Littlejohn (1993) classified various types of axial failure when using grouted bolts in one or more of the manners: the bolt, the grout, the rock, the bolt-grout interface or grout-rock interface. The type of axial failure depended on the properties of individual elements. The steel bar governed the axial behaviour of the bolt, which was much stiffer and stronger than the grout and rock. If the bolt had sufficient length to transfer the entire load to the rock it would fail. The shear stress at the bolt-grout interface was greater than at the grout-rock interface because of the smaller effective area. If the grout and rock were of similar strengths, failure could occur at the bolt-grout interface. If the surrounding rock was softer then failure could occur at the grout-rock interface.
From pullout tests of cable bolts in the laboratory and in the fields, Hyett et al (1992) have identified two failure modes in cementitious grouted cable bolt. One mode involved radial splitting of the concrete cover surrounding the cable, and the other shearing of the cable against the concrete. The radial splitting mechanism was induced by the wedging action between the lugs of the bar and the concrete. This exerted an outward pressure on the inside of the concrete annulus that was balanced by the induced tensile circumferential stress within the annulus. However, if the tensile strength of the cement was exceeded, radial splitting occurred, the circumferential stress in the concrete annulus reduced to zero as well the associated reaction force at the steel-concrete interface, so resulting in failure. The shearing mechanism involved crushing of the concrete ahead of the ribs on the bar, eventually making pullout along a cylindrical frictional surface possible. It could be concluded that as the degree of radial confinement increased the failure mechanism changed from radial fracturing and lateral displacement of the grout annulus under low confinement, to shear of the cement flutes and pullout along a cylindrical frictional surface under high confinement.