Non-destructive testing on anchorage quality of hollow grouted rock bolt for application in tunneling, lessons learned from their uses in coal mines

Abstract

Hollow grouted rock bolts are widely used to reinforce the surrounding rock mass in underground engineering, such as deep roadway of coal mines, deep tunneling. The anchorage capacity of hollow grouted rock bolts greatly influences the support quality. However, the anchorage capacity of hollow grouted rock bolt is usually destructively measured. In this paper, the grout ratio, defined as ratio of the grouted volume to the over-coring volume of anchorage, is introduced to measure the anchorage capacity. The quantitative relationship between the grout ratio and anchorage capacity of hollow grouted rock bolt by pull-out tests is established. Then, the non-destructive inspection on grout ratio of anchorage for hollow grouted rock bolts by stress wave reflection method is theoretically and experimentally investigated. By combining the two steps, a new non-destructive inspection method on anchorage quality of hollow grouted rock bolts is proposed. The results indicate that: (i) The anchorage capacity increases with the grout ratio increases up to 75% beyond which the benefits are lessened. (ii) The stress wave propagating along the bolt rod reflects and transmits at each interface impedance mismatch. The reflected signals can be used to identify the start and end of anchorage, and void part in anchorage. (iii) The results of wavelet transform analysis indicate the low-pass filters employed to reflected signals can be used to identify the start of anchorage, while the high-pass filters can identify the void part and the end of anchorage. (iv) A new supporting capacity monitoring method combined with pull-out tests and non-destructive method on hollow grouted rock bolts has been proposed, which can be used to inspect the on-site hollow grouted rock bolts in roadway on a large scale.