Excavation design in stratified rock

Abstract

A stratified host rock mass for an ore deposit is not uncommon in mining practice. An orebody in a sedimentary environment is frequently conformable with the surrounding rock, in which the stratification is conferred by bedding planes or related depositional features. The main geometric characteristics of these features are their planar geometry and their persistence. They can be assumed to be continuous over plan areas greater than that of any excavation created during mining. There are two principal engineering properties of bedding planes which are significant in an underground mining context. The first is the low or zero tensile strength in the direction perpendicular to the bedding plane. The second is the relatively low shear strength of the surfaces, compared with that of the intact rock. Both these properties introduce specific modes of rock mass response to mining, which must be considered in the excavation design procedure. An associated issue is that, for flat-lying stratiform orebodies, the typical mining method involves entry of personnel into the mined void. The performance of the bed of rock spanning the excavation, i.e. the immediate roof, then assumes particular importance in assuring maintenance of a geomechanically sound, local mining environment.