# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 1 Introduction  
Hydro-mechanical coupling effect should be considered in rock engineering, petroleum extraction, and enhanced geothermal exploitation. Yet the mechanical properties and cracking behavior of rock mass under hydro-mechanical coupling have not been comprehensively understood.  
In the last decades, a series of related researches have been conducted to investigate the basic failure mechanism of jointed rock mass [1; 2; 3; 4; 5; 6; 7]. Scholars have summarized the crack types of fractured rock mass under uniaxial compression, and crack types initiated from the pre-existing flaws have been proposed.  
[PERSON] and [PERSON] [8; 9] investigated the cracking behaviors in modeled gypsum and Carrara marble specimens with a single flaw under uniaxial compression by a high-speed camera. Seven different crack types were identified based on their geometry and tensile or shear propagation mechanism. [PERSON] [10] analyzed the failure mode and cracking process of sandstone specimens containing a single flaw under uniaxial compression, and proposed nine different crack types based on their geometry and crack propagation mechanism (tensile, shear, lateral crack, far-field crack, and surface spalling). In addition to uniaxial or biaxial compression tests [2; 7; 8; 9; 10; 11; 12; 13; 14], triaxial compression tests [15; 16; 17; 18; 19] are also used to study jointed rock masses. The cracking behavior is more complex and difficult to observe under three-dimensional conditions. However, many researches using three-dimensional conditions, judged the crack types by crack geometry [17; 18; 19; 20; 21]. They either refer directly to the crack types obtained by uniaxial or biaxial compression tests or they lack the detailed analysis of the mechanical (tensile/shear) properties and geometric characteristics of three-dimensional cracks.  
Some new techniques can help determine crack geometries and propagation mechanisms. To investigate the three-dimensional (3D) cracking behavior of rock, X-ray computed tomography (CT) has been adopted to detect internal cracks [22; 23; 24; 20; 25]. The X-ray CT scanning overcame the difficulties in observations within nontransparent materials, and the spatial geometry can be determined in a nondestructive way. Additionally, the microscopic observation by a scanning electron microscope (SEM) provides a basis for judging the tensile or shear properties [26; 27; 28; 29; 21]. These studies provide evidence that CT scanning and SEM observation are effective ways to explore internal crack behavior and propagation mechanism (shear/tensile). However, hydro-mechanical coupling was not taken into account in these studies.  
From the above introduction, crack geometries, propagation mechanisms, as well as the failure mode of jointed rock mass under hydro-mechanical coupling have not been adequately studied in past researches. This paper makes an effort to fill this gap by CT scanning and SEM observation. A conventional triaxial test, hydraulic coupled triaxial test, and a series of uniaxial compression tests were conducted, in which the cylinder gypsum specimens with a single pre-existing flaw were employed. Then the mechanical and cracking behavior was analyzed, and detailed investigations based on CT and SEM were carried out. Finally, nine types of cracks that led to specimen failure were summarized, and the effect of water pressure and confining pressure was discussed.