## 5 Discussion  
Based on the experimental results in this study as well as the data collected from the references, it has been found that the variation behaviors of cohesion and the internal friction angle have two different types for the granite samples after high-temperature and water-cooling treatment, while the tensile strength changes in the similar way. It is necessary to understand the mechanism of these behaviors. As shown in some studies, this mechanism should be closely related to the mineral composition in the samples [39; 40; 41; 42].  
For each single group of the experiments, the samples have almost the same mineral components and micro-structures, and the treatment method is also the same, so the variation of the strength parameters should be mainly dependent on the damage evolution or crack propagation owing to the heating and cooling effects. Reference [43] explained that the behaviors of cohesion weakening and friction strengthening are resulted from the crack propagation during the damage process of the rock. Therefore, it is reasonable to explain the two types of cohesion and the internal friction angle variation behaviors with different cracking process during the heating and cooling treatment.  
For the continuous decreasing behavior of cohesion (type I), it should be resulted from the growing cracks in the granite samples with increasing treatment temperature before the water cooling. However, cohesion of some samples increases when the treatment temperature is not very high (type II). This behavior should be related to the expansion of the minerals and closure of the cracks owing to the heating treatment before the water cooling. References [44; 45] also reported that the strength values of granite samples have an increase with the increasing temperature within a certain range, and reference [44] presented the decreasing width of cracks with increasing temperatures ranging from 65 \({}^{\circ}\)C to 160 \({}^{\circ}\)C in SEM images.  
Reference [43] shows that the growth of cracks may lead to the increase of internal friction angle, therefore the increasing behavior of the internal friction angle (type II) should be due to the growing cracks with increasing treatment temperature before the water cooling. For type I behavior of the internal friction angle, it does not change significantly. This should be owing to the limited crack number in the samples (e.g., Fangshan granite, Sichuan granite, Wuyi granite, and Rizhao granite--1 & 2 samples), as the decreasing of cohesion is not very significant for these type of samples, either.  
However, it should also be noted that although the variations of cohesion and the internal friction angle show different behaviors, their tensile strength values drop in the similar way with the increasing treatment temperature. [PERSON] (1967) observed that the crack initiation and propagation stresses of norite under uniaxial tension are 95% and 97% of the peak tensile strength, respectively [46]. [PERSON] et al. found that AE (acoustic emission) initiation stress level of medium grained granite in direct tensile tests with a single-notched plate is 98% of the peak tensile strength [47]. [PERSON] also concluded that under overall tensile loading, the stress levels of crack initiation and crack propagation are very close to the peak strength [48]. This means that the tensile strength is more sensitive to the induced cracks. Consequently, for the type I granite samples, even very limited amounts of induced cracks may lead to a decrease of the tensile strength.