Mechanisms of Bolt failures

The primary role of the bolt reinforcement is to minimise displacement of the existing fractures by providing confining stress to the fracture surfaces.  In some cases, rockbolts may minimise some residual fracture formation within the moving strata. Rockbolts installed across the bedded, jointed or fractured strata is capable of resisting axial and shear deformation (Figures 13 and 14). The fully grouted bolt function can be classified into three separate categories: the axial resistance of the rockbolts subjected to tensile load; the shear resistance of the bolts installed across a potential shear plane and the combination of both axial and shear resistance of the bolts undergoing pull and shear loads.

Hence, there are three types of rockbolt failure mechanisms: the axial failure where bolts fail due to significant axial load; the shear failure where bolts yield due to large shear load; the combined axial and shear failure due to both axial and shear load. Figure 2.14 shows two bolts in an underground mine yielded due to a combination of pull (joint opening) and shear at rock joints.

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Axial failure modes of rockbolts

Among the three failure mechanisms of rock bolts, axial failure was extensively studied using push and pullout tests of short encapsulated rock bolts. The failure modes of rock bolts subjected to axial load are shown below.

Aydan (1989) showed that an increase in bearing capacity was attributable to the normal compressive stress resulting from the geometric dilation of the surface. This suggested that shearing might occur along one of the surfaces of weakness in the rock bolt system (grout-rock interface and bolt-grout interface), and classified the failure modes in the push and pull tests as follows:

  • Failure along the bolt-grout interface. This occurred in every test on bars with a smooth surface and deformed bars installed in a large borehole.
  • Failure along the grout-rock interface. This occurred in deformed bars installed only in smaller diameter boreholes.
  • Failure by splitting of grout and rock annulus
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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.

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