In the realm of plasticity theory, a new proposed collapse mode, based on the laminar characteristics in Hoek–Brown rock masses, is employed to estimate the stability with referring to the progressive failure mechanism, by which the analytic solutions for characterizing the profiles of detaching blocks in tunnels with arbitrary cross-sections are derived with the help of upper bound theorem and variational principle. The results and conclusions in previous research are introduced to verify the agreement and validity of the proposed mechanism in this literature, i.e., no additional collapsing blocks would take place if the tunnels are constructed in homogeneous rock stratum, and one exception is that such issue, such as deterioration of buried rock banks with the mechanical parameters being changed which is what has been discussed in the preceding sections, would induce additional detachments apart from the initial block.