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7       Excavation design in massive


                                        elastic rock




                                        7.1  General principles of excavation design

                                        Mining excavations are of two types – service openings and production openings. Ser-
                                        vice openings include mine accesses, ore haulage drives, airways, crusher chambers
                                        and underground workshop space. They are characterised by a duty life approaching
                                        the mining life of the orebody. It is therefore necessary to design these openings so
                                        that their operational functions can be assured and maintained at low cost over a
                                        relatively long operational life.
                                          Mine production openings have a temporary function in the operation of the mine.
                                        These openings include the ore sources, or stopes, and related excavations such as
                                        drill headings, stope accesses, and ore extraction and service ways. In these cases, it
                                        is necessary to assure control of the rock around the excavation boundary only for the
                                        life of the stope, which may be as short as a few months.
                                          The issue considered in this chapter is the design of service openings and produc-
                                        tion openings subject to entry by personnel. (The design of non-entry excavations is
                                        considered later in conjunction with the related mining methods.) A logical frame-
                                        work is presented for the design of these openings. Although it is proposed for general
                                        application in the design of permanent mine openings, it can also be regarded as a ba-
                                        sis for the evolution of designs for specific temporary openings. Such an evolutionary
                                        process could be associated with the implementation of an observational principle of
                                        excavation design.
                                          Inprinciple,excavationdesigninmassiveelasticrockrepresentsthesimplestdesign
                                        problem posed in mining rock mechanics. However, only in recent years have some
                                        key questions related to rock strength, fracture and failure and their effect on the
                                        behaviour of rock in the boundary and near field of an excavation been resolved.
                                        These matters were considered in Chapter 4, and in particular in Section 4.5. The
                                        case considered in this chapter involves a single excavation, which is taken to imply
                                        that the opening will not be mined in the zone of influence of any existing opening.
                                        The rock mass is considered massive if, on the scale of the excavation, the volume
                                        of rock to be mined to create the opening is traversed by only one or two persistent
                                        structural features prone to slip or separation. However, many observations (Brady,
                                        1977, Martin et al., 1999, Diederichs, 2002) suggest that the proposed methods for
                                        massive rock, involving comparison of the state of rock stress and local rock mass
                                        strength, are also applicable in moderately jointed rock masses.
                                          The rock mass strength properties are assumed to be defined by compressive failure
                                        criteria of the types discussed in Chapter 4, with an in situ crack initiation and damage
                                        stress described by   ci and the constant deviatoric stress criterion (Martin et al., 1999)
                                        and an in situ long-term strength described by   cd and a modified Mohr-Coulomb cri-
                                        terion (Martin et al., 1999). Both the damage criterion and the long term strength
                                        criterion may be represented by appropriate m and s parameters in the generic



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