Focused on the failure modes of prestressed anchor cables

according to Hoek-Brown failure criterion and associated flow rule

the ultimate pullout force and failure mechanism of anchor cable were deduced through upper bound limit analysis

where bond failure of anchorage body and anchored rock mass failure were considered respectively. The influences of grouting pressures

lengths of anchorage section and rock parameters on ultimate pullout forces and failure mechanisms were analyzed. The results reveal that

increasing anchorage length in a certain extent and using pressure grouting are effective measures to improve ultimate pullout forces of anchor cables. When anchored rock mass failure occurs

ultimate pullout forces will increase with the improving of empirical parameter A

tensile strength

compressive strength and unit weight of rock mass

while it will decrease with the improving of empirical parameter B. The shape of failure surface of anchored rock mass looks like horn

which is consistent with the conclusions of existing literatures; and parameter B is a key factor affecting the shape of failure surface of rock mass; it determines the curvature of failure curved surface; and curved surface will degenerate into circular conical surface when B is equal to 1.