Experimental Study of the Effects of Borehole Size, Borehole Roughness, and Installation Pressure on the Pull Load Capacity of Inflatable Rockbolts

An inflatable rockbolt is a steel tube that has been rolled into an omega shape. During installation, water is injected into the tube, which expands the profile of the rockbolt, generating a radial force against the borehole wall. This reinforces the rock mass through the friction at the bolt–rock interface. This study aims to quantify the effects of borehole size, borehole roughness, and installation water pressure on the pull load capacity of the bolt. This was achieved through a series of pull tests under laboratory conditions. The test results showed that the load capacity was higher in boreholes that were either close to the initial profile diameter of the bolt or the fully unfolded diameter of the bolt, that were 26.8 mm and 38.7 mm, respectively, in the study. The load capacity and the radial stiffness of the bolt were lowest in the medium-sized boreholes (33 and 33.5 mm). In small boreholes, the shoulders of the bolt tongue are tightly compressed such that the outward elastic deformation of the bolt tube is locked in after installation. This deformation locking enhances the load capacity of the bolt. In addition, the load capacity was found to be higher in percussively drilled boreholes than in diamond-drilled boreholes. The additional friction angle of the percussive boreholes was back-calculated to be approximately 5.83°. The load capacity was also found to increase as the installation water pressure increases in the range of pressures tested. It was observed that the inflatable bolt was clamped against the borehole in three zones: on the two tongue shoulders and the side of the bolt opposite the tongue.