Mechanical Behavior and Failure Mechanism of Lacustrine and Marine Shales under Combined Static and Dynamic Loadings

Rock engineering applications and geological energy recovery demand a thorough evaluation of the strength characteristics and fracture mechanisms of shales under dynamic impact conditions and hydrostatic in situ stress. The modified split Hopkinson pressure bar test was used to conduct coupled static and dynamic loading for marine and lacustrine shales under different confining pressures (P_c). Mechanical properties and failure modes were elucidated through energy evolution patterns and characterizations of the surface morphology, inorganic mineral, and total organic carbon on fractured surfaces. A model of the Pearson correlation coefficient was established to analyze the factors. The results show that the characteristics of stress–strain curves changed from ductile to brittle under the effect of P_c, along with deformation hardening and improved potential to resist impact loading. The deformation and failure of shales under P_c were restrained, and the fractured surfaces were intact and flat. The dependences of the yield strain, elastic modulus, and ultimate strain on P_c were weaker in marine shale than in lacustrine shale. Peak energy lagged behind peak stress in the absence of P_c, while the two were synergistic under P_c. An increase in transmitted energy indicated nonreflective or weakly reflective propagation of the stress wave. Mechanical properties slightly responded to compositions under unconfined and high confining pressures (P_c = 25 MPa) induced by rapid fragmentation and difficulty in failure, respectively. The correlation between compressive strength and compositions was stable after applying P_c. The understanding of the rock failure behavior based on the fractured surface and transmitted energy evolution facilitates the control of gas/rock outbursts and the optimization of enhanced shale gas exploitation techniques.