Stability of unlined circular tunnels with surcharge in heavily fractured rock mass

This paper presents the stability of an unlined circular tunnel in heavily fractured rock mass subjected to surcharge loading on the ground surface. The rock mass is considered to be homogeneous and isotropic, and obeys the Hoek–Brown failure criterion. The stability is defined in terms of the maximum surcharge load that can be supported at the ground surface for different input rock mass parameters (uniaxial compressive strength, geological strength index, unit weight, Hoek–Brown material constant), tunnel geometric parameters (tunnel cover and diameter) and disturbance factor owing to stress relaxation and blasting damage. Finite-element lower-bound limit analysis based on the distribution of stresses in a statically admissible stress field is performed with the implementation of power cone programming as a convex optimisation technique to maximise the collapse load at the ultimate failure. The use of the generalised Hoek–Brown (⁠$GHB$⁠) criterion over the original Hoek–Brown criterion has been demonstrated and the results are presented in a non-dimensional form for various combinations of the aforementioned input parameters for rock mass considering the $GHB$ criterion.