Ground supports for tunnels in weak rocks

The various factors and how they influence the loading and deformation of supports needed in deep tunnels in weak rocks are presented. Broadly, these are the rock and water conditions and their response to excavation, the excavation and support timings and procedures, and the support details and their workmanship. Particular attention is drawn to the need to consider the truly three-dimensional influence of rock discontinuities on excavation collapse mechanisms with the aid of models and field observation as a guide to support and safety requirements; to consider during construction the positioning and timing of support placement in relation to the rate of advance of the face (another three dimensional effect); to allow a degree of rock yielding by means of yielding supports on the basis of concepts developed from simple yielding rock/support interaction theory combined with construction monitoring; and to consider quantitatively the real load-deformation properties of the structural supports and their in...