Experiment study on loose characteristics of underground excavation in block-fractured rock mass

Based on the physically similar simulation experiments for underground excavation in block-fractured rock mass, the kinematic behaviors of overlying rock stratum and the change laws of loose zone with different dip beddings are obtained. A simple mathematical model for the limit dimension of loose zone is established. When the dip of bedding is less than the friction angle of rock, the rock stratum will separate and some rock blocks in every rock stratum may slide along joint surface as a whole. Left and right limits of loose zone are two step-surfaces consisting of beddings and joints. The upper limit is one of the bedding planes, which can be calculated by the model of Voussoir beam. The dip of step-surface is determined by the structural characteristics of block-fractured rock mass. All rock blocks in the loose zone is a movable block-group in block theory. When the dip of bedding is greater than the friction angle of rock and less than the limit angle of stability, rock stratum will not separate. All rock blocks in loose zone will fall freely as a whole. Right and left limits of loose zone are two step-surfaces. The upper limit is one bedding surface. In the vicinity of limit of loose zone, some blocks can rotate, and form an effective pressure arch which guarantees the stability of the overlying rock stratum. When the dip of bedding is greater than the limit angle of stability, loose zone can be identified as the area between two sets of bedding surfaces. In the sliding process of rock mass, the force status of some rock blocks may change. Rotation behavior of rock blocks results in block squeeze phenomenon, and prevents sliding of loose rock mass.