**1. Introduction**

New, and high-efficiency technology for rock breaking is required in the field of tunnel excavation, mining and mineral processing. The technology of microwaveinduced fracturing of hard rock has been considered as a potential method for

#### **Figure 1.**

*Schematic diagram of microwave-assisted mechanical rock cutting [4].*

rock, breaking due to various advantages including high efficiency and having no dust or noise pollution [1–3]. Realizing microwave-assisted mechanical rock cutting (**Figure 1**) using the microwave-induced hard rock fracturing technique can prolong the mechanical life and improve the efficiency of rock breaking operations [4–7]. At present, tunnel boring machines (TBMs) and shield machines have been increasingly used in tunnel excavation. The shield machine is subjected to a series of problems such as deformation of the tool apron and severe cutter wear due to the presence of boulders [8–10]. During the tunneling of hard rocks by TBMs, the disc cutter is worn and frequently changed-out, thus increasing the cost of maintenance and influencing construction progress [11–14].

The design of cutter heads of shield machines and TBMs is closely related to the properties of rocks and prevailing geological conditions [15, 16]. The mechanical strengths (uniaxial compressive strength, tensile strength, and point load strength, etc.) of rocks are important parameters influencing the service life and penetration of disc cutters on TBMs [17, 18]. Microwave treatment can significantly decrease the strength of rocks [3, 19–22], and thus can improve the penetration and life of disc cutters. Therefore, by introducing microwave heating technology into TBMs or shield machines, hard rocks or boulders can be pre-fractured through microwave irradiation. In this way, cutter wear can be reduced to increase efficiency in tunnel excavation.

Some scholars have carried out numerous experiments and numerical research into the mechanism governing the microwave-induced fracturing of rocks (or ores) and the influence of microwave treatment on the mechanical properties of rocks (or ores) [23–25]. Under the effect of microwave treatment, new intergranular and transgranular fractures are generated in rocks [26] to lead to the reduction of work index [27, 28] and strengths (including uniaxial strength, Brazilian splitting strength, and point load strength) of rocks [2, 29, 30]. More seriously, rocks are cracked and crushed or molten to cause rocks (or ores) to lose all of their bearing capacity [29, 31]. Hassani et al. [3, 6, 32] and Nekoovaght et al. [33, 34] studied the influence of microwave power and irradiation time on the strength of different kinds of rocks by using a frequency of 2450 MHz multi-mode cavity. In addition, they also studied the influence of the distance between the microwave antenna and the rock on the heating characteristics by experiments and numerical comparison.

**149**

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking…*

Peinsitt et al. [35] explored the effects of microwave irradiation on the strength, acoustic velocity, and heating effect of three types of dry and saturated rocks using a 2450 MHz multi-mode cavity. By using an open-ended waveguide set-up at a frequency of 2,450 MHz, Hartlieb et al. [36–38] explored the failure mechanism and thermal physical characteristics of different types of rocks. Lu et al. [29, 39, 40] studied the mechanism, temperature distribution, and crack propagation of microwave-

At present, the technology of microwave-induced fracturing of hard rock is still in the laboratory research stage. By summarizing the research results of relevant scholars in this field, this work generalizes the mechanism of microwave heating of rock, microwave heating system, heating characteristics, and the effect of micro-

When a dielectric material is subjected to an alternating current, it absorbs electrical energy, which is dissipated in the form of heat (the dielectric loss). The dielectric constant of the material consists of the real part and the imaginary part,

εε

"

''

ε

ε

= −' *j* (1)

∂ = (2)

<sup>2</sup> *P*=2 "E f πε ε<sup>0</sup> (3)

Q T = ∆ *Cm* (4)

 ε

where the real part (ε') is known as the dielectric constant. The imaginary part

The loss tangent (tan δ) is the ratio of the imaginary part (ε") to the real part

tan '

It measures the ability of the dielectric to store energy and convert it into heat. The microwave absorption capacity of electrolyte material is related to its dielectric properties. The microwave heating mechanism of minerals and rocks in the electromagnetic field is usually expressed by the power density, which can be

where *P* is the loss power density deposited within the sample; *E* is the electric field and *f* is the microwave frequency; ε0 is the dielectric constant of free space

The temperature of the dielectric material increases when it absorbs microwave

energy [41]. According to the laws of thermodynamics the amount of energy required to increase the temperature of a material to a given amount is calculated by

*DOI: http://dx.doi.org/10.5772/intechopen.95436*

heated rocks using a multi-mode resonator.

**2. Principle of microwave heating rocks**

(ε") is known as the loss factor [41].

expressed by the following Equation

(8.85 × 10−12 F/m) [42].

the following equations

as shown below

(ε'), i.e.

wave heating on rock cracking and mechanical properties.

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking… DOI: http://dx.doi.org/10.5772/intechopen.95436*

Peinsitt et al. [35] explored the effects of microwave irradiation on the strength, acoustic velocity, and heating effect of three types of dry and saturated rocks using a 2450 MHz multi-mode cavity. By using an open-ended waveguide set-up at a frequency of 2,450 MHz, Hartlieb et al. [36–38] explored the failure mechanism and thermal physical characteristics of different types of rocks. Lu et al. [29, 39, 40] studied the mechanism, temperature distribution, and crack propagation of microwaveheated rocks using a multi-mode resonator.

At present, the technology of microwave-induced fracturing of hard rock is still in the laboratory research stage. By summarizing the research results of relevant scholars in this field, this work generalizes the mechanism of microwave heating of rock, microwave heating system, heating characteristics, and the effect of microwave heating on rock cracking and mechanical properties.
