of the tunnel cross section, is investigated in this thesis. The influence of rock mass quality Q and the tunnel dimension is also studied. Finally, an approach to determine the new seismic support system using existing Q-system design chart and expected peak ground acceleration at the tunnel site is presented. The earthquake loading is modeled through quasi-static seismic loading in Phase2, a finite element modeling program by Rocscience, Inc. for design of underground structures and slopes. The quasi-static assumption is valid in rocks, as due to their higher velocity, the wavelength of shear waves is > 20D, where D is tunnel diameter. At this scale, the dynamic interaction between the tunnel and the passing seismic waves is minimal and thus validates the quasi-static assumption. The seismic coefficient, a unitless vector dependent on the peak ground acceleration (PGA), serves as a representative parameter for the expected critical earthquake. Four rock mass classes with Q = 1 - 40, to represent "very poor" to "very good" rock masses are modeled by varying the deformation modulus and Mohr-Coulomb parameters, determined from empirical relations. The increase in support pressure (represented by axial force) is investigated as function of rock mass quality Q and tunnel dimension. A model comprising of a 10 m diameter tunnel at 60 m depth surrounded by rock masses with Q ranging from 1 - 40 is used to investigate the influence of rock mass quality Q. While the seismic loading is unchanged, the magnitude of the axial force on the lining and the net increase due to seismic loading, referred to as seismic axial force, increases as the rock mass quality decreases. To check the influence of tunnel diameter, the diameter of a circular tunnel at 60 m depth and under fixed seismic loading is increased from 5 m to 20 m at 5 m interval. The magnitude of the axial force and the seismic axial force increases with tunnel diameter for rock mass with Q = 1 ("very poor" rock mass). Conversely, the increase in magnitude of axial force and seismic axial force on the lining is relatively insignificant for rock mass with Q = 40 ("very good" rock mass). Inferred from the above findings, an approach to determine the seismic support pressure by using the concept of Qseismic, first introduced by Barton (1984), is presented. During earthquakes, the required support pressure is expect