Based on extensive field investigation and instrumentation, the characteristics of deformation and failure of underground roadways within soft rocks in collieries are formulated and presented graphically and quantitatively in this paper with a particular weight being placed on the deformational behavioral characteristics with time after roadway excavation. In view of the characteristics described, principles regarding roadway support within soft rocks are briefly outlined
Soft rock, according to the International Society for Rock Mechanics classification scheme for intact geotechnical materials [1] and I. W. Johnston (1993) [2], refers to a group of geotechnical materials for which the uniaxial compressive strength falls approximately in the range 0.5~25 MPa. This group of materials comprises of extremely weak rock, very weak rock and weak rock. Geotechnical characteristics of soft rock, such as slaking, swelling, compressibility, time dependence, volume change, etc. have been well discussed in literature [3,4,5]. However, engineering practice in soft rocks needs far more understanding of rockmass behavior than that the knowledge on intact soft rock could possibly provide. As a host rock where a roadway is accommodated, the behaviour of soft rock surrounding the roadway is a comprehensive reflection of intact rock properties, ground pressure or stress condition, rock mass structure, dimension and orientation of roadway and hydrological condition of rockmass, mining activities taking place in the proximity of the roadways and artificial rock support measures used. It is therefore, the behavior of a roadway within soft rock should be tackled by treating the geometry and dimension of roadway, support approaches and geotechnical characteristics of the rockmass surrounding the roadway as a behavioral entirety. Soft rocks in underground coal mines are usually argillously cemented, prone to deterioration when water is met as was extensively discussed in literature [6].
