In view of the great demand in coal mines for supporting systems which can incorporate large displacements, the idea of yielding cable bolts is very impressive.

One of the ways in which cable bolts may be allowed to yield, to incorporate large displacement and still have high residual strength, is to introduce small 'cages' (called mini cages) along the length of the cable. On loading of such a mini cage cable bolt, the cages collapse, accommodating most of the displacement, without straining the cable element much.

The performance of a mini cage cable bolt may be predicted in theory, but in the field variable performances can be expected. For example the anchorage between the resin and the bolt may break on the collapse of the cages or the anchorage between the resin and the coal may break or the coal core surrounding the cable bolt may break free of the surrounding core, depending on which is the weakest part of the system.

Numerical modeling methods are applied to analyse the assumed behaviour of the yielding cable bolt. But for field verification overcoming or pull tests are not only expensive but also very inappropriate for the purpose, as the behaviour of the bolts is to be studied in service and thus in-situ.

Thus NDT of yielding cable bolts is required. This may be done by passing longitudinal and transverse elastic waves through the cable bolt and receiving the signal by placing sensors in the coal around the bolt.

Principles of seismic data analysis and interpretation may be applied to analyse the data collected. This three dimensional analysis will give information about the change in length of the bolt, the anchorage condition of the bolt to the surroundings and also the difference in the behaviour of the system depending on where the mini cages are placed along the length of the bolt. Comparison of these results with NDT results on some conventional bolts will decide the ability, or the lack of it, of such yielding cable bolts with mini cages to accommodate large displacements and still offer high residual strengths.