## 2 Samples and Methods
### Experimental Procedure  
#### 2.2.1 Compression Experiments  
Forty-eight shale samples were dried in a thermostatic drying oven at 85 \({}^{\circ}\)C for 24 h. In order to study the difference in mechanical properties between bedding shale and lamellar shale, 24 rock samples were selected for uniaxial mechanical tests. The 24 rock samples have four axial direction-lamina (bedding) angles (\(\beta\)), each of which has three bedding shale samples and three lamellar shale samples. First, according to the \"Regulation for testing the physical and mechanical properties of rock--Part 5: Test for determining the water absorption of rock\" (Chinese Standard, Standard ID: DZ/T 0276.5--2015), two samples from each \(\beta\) of bedding shale and lamellar shale were hydrated in distilled water at 80 \({}^{\circ}\)C for 24 h and 48 h, respectively. Then, the total of 24 samples after drying following the 24-h and 48-h hydration tests, were subjected to uniaxial compression test. The testing method is according to the \"Regulation for testing the physical and mechanical properties of rock--Part 18: Test for determining the uniaxial compressive strength of rock\" (Chinese Standard, Standard ID: DZ/T 0276.18--2015).  
In the remaining 24 lamellar shale samples after drying, two samples from each \(\beta\) were taken for 24-h hydration test, and two samples from each \(\beta\) were taken for 48-h hydration test. In this way, two sets of samples were obtained. One piece of a dry sample, one piece of 24-h hydration sample and one piece of 48-h hydration sample under each \(\beta\) in a set. To further explore the effect of hydration on mechanical properties of lamellar shale, the two sets of samples were subjected to triaxial compression tests with confining pressures of 15 MPa and 30 MPa, respectively. The testing method is according to the \"Regulation for testing the physical and mechanical properties of rock--Part 20: Test for determining the strength of rock in triaxial compression\" (Chinese Standard, Standard ID: DZ/T 0276.20--2015).  
The uniaxial compression tests and triaxial compression tests were all conducted using a triaxial rock mechanics testing apparatus (RTR--1000, GCTS Ltd., Phoenix, AZ, USA). The axial loading rate was 0.03 mm/min. This loading rate was determined according to the strain control loading method in the two Chinese standards as mentioned above. Compared with the stress control loading method [31], the strain control loading method has an advantage that the stress-strain curve of residual stress stage can be easily obtained. During the experiments, the axial stress, axial and radial deformation data of the sample were recorded in real time. The calculation methods of the compressive strength (deviatoric stress), Poisson's ratio, elastic modulus and other mechanical parameters refer to the literature [32]. The shale anisotropic magnitude is described by \(R_{C}\)[33]:  
\[R_{C}\frac{\sigma_{ci(90)}}{\sigma_{ci(\min)}} \tag{1}\]  
where \(\sigma_{ci(90)}\) is the uniaxial compressive strength of the sample for \(\beta\) = 90\({}^{\circ}\), and \(\sigma_{ci(\min)}\) is the minimum value of \(\sigma_{ci}\) obtained from oriented samples of uniaxial compression tests.