Ultimate collapse of cross-jointed, thinly bedded, horizontal roof strata

Summary

Small scale physical model studies of the collapse of mine roofs excavated in horizontally bedded strata containing uniformly spaced, rough, vertical cross-joints revealed a new type of failure mechanism, of which the classic Voussoir arch is shown to be a special case. The mechanism involves vertical displacement of a central section about two peripheral ‘hinges’. Hence, the author has called it the ‘2-hinge’ collapse mechanism.

A numerical model of the mechanism was developed, which allows for progressive rock failure at the hinge points and for external loadings across the roof span. Results from this analysis show that the collapse mechanism is very sensitive to bed thickness and rock strength. Failure by this mode is more likely to develop in weak, thinly laminated beds with low horizontal stresses.

It is deduced from this investigation that, in a geological environment where this type of failure is likely, bolts installed close to the sides (ribs) of openings should have high shear stiffness, for example full column, resin bolts. Bolts in the central region, where vertical movement predominates, need adequate pull-out resistance and, if some corrosion protection is necessary, may be grouted with a less expensive cement or plaster-based grout.