ROCKBOLT BEHAVIOR UNDER DYNAMIC LOADING - FIELD-TESTS AND MODELING

Knowledge of rockbolt response under dynamic loading conditions is imp ortant when rockbolts are used to support ground subjected loads from blasts, earthquakes or rockbursts. Dynamic strain measurements from st rain gauges mounted on end-anchored rockbolts and velocity measurement s from geophones mounted on the rock surface were recorded during two different blasts. These blasts caused peak particle velocities in the rock exceeding 1 m/sec but maximum dynamic stresses in the bolts were well below the yield point of the steel. The measurements were analyse d by analytical methods and a numerical model which was developed to e xplain the measured rockbolt variations resulting from the dynamic loa ding caused by the blasts. The numerical model developed for this stud y is a one-dimensional finite difference representation of a mechanica l system involving the following components: the wave source, the rock mass, the mechanical rockbolt, and the anchor and head connections. Th e measured strains on the rockbolts and the dynamic movement of the ro ck surface were closely simulated by the numerical model. Both axial a nd lateral free vibrations were excited in the rockbolts by the dynami c loading and the vibrations persisted much longer than the ground mot ions. Damping al the connections between the rockbolt and the rockmass at the anchor and the plate influence the transfer of the stress wave from the rock to the bolt and the resulting frequency of vibrations i n the rockbolt.