1. Introduction
The persistent demand for mineral resources is fueling a future industry of deep-sea mining, in which the vigorous exploitation and utilization of marine resources has become a national strategy of many countries in the world. Seafloor Massive Sulfides (SMS), formed by the hydrothermal or chemical processes on the seafloor, are rich in precious and base metals such as gold, silver, manganese, iron, cobalt, etc. [
1,
2,
3,
4,
5]. Currently, SMS deposits are primarily exploited by a spiral drum with cutting picks. During the cutting and crushing process, the cutting picks are bound to suffer from wear and tear, thus, it is necessary to study and predict the force on the cutting picks. However, due to the difficulty in obtaining the SMS samples in current experimental studies and the limitations of seabed exploration techniques, it is difficult to obtain the accurate mechanical properties of SMS and conduct the physical researches. Therefore, the numerical simulation for mining SMS using the discrete element method has become particularly significant.
Currently, little data is available on the mechanical properties of SMS samples. Kuiper et al. [
6] carried out experiments of crushing material samples to the breaking point, aiming to determine the compressive strength and tensile strength of the rocks under different high-pressure conditions. The unconfined compressive strength test and Brazilian test strength (BTS) test were performed to analyze the physical and mineralogical properties of insufficient SMS samples in [
7], in which the result revealed that the geotechnical properties of SMS samples were related to the mineralogical characteristics. However, the real SMS samples in [
6] were not taken into account, and the SMS samples in [
7] are relatively few. Therefore, the further investigation on the mechanical properties of sufficient SMS samples is necessary.
In order to investigate the cutting mechanism and the variation regularities of the cutting force in the crushing process, large amounts of researches on the rock cutting experiments have been carried out. Grima et al. [
8] conducted the cutting mechanism experiments in high-pressure vessels to simulate the cutting process and proposed an analytical model to predict the cutting force under high pressure. In addition, the proposed model was in good agreement with the experimental results. Kaitkay et al. [
9] carried out experiments to study the variation of the cutting force in terms of the cutting depth, rake angle and hydrostatic pressure under different hydrostatic pressures. It was concluded that the cutting force increased with the increase of the negative rake angle and hydrostatic pressure. Combining the rock mechanics with the fluid mechanics, Helmons et al. [
10] proposed a new method based on the discrete element method (DEM) to study the high deformation behavior in the seabed mining process, so as to estimate the local deformation rate. Moreover, Alvarez et al. [
11] developed a test unit to fully understand the high hydraulic cutting process and evaluated the force and power during the cutting process.
Due to the high cost of the high-pressure rock cutting experiments and the difficulty in directly observing the removal process, the numerical simulation of the cutting process has aroused great interest in the last decade [
12,
13]. As postulated by Estay and Chiang [
14], a new rock fracture modeling method based on the basic principle of the discrete crack model (DCM) and the fracture criterion was proposed to determine whether or when the rock was crushed. Considering the two-dimensional (2D) and three-dimensional (3D) models, the discrete element method (DEM) was applied to simulate the cutting process of the rock. Combining the unconfined compressive strength test with the indirect tensile strength test, the theoretical derivation and calibration method of the discrete element model were briefly summarized [
15]. The discrete element codes PFC2D and PFC3D were adopted to investigate the rock cutting process under various hydrostatic pressures [
16] and the force on the cutting picks was monitored [
17,
18]. Based on the continuum mechanics and discrete modeling method, Stavropoulou [
19] used the numerical code FLAC2D to simulate the cutting progresses of four marbles, and the results showed that the cohesive force and the internal friction were the key parameters affecting the drilling resistance. The UDEC program was successfully utilized to simulate the rock cutting process and obtain failure conditions [
20]. Onate et al. [
21] combined the discrete element method (DEM) with the finite element method (FEM) to study the dynamic problems of the geomechanics. Furthermore, the explicit finite element program, LS-DYNA, was successfully employed to explore the changes of the forces on the cutting heads [
22] and the lateral pressure on the tool forces [
23]. Rahman et al. [
24] applied EDEM and MSC.ADAMS to simulate the steady-state cutting process and investigate the dynamic interaction, respectively. EDEM has been proved to be able to fairly simulate the macro behavior of sand particles in [
25]. However, the above researches are mainly focused on ordinary rocks that are quite different from SMS.
Summarizing the aforementioned, the existing researches clearly show that the investigations on the mechanical properties of SMS and the cutting forces under high hydrostatic pressure are limited and the further investigations are necessary. Therefore, this work starts with the uniaxial compressive strength (UCS) and triaxial compressive strength (TCS) tests to obtain the mechanical properties of sufficient SMS samples. Then, the micro properties are determined through a calibration process. Eventually, the cutting processes of single and two cutting picks are simulated, and the force variation on the cutting picks in the process is obtained.