Stability of Twin Elliptical Tunnels in Hoek-Brown Rock Mass

Stability of twin and single elliptical tunnels in Hoek-Brown rock masses subjected to surcharge pressure is estimated by lower bound finite element limit analysis with power cone programming. The tunnels are considered to be unlined. The maximum normalized surcharge pressure (q(s)/& sigma;(ci)) is evaluated and its positive value implies that the tunnel is stable, where q(s) is the intensity of the surcharge pressure and & sigma;(ci) is the uniaxial compressive strength of the intact rock. The effect of different Hoek-Brown parameters, uniaxial compressive strength of rock mass, cover depth ratio, the aspect ratio of the tunnel profile, and clear spacing between twin tunnels on the magnitude of q(s)/& sigma;(ci) of elliptical tunnels have been studied. For a few cases, the stress state proximity to failure has also been presented to study the failure patterns of twin elliptical tunnels. Following regression analysis design equations are provided to estimate the magnitude of q(s)/& sigma;(ci) for dual as well as single elliptical tunnels.