Full scale linear cutting tests are regarded as a reliable basis for performance prediction of hard rock Tunnel Boring Machines (TBM). However, obtaining suitable specimen is difficult and the effort to perform a feasible number of cutting tests is significant. This paper presents a newly developed small scale rock cutting test apparatus which can be installed in conventional hydraulic testing machines typically available in common rock mechanics laboratories. The specimen are obtained from drill cores which are available at early project stages. Results are compared with full scale rock cutting tests and geotechnical standard parameters. Consequently, the new test might prove a valuable tool for performance prediction of mechanically excavated underground structures.
Accurate performance prediction is one of the main concerns in determining the construction time of excavations utilizing mechanical excavators such as tunnel boring machines (TBM). Typically, performance prediction models are based on geotechnical parameters like uniaxial compressive and indirect tensile strength as their main input parameters. The most established model of this kind is the semi-empirical/ semi-theoretical performance prediction model of the Colorado School of Mines (Rostami 1997). It was developed by combining basic theoretical relationships with laboratory data obtained from full scale rock cutting tests.
Besides the use of models, the best net penetration rate prediction can be obtained by conducting full scale linear cutting tests. The first well-known linear cutting machine was built in the 1970's at the Earth Mechanics Institute of the Colorado School of Mines (Ozdemir et al. 1976). Today, there are several linear cutting rigs which are used for the evaluation of rock cuttability (e.g. Balci & Bilgin 2007, Jeon et al. 2006, Abu Bakar & Gertsch 2012).
Inspite of providing reliable results, the conduction of full scale cutting tests is costly and time-consuming. In addition, large rock blocks are needed which are typically not available during an exploration phase of a tunnel project. Hence, full scale cutting tests are rarely carried out for performance prediction purposes before TBM excavation starts. In order to overcome these downsides, cutting tests on a smaller scale might prove useful because rock failure mechanisms are identical to the real excavation process except from scale effects.