Rock bolt is primary support in modern tunnel engineering and has crucial role of stabilizing the excavated space in jointed rock mass by anchoring the ground with cohesive and frictional strength. Spiral bolt is rock bolt type support and recently developed by Hirata et al. 2005. It has spiral shape of twisted flat bar (width 32 mm by thickness 9 mm), which has around 40% less area compare to 25 mm ordinary deformed bar rock bolt. As spiral bolt has less weight than rock bolt, it can be easily handled and quickly installed in tunnel or rock slope. With the same reason, the material cost can be saved if rock bolt is replaced by spiral bolt.

In order to be applied as a replacement of the rock bolt, it has to be proved that support capacity of spiral bolt should be superior or equivalent to that of rock bolt as well as better workability and cost effectiveness. Therefore, investigation on the overall capacity of spiral bolt and rock bolt became required in order to examine the effect of shape and areal differences.

In this study, support capability of rock bolt and spiral bolt has been investigated for pull-out and shear analysis. For pull-out analysis, we made two different models for various rock types:

  1. cylindrical shape mortal with centered deformed bar rock bolt, and

  2. cylindrical shape mortal with centered spiral bolt (twisted flat bar).

For shear analysis, we made two different models for various rock types with single weak layer:

  1. centered deformed bar rock bolt with interface and

  2. centered spiral bolt with interface.

The pull-out simulation shows that the stress-strain behaviours of rock bolt is perfect plastic regardless of rock type, however strain hardening behaviour is apeared for spiral bolt which is encapsulated in hard rock, result in higher pull-out strength. Test results show equivalent capacity for weathered soil and rock, and even superior for soft and hard rock. Shear test shows shear resistance of spiral bolt is equivalent to rock bolt for various rock types. In conclusion, with the effect of spiral shape, spiral bolt is better support in mechanical behaviour as well as economical feasibility than rock bolt.

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