A Knowledge of the mechanical properties of weak rocks such as shale, sandy shale and coal is essential for ground control in undeground coal mining. Most tests which are usually used to determine the strength of rock require the specimen to be in the precisely prescribed shape. However, with weak rocks it is often difficult, or practically impossible in some cases, to prepare the specimen as required by these tests.
From this point of view, rod penetration test has been carried out, because in this method an unprepared specimen can be used.
The results of this study are summarized as follows:
1. The size of specimen should be at least 10 times the rod diameter.
2. Size effect of rod on penetration strength S_rp is caused by difference in fracture mechanism of rock around the top the penetrated rod.
3. Penetration modulus E_rp is not dependent on the rod size.
4. Uniaxial strength Sc and tangent Young's modulus at 50% of compressive strength E₅₀ can be estimated with penetration strength S_rp and penetration modulus E_rp as follows; S_c=0.11·S_rp, E₅₀=2.3·E_rp.
5. It is possible to estimate the strength reduction with water quantitatively by this test.