DYNAMIC RESPONSE OF AN UNDERGROUND TUNNEL TO SEISMIC WAVES

This study investigates the interaction of seismic waves with a cylindrical tunnel cavity embedded in a semi-infinite poroelastic medium. When seismic waves scatter from an underground tunnel, the magnitude and location of maximum tangential stresses on the tunnel cavity varies upon angle, frequency, and kind of the incident wave. The overburden thickness (depth of the tunnel) and the bulk properties of the surrounding medium also affect the response of such systems to dynamic excitations. Using Graf's addition theorem for the cylindrical Hankel functions, the multiple-scattering between the tunnel wall and the free surface is expressed in form of infinite series. To model the underground medium, the Biot dynamic model of poroelasticity is employed. A crossover frequency f(x) at which the wavelengths become comparable to the size of the tunnel, the Biot critical frequency, and the crossover frequency at which fluid diffusion length is of the order of the tunnel size are introduced. Results are discussed at the crossover frequency f(x). It is shown in this paper that neglecting the effect of the free boundary or using equivalent effective elastic medium approximations may lead to significant errors. A limiting case involving an elastic halfspace containing a long cylindrical cavity is considered and fair agreement with a previous study is established.