Back analysis of grouted rock bolt pullout strength parameters from field tests

This paper focuses on the grout cohesive strength and the grout friction angle of rock bolt, which have a significant influence on the pullout force and are difficult to estimate. Traditional method estimate the two parameters from the results of pull-out test conducted under different confining pressures. St. John and Van Dillen gave two approximate empirical formulas in 1984. In this text, a new method was proposed to back calculate the grout cohesive strength and the grout friction angle based on given field pullout force of rock bolt. In order to verify the method, a numerical model was built by FLAC3D to approach the two parameters by the principle of dichotomy. The convergence result had been proved to be right by numerical pull-out tests.

► The method to inverse parameters of rock bolt based on the principle of dichotomy was introduced.
► The cohesive strength and friction angle of grout could be inversed based on field pullout force of rock bolt.
► The inversed parameters had been proved to be feasible in researches on rock bolt.