It is essential to study the mechanical properties of saturated rock under different loading and unloading paths for strength calculation, safety assessment and disaster prevention; however, current literature rarely mentions conventional triaxial loading and unloading conditions. To analyze the mechanical properties, strain energy evolution characteristics and failure mode, a series of conventional triaxial unloading tests (with axial loading rate v_a of 0.06–6 mm/min and circumferential unloading rate v_u of 0.1–10 MPa/s) and conventional triaxial compression tests were carried out on saturated granite. The test results showed that the damage sources of specimens in the conventional triaxial unloading test were mainly related to circumferential deformation, while in the conventional triaxial compression test, it was related to the axial deformation. Under the same v_a, the confining pressure and axial stress at the failure point decreased with the increase of v_u, and the stress coordinate of the failure point was located outside the conventional triaxial compression envelope of σ₁–σ₃. As v_u increases, except for the variation of circumferential strain energy ΔU_c decreasing slowly, the trend of strain energy changes must be determined together with v_a. As v_a increases, the relationship between the magnitude of each energy changes from ΔU_a > ΔU > ΔU_d > ΔU_e > ΔU_c to ΔU_d > ΔU_a > ΔU > ΔU_e > ΔU_c, while the change of dissipated energy is dominated by v_u and v_a together to become dominated by v_a. In addition, with the increase of v_u and v_a, the damage pattern of the specimen also changes from shear damage in a single shear plane to mixed damage with tensile strain failure and shear plane during which the dilation angle of the specimen increases in total except for v_u = 10 MPa/s, v_a = 0.6 mm/min and 6 mm/min.

In recent decades, researchers have paid more attention to the indirect tensile test than to the direct tensile test (DTT) of rocks, mainly due to difficulties in the alignment and the stress concentration at the end of an intact cylindrical specimen. In this paper, a new flattened cylinder specimen and a clamp device were designed to obtain the true tensile strength of the rock in DTT. Stress distributions of the specimen with different lengths (l) and cutting thicknesses (t) were analyzed, and damage processes of the specimen were monitored by the Digital Image Correlation (DIC), the fractured sections were also scanned. Different mechanical parameters were also obtained by the DTT of the flattened cylinder specimens and the intact cylinder specimens, as well as the Brazilian disc. Research results show that the tensile strength obtained by DTT is smaller than the Brazilian disc and is slightly greater than the intact cylindrical specimen. The flattened cylinder specimen with 0.20 ≤ 2t/D < 0.68 and 0.10 ≤ l/D ≤ 0.20 is recommended to measure the true tensile strength of rock material in DTT. This new shape of the specimen is promising to be extended in the uniaxial or triaxial direct tension test.

Prediction on the crack propagation process of brittle rock under Thermo-Hydro-Mechanical (THM) loading conditions is very important for crack-arrest design and disaster prevention in deep rock engineering. Although currently existing fracture criteria can successfully predict crack initiation under arbitrary loading conditions, very few quantitative methods are available for predicting the crack propagation process of brittle rock under single loading condition, let alone THM loading conditions. In this paper, a new THM coupling fracture theory is established to predict the crack propagation process (including the crack initiation, stable and unstable propagation of Mode I or Mode II fracture), where the calculation formulae of Mode I and Mode II wing-tip stress intensity factors are deduced by a new superposition method with higher accuracy than the conventional superposition method and simpler form than the complex function method. Uniaxial compression tests and self-designed THM coupling tests of pre-cracked specimens are conducted to measure stress-strain curves and fracture trajectories under different loading conditions. Research results show that mechanism of crack initiation is the same as that of unstable propagation in both uniaxial compression and THM coupling tests. As the temperature and hydraulic pressure are increased and the confining pressure is decreased, the mechanisms of crack initiation and unstable propagation are changed from Mode II to Mode I. Mode I fracture has smaller loads of crack initiation load and unstable propagation and larger length of crack stable propagation than Mode II fracture. Test results agree very well with prediction ones, which proves the validity of the new THM coupling fracture theory. This fracture theory can be further extended to predict the multi-crack propagation process under THM loading conditions.

Based on our direct shear creep experiment and the direct shear rheological constitutive model, a semi-empirical time-dependent parameter of the shear strength is obtained by Mohr–Coulomb shear strength theory, and different time-dependent traction force calculations between deep-sea sediment and a tracked miner are conducted by the work-energy principle. The time-dependent traction force calculation under its influencing factors, including the time, track shoe number, and grounding pressure, are analyzed and proved to be valid by the traction force experiment of a single-track shoe. The results show that the time-dependent cohesion force obtained by a semi-empirical way can be easily used to deduce the time-dependent traction force models under the different grounding pressure distributions and applied into deep-sea engineering application conveniently; the verified traction force models by the traction force experiment of a single-track shoe illustrate that traction force under the decrement grounding pressure distribution is the worst among the four kinds of grounding pressure distributions and suggested for evaluating the most unfavorable traction force and calculating the trafficability and stability of the deep-sea tracked miner.