Experience with inflatable friction rock bolts on the Cross River Rail project.

Friction rock bolts offer a significant advantage over grouted bolts in that they do not require grouting and thus provide immediate ground support upon installation. Inflatable friction bolts were developed around 40 years ago, with various versions of this type of bolt now offered by several manufacturers. The bolt is unique in that it is expanded with high pressure water during installation. This expansion generates the shear strength of the frictional interface with the rock. Pull testing was performed on inflatable friction bolts both prior to and during construction of the Cross River Rail project in Brisbane, which commenced construction in early 2020 with tunnelling completed at the end of 2021. Pull testing involves applying an axial force to the bolt head to confirm proper installation and can be used to determine geotechnical strength and stiffness parameters. Bolts for initial site trials were partially sleeved such that only a short length of bolt was inflated. This detail allowed the frictional interface between the bolt shaft and borehole wall to be tested to relatively high shear stresses, and thus allowed measurement of the ultimate geotechnical bond capacity. The test methodology also involved measurement of the axial displacement of the bolt relative to the applied load. This permitted bond shear stiffness to be assessed. This is a parameter utilised in numerical analysis and design of tunnel support and relates to the rate of load transfer between the rock bolt and surrounding rock mass. It has a significant impact on the magnitude of calculated bolt forces and a lesser influence on tunnel deformation. Both pre-production and production pull tests were completed in rock mass types including Neranleigh Fernvale Group, Brisbane Tuff and Aspley Formation bedrock. The measured bond strength values confirmed design assumptions that were based on a comprehensive literature review. The results allowed the contractor to proceed with procurement of the plant and materials needed to install the bolts with the confidence that they were an appropriate ground support element for the anticipated ground conditions. Advantages and limitations of this type of bolt are discussed, together with durability considerations, installation methods, production rates, construction challenges, test methods, and back-analysis of key design parameters. [ABSTRACT FROM AUTHOR]