Paper contribution

Updated knowledge of inflatable rock bolts

Charlie Li

Proceedings
EUROCK 2026
Pages
273-278
Submitted
28 January 2026
Accepted
15 June 2026
Published
12 August 2026

Summary

Abstract

Inflatable rock bolts can quickly stabilize loosened rock blocks in underground excavations. An inflatable bolt is made of a steel tube that is folded into an omega shape. During installation, high-pressure water is injected into the tube to expand it to the borehole size. A primary contact stress is established on the borehole wall after installation, resulting in frictional resistance that prevents rock blocks from moving. It is recommended in current installation specifications that bolts be installed in boreholes slightly smaller than the original diameter of the steel tube, and small boreholes are generally not preferred. Recent theoretical and experimental studies of inflatable bolts have provided new knowledge, including the optimal borehole size and the relationship between the radial stiffness of the partially unfolded tube and the diameter of the unfolded tube. These studies show that the pullout load is high in both large and small boreholes. In large boreholes, the high pullout load is due to the high radial stiffness of the bolt tube, while in small boreholes it is due to locked contact stress. Middle-sized boreholes should be avoided because the stiffness of the bolt is lowest, and the locked contact stress can also be very low in such boreholes. The high pullout load in small boreholes, which contradicts theoretical predictions, is a new finding. It is attributed to locked contact stress caused by the tight contact of the shoulders of the tongue flanks of the bolt in small boreholes. Therefore, small boreholes can be used for inflatable bolts.

Contributors

Authors

  1. Charlie Li NTNU-Norwegian University of Science and Technology, Norway ORCID 0000-0003-2618-3231

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References

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