How to improve mechanical hard rock excavation?

It is important to have reliable prediction methods for mechanical excavation tools. I.e. when pre-damage methods like high-power microwaves, lasers, water jets or others are applied theoretical investigations on the possible improvements are inevitable. Montanuniversitaet Leoben performed two different studies on the subject (one for roadheaders, one for full-face tunnel boring machines). See the abstracts and results her:

Fracture pattern after pre-damaging of rocks (left: real; right: simplified sketch)

Roadheader study

Theoretical Investigations on the Influence of Artificially Altered Rock Mass Properties on Mechanical Excavation; Hartlieb and Bock (2018); DOI: 10.1007/s00603-017-1355-3

This study presents a theoretical analysis of the influence of the rock mass rating on the cutting performance of roadheaders. Existing performance prediction models are assessed for their suitability for forecasting the influence of pre-damaging the rock mass with alternative methods like lasers or microwaves, prior to the mechanical excavation process. Finally, the RMCR model was chosen because it is the only reported model incorporating a range of rock mass properties into its calculations. The results show that even very tough rocks could be mechanically excavated if the occurrence, orientation and condition of joints are favourable for the cutting process. The calculated improvements of the cutting rate (m³/h) are up to 350 % for the most favourable cases. In case of microwave irradiation of hard rocks with an UCS of 200 MPa a reasonable improvement of the performance by 120% can be achieved with as little as an extra 0.7 kWh/m³ (= 1% more energy) compared to cutting only.

Different damage modes and possibilites for pre-fragmentation

TBM study

Pre-Conditioning of Hard Rocks as Means of Increasing the Performance of Disc Cutters for Tunneling and Shaft Construction; Hartlieb and Rostami (2018)

Excavation of very hard intact rock by various mechanized systems is challenging and requires high cutting forces. This refers to tunneling through rock that is in excess of 200–300 MPa, which has also been considered difficult tunneling conditions by the ITA Workgroup 14 that is specialized in mechanical excavation. There are innovative concepts to improve the cuttability of hard and abrasive rocks by using pre-conditioning systems for introducing micro-fractures in the rock. This includes high-power microwave irradiation which leads to weakening of the rock by introducing micro fractures in the medium. The initial results of treating rock surface with a 24 kW microwave shows initiation of some micro-crack network and the preliminary results will be discussed in this paper. The findings will be linked to possible improvements of tunnel/shaft boring machines (TBM, SBM) performance, especially with respect to possibility of deploying a hybrid system to aid the center and gage cutters by pre-damaging the rock.

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