ABSTRACT
Entries are “tunnels” of rectangular cross-section. In stratified ground, entries have roof spans that may be determined by beam analysis, provided the immediate roof separates from the overlying strata. This situation is often the case in softrock mines (e.g. coal, salt, trona, potash). Service life is variable, but mainline entries have long service lives, perhaps 20 years or more. Other mining entries may be used for a year or less. In any case, entry roof span is just entry width. If roof strength is inadequate for achievement of an acceptable safety factor, then support or reinforcement is required, usually in the form of roof bolts. The term “roof” bolting implies bolting in stratified ground or softrock mining, for example, coal mining; “rock” bolting is used in hardrock mining and tunneling. There is more than a semantic difference between the two mining environments. If the immediate roof layer is thick relative to entry width or roof span, then bed separation is unlikely. In the absence of bed separation, design analysis requires investigation of stress concentration about the considered entry. This is likely to be the case in hardrock room and pillar mines and excavations for other purposes such as underground manufacturing and storage facilities. In either situation, the usual design criteria are factors of safety in tension and compression,
FSt = To σt
FSc = Co σc
(5.1)
whereTo andCo are unconfined tensile and compressive strengths, respectively; σt andσc and peak tensile and compressive stresses. A complication that may arise is strength anisotropy, for example, tensile strength parallel to stratification planes (“bedding”) may be significantly different from tensile strength perpendicular to bedding. Shear strength may also be a factor where shear strengths across and along bedding differ significantly.
