Granite is considered as one of the most important types of host rock for geological disposal of high-level radioactive waste (HLW) [
1,
2,
3,
4,
5,
6,
7,
8,
9]. An appropriate modelling on the mechanical behavior of granite is of great importance for site selection and design of the repository [
9,
10,
11,
12,
13]. Specially, it should be noted that the heat induced by the nuclear waste may have considerable influences on the mechanical behavior of the host rock, so the thermal effect cannot be ignored [
8,
12,
14,
15,
16,
17].
There have been extensive studies on the mechanical behaviors of the host rock for HLW disposal [
1,
9,
18,
19,
20,
21,
22]. In many studies, simultaneously mobilized Mohr-Coulomb strength parameters (cohesion and friction angle) were used in the modelling of the underground excavation [
23,
24,
25,
26,
27]. In order to describe the plastic strain softening behavior of the rock, they assumed that both cohesion and friction angle degrade from the initial value to the residual value with the increasing plastic strain, and piecewise linear models were usually adopted [
23,
24,
25,
26,
27]. However, based on a series of theoretical analyses and laboratory experimental studies on cohesive soil, granite, marble, limestone, etc., it has been found that the geo-materials always show non-simultaneous mobilization of strength components, i.e., cohesion will be degraded and friction angle will be enhanced with the increasing damage or plastic strain during the failure process [
26,
28,
29,
30,
31,
32,
33,
34,
35]. This is owing to the development of cracks inside the rock decreases the cohesive strength, while the induced crack surfaces make the frictional strength increases [
26,
29,
36]. Accordingly, the cohesion weakening friction strengthening (CWFS) model was proposed, and this model with linear equations was used for modelling the failure process of URL Mine-by tunnel [
26,
29,
36]. A comparison study shows that CWFS model can capture the failure extent and depth of failure (DOF) of this circular excavation better than the other widely used models such as elastic model, elastic-perfectly plastic model, elastic-brittle model, etc. [
26]. Thereafter, more linear CWFS models are used in the researches and give reasonable simulations on the stability of underground openings, pillars, as well as the process of crack propagation [
30,
37,
38,
39]. Nonetheless, it was pointed out that the linear CWFS model may result in a problematic behavior of the stress – strain curves, and a fitted non-linear CWFS model with smooth curves was proposed, which was proved to be able to capture the gradual damage process better [
31]. More recently, reference [
40] proposed the guidelines for the parameters selection for CWFS modelling analysis of excavations. Up to date, the concept of CWFS analyses has widely been accepted in modelling the failure of brittle rocks.
However, the studies on the mobilization of strength parameters mainly focused on the Mohr-Coulomb criterion. As an actual fact, Hoek-Brown criterion is also widely used in the modelling of field rock engineering [
41,
42]. There were some studies using piecewise linear models with simultaneous mobilization of Hoek-Brown strength parameters (
and
) to analyze rock behaviors [
23,
24,
26,
43]. Nevertheless, according to the above-mentioned analyses on the non-simultaneous mobilization of cohesion and friction angle, we should notice whether the Hoek-Brown strength parameters may also be mobilized non-simultaneously during the damage and failure process of rock. If the answer is yes, what is the characteristics of this mobilization? What is the relationship between the mobilized Hoek-Brown strength parameters and the rock damage or plastic strain? This has not been investigated in the published researches, and it is required to conduct a detailed study.
In the recent several decades, extensive field and laboratory researches have been carried out in the site selection of HLW disposal in China [
1,
2,
4,
10,
15,
20,
22,
44,
45]. Alxa candidate area in Inner Mongolia is one of the three candidate areas with large volume of granitic rock.
Figure 1 presents the location of Alxa area with two sub-areas (TMS and NRG), as well as the main geological structures around this area. More detailed information about Alxa area has been provided in reference [
1]. Field investigations on the outcrops have been conducted and four boreholes (named as TMS01, TMS02, NRG01 and NRG02) with the depth of 600 m have been drilled. Laboratory experiments on the cored samples have also been carried out for studying the mechanical properties of the rock. These researches show that the granite around NRG01 borehole shows the best rock mass quality in Alxa candidate area [
1]. Nonetheless, further studies should still be conducted on NRG01 granite samples with coarse grains for a better modelling on the mechanical behaviors. What is the characteristics of the mobilization of cohesion and friction angle for NRG01 granite samples? How will the heat produced by the nuclear waste affect the mechanical behavior of NRG01 granite samples during the damage and failure process? Will the mobilization of Hoek-Brown strength parameters occur for NRG01 granite samples in a simultaneous or non-simultaneous way? Is there any suitable equations to describe this mobilization? What is the mechanism?
Based on a series of systematic uniaxial and tri-axial compression experiments on NRG01 granite samples treated by different temperatures, mobilization of both Mohr-Coulomb and Hoek-Brown strength parameters have been analyzed in details. This paper is organized as follows: In
Section 2, the physical and mechanical properties of the samples, the experimental setup and methods will be introduced. The experimental results will be presented in
Section 3.
Section 4 will provide the systematic data analyses and discussions on both the mobilization of Mohr-Coulomb and Hoek-Brown strength parameters during the failure process of NRG01 granite samples under different heat treatments. Based on the above-mentioned analyses and discussions, some conclusions will be drawn in
Section 5.