Page 242 - Rock Mechanics For Underground Mining

8 Excavation design in

stratified rock

8.1 Design factors

Tabular orebodies hosted by stratified rock masses are quite common in mining prac-
tice. An orebody in a sedimentary setting is typically conformable with the surround-
ing rock in which the stratification is associated with bedding planes, foliation 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 di-
rection 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 strati-
form 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 maintaining geomechanically sound and
operationally safe mine workplaces.
Excavationsinastratifiedrockmassareusuallyminedtoacross-sectionalgeometry
in which the immediate roof and floor of the excavation coincide with bedding planes,
as illustrated in Figure 8.1. Factors to be considered in the design of such an excavation
include:

(a) the state of stress at the excavation boundary and in the interior of the rock
medium, compared with the strength of the anisotropic rock mass;
(b) the stability of the immediate roof;
(c) floor heave in the excavation.

Figure 8.1 An excavation in a strat-
ified rock mass, with geometry con-
forming with dominant rock structure.

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