# The Law and Mechanism of the Effect of Surface Roughness on Microwave-Assisted Rock Breaking
## 1 Introduction  
Hard-rock excavation has been a key problem in underground space excavation and resource exploitation, and it has been the focus of study by many scholars for a long time. Traditional rock underground space excavation is mainly performed using the drilling and blasting methods and the mechanical method [1, 2]: The drilling and blasting methods induce large disturbances in the original rock, which can easily cause damage to the surrounding rock, and have low construction accuracy. With the rapid development of mechanical excavation, the mechanical method has overcome the deficits of most drilling and blasting methods, but it still exhibits large wear and low efficiency when excavating hard rock. Therefore, the development of an energy-saving, efficient, and economical hard-rock crushing method is a necessary part of the development of underground engineering excavation technology [3]. The strength of rock is significantly weakened after microwave irradiation; thus, some scholars have proposed the application of microwave radiation to mechanical rock breaking with many studies having demonstrated significant results. Microwave heating has the characteristics of bulk heating, rapid speed heating, energy saving, high efficiency, easy control, and safety. Microwave irradiation can be used as anauxiliary means of rock breaking and applied to hard-rock excavation engineering [4, 5]. Some preliminary research results on microwave-assisted rock breaking are as follows: [PERSON] et al. [6, 7, 8] explored the mechanism of microwave radiation damage to rocks and found that the higher the mineral iron content in the rock, the more serious the rock damage after microwave irradiation; [PERSON] et al. [9, 10, 11] found that the microwave irradiation power and irradiation time are the main factors affecting the damage and strength weakening of the internal structure of rocks, and when the power increases to a certain extent, the influence of irradiation time on the strength of rock and the weakening degree of the internal structure is gradually significant; [PERSON] et al. [12, 13] studied the effect of microwave heating pathways on the effect of microwave radiation on rock and found that, when the microwave energy input was consistently maintained, the rock-breaking effect was better when following the irradiation pathway of 'first low and then high'; [PERSON] et al. [14, 15] found that the response of rocks to temperature increases under microwave irradiation was mainly related to the dielectric properties and mineral content of the rocks themselves; [PERSON] et al. [16, 17] found that the effect of microwave irradiation can be improved by increasing the water content of rock mass, but the effect will decrease with an increase in water content; [PERSON] et al. [18, 19, 20] found that, under different stress conditions, a complex crack network dominated by tensile cracks was generated in the rock samples after microwave-induced borehole rupture under different stress conditions, and the thermal fracture process could be divided into a silent period, calm period, dense period, and continuous period; [PERSON] et al. [21, 22, 23] found that the microwave irradiation law of different irradiation distances decreased when the irradiation distance increased. The microwave irradiation surfaces of the rock samples in the abovementioned studies were set to be flat and smooth, which is inconsistent with actual engineering situations. At present, research on the laws and mechanisms of the effect of microwave irradiation on rough rock surfaces is still lacking. The tunnel boring machine (TBM) creates rough rock surfaces, which will inevitably influence the microwave irradiation effect. In previous studies, the sample surfaces were assumed to be smooth and flat, which is far from the actual engineering scenario. In the author's preliminary experiments, it was found that rough rock surfaces have a certain influence on the effect of microwave-assisted rock breaking.