Loosened zone mechanism in soft rock excavation and deformation control techniques

Abstract

For underground mining, excavation support in soft rocks is a long-standing serious problem. Although many researches have been carried out to solve the problems, due to the complexity of soft rock engineering, support design for soft rock excavations is still based on empirical method or numerical simulation. In addition, there is no feasible quantitative index in defining soft rock engineering. By discussing the characteristics of soft rock engineering, this paper uses a quantitative index - the ultimate loosened zone thickness of surrounding rocks (represented with Lp) to define soft rock engineering, which reflects the combined effects of stresses and strengths of surrounding rocks. According to the results of lab test and in situ measurement, the nonlinear deformation mechanism of loosened zone and its influential factors are analyzed, and especially the dilatant deformation mechanism and the reasonable control techniques are discussed in detail. It is convenient to use the resultant index reflecting the stability of surrounding rocks, i.e. the loosened zone thickness, to define the soft rock engineering (Lp =150 cm). For any soft rock engineering regardless of its causes, the loosened zone thickness is greater than 150 cm, and the compressed arch theory is used in support design. It is impossible to restrict absolutely the nonlinear deformation of loosened zone. A certain amount of deformation should be allowed to release energy accumulated in rocks, so as to diminish the support reaction. On the other hand, the excessive deformation should be avoided so as to keep the proper working space and stability of excavation. The pure stress control or high strength support is not advised. Based on the analyses of field measurements of loosened zone and on the discussion of reasonable support patterns, an industrial test was conducted by substituting original 29# Toussaint-Heintzmann yielding arch with the support using shotcrete and bolting wire mesh, at 11th, 12th, 13th level, in Zhao Ge Zhuang coal mine (Jing and Fu, 1996). Some successful results are achieved: reduction of repair rate, acceleration of construction speed and annual economic benefit of more than five million RMB. It has proven that the support using shotcrete and bolting wire mesh is economical, effective and practical for deep excavations. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.