Experimental and numerical analysis of fully grouted long rockbolt load-transfer behavior

Abstract

As a simple supporting structure, rockbolts are responsible for maintaining the integrity and stability as well as controlling the deformation, displacement, and crack development in the surrounding rock mass. Knowledge of rockbolt load-transfer behaviors constitutes valuable information, as the rockbolt functionality is affected by load-transfer processes. A series of pullout tests was conducted on a rockbolt with a long embedment length (1500 mm). The critical rockbolt embedment length at which yielding or necking occurs together with the friction coefficient at the residual axial stress was analyzed numerically. Additionally, the failure modes of rockbolt-grouted systems and the effects of several parameters, including the rockbolt diameter (18, 20, and 25 mm) and cement mortar strength, were assessed. The rockbolt strain and energy absorption during the pullout process were also investigated. The identified rockbolt failure modes, which are dependent on the cement mortar strength and rockbolt diameter, include pullout accompanied by partial concrete splitting failure parallel to the rockbolt. Under the same cement mortar strength, the maximum load increases and the rockbolt displacement at the maximum load decreases as the rockbolt diameter increases. When diameter is held constant, the maximum load and corresponding displacement increase as the cement mortar strength increases, but the residual load is irregular. The energy absorbed by the rockbolt in the pullout test is related to the cement mortar strength and failure mode. With an increasing embedment length, the failure process is converted from a completely damaged interface with little simultaneous damage to the cement mortar at the free end to the expansion of the cracks at the loaded and free ends toward each other. The interface is completely damaged when the cracks at both ends become connected.