Experimental study on artificially induced crack patterns and their consequences on mechanical excavation processes

This paper discusses the influence of pre-conditioning by microwave irradiation of granite on crack formation and subsequent mechanical cutting of granite with UCS of 210 MPa and CAI of 4.2. With the help of micro- and macroscopic analyses (CT-scans, linear cutting test rig) it will be shown how high-power microwave irradiation with 24 kW influences the structure of the rock and leads to local damage directly in the area of the beam and to global damage in the area surrounding area. The result is a pronounced network of cracks stretching out over the entire surface of the rock sample with a depth wise extension of approx. 100 mm. This artificially introduced crack network will lead to a significant reduction in required forces when cutting the material (6.04 to 4.29 and 6.26-5.22 kN for 8 and 12 mm cut spacing, respectively) and consequently also to a reduction in the specific energy consumption for the cutting process (50 down to 35 kWh/m³). Additionally the size distribution of the fragments will be changed to coarser sizes with ×50 changing from 0.97 and 2.05 to 1.51 and 2.39 mm for 8 and 12 mm cut spacing, respectively.