Rock drilling tests: the Sievers' Miniature Drill test
The design of an underground excavation includes the delicate phase of estimating the drillability of the rocks. The Sievers ‘Miniature Drill Test is a drillability test that evaluates, using an index called Sievers’ J-Value (SJ), the ability of a rock to be pierced by a metal tip.
The Sievers drillability test was developed by the Swedish H. Sievers in the 1950s and subsequently implemented by the Norwegian University of Science and Technology (NTNU) and the Norwegian research company SINTEF.
The Sievers perforability test provides an estimate of the “hardness” (or resistance to indentation) of a rock surface: it is performed on a cylindrical specimen, the bases of which are plane parallel and obtained by cutting with a diamond saw.
The Sievers’ Miniature Drill Test is carried out thanks to a special drill – similar to the one shown in the figure above – equipped with a counterweight: the latter keeps Vidia points in pressure against the surface of the specimen for the entire duration of the test; this therefore consists in making a number of holes ranging from 4 to 8 and measuring their depth. The figure below shows an example of what the rock specimen looks like once the test is complete.
The average of the depths of these holes leads to a coefficient, known as Sievers’J-Value (SJ), which will in turn be used for the calculation of the Drilling Rate Index (DRI) and Cutter Life Index (CLI).
To obtain the perforability indices indicated by the NTNU-SINTEF standard, it is necessary to perform other drills, in addition to the Sievers’ Miniature Drill Test: the Brittleness test, the Abrasion Value test and the Abrasion Value Cutter Steel test.