The present invention discloses a rock bolt having a tendon (1, 201) which is able to yield and thereby control movement of unstable rock strata into which the bolt is installed. The tendon has a portion which is provided with a grout slippage mechanism, preferably in the form of a tube (8) which surrounds the tendon. A two part grout anchor (10, 100, 200) is clamped to the tendon. The grout anchor is embedded in grout and is therefore essentially immovable. The tendon passing through the anchor is worked and thereby dissipates energy. The tendon can be either a multi-strand cable (1) or a solid bar (200). Various forms of anchor (10, 200) are disclosed with either complementary recess(es) (14) and protrusion(s) (15) or opposed protrusions (115) which form pinch point(s). A one piece grout anchor (300) is also disclosed.