## 1 Introduction  
Red clastic rocks weathered in laterite from the Mesozoic-Cenozoic era is termed \"red-bed\" [1]. The red bed mainly incorporates lacustrine deposits, fluvial facies deposits, and piedmont diluvial deposits. Moreover, the red bed is primarily composed of mudstone, sandstone, siltstone and shale. Figure 1 shows the state of red-bed soft rock in different scales. Normally, the meso-structure of red-bed soft rock shows discontinuity and anisotropy [2] caused by original defects (e.g., pores and cracks) [3]. Meanwhile, the red-bed soft rocks are rich in clay mineral (e.g., montmorillonite and kaolinite) which is sensitive to hydration-induced damage [4]. Hence, the special characteristics of red-bed soft rocks occasionally bring engineering disasters [5; 6; 7; 8; 9] (e.g., structure collapse in deep soft rock tunnel, slope slide, water infiltration induced deterioration, and creep induced large deformation problems). The study on deterioration mechanism is important to academic research and also the engineering guidance. However, the above-mentioned factors make it difficult to study the failure process in a precise way. In addition, the water induced failure propagation in red-bed engineering is regarded as a multi-scale issue [10], including the dissolution and slide of clay minerals in meso-scale, the crack propagation and coalescence in meso-scale, and the strength deterioration in macro-scale. Dealing with such a complex process, we need to find an applicable method. However, researches on the deterioration process in red-bed soft rock are limited either on meso-scale or on macro-scale. Nowadays, the researches of red-bed soft rock's failure process mainly involved in triaxial tests under ultimate environments and meso-mechanical analysis basing on image technologies. On the one hand, basing on mechanical experiments, [PERSON] et al. [11] studied the influence of micro parameters, (including porosity, particle radius, elastic modulus), on the strength of porous sandstone. [PERSON] et al. [12] proposed a stress-strain relation that is more applicable to porous soft rock by uniaxial and triaxial tests. [PERSON] et al. [13] studied the pre-peak mechanical behavior of red-bed soft rock through loading, tensile, and shear tests under low confining pressures. Meanwhile, many scholars have studied the failure mechanisms of red-bed soft rock considering water corrosion [14; 15; 16; 17; 18; 19; 20]. When red-bed soft rocks encountering poor engineering conditions, [PERSON] et al. [21] conducted triaxial tests of porous soft rock under dry and wet environments to obtain the law of permeability variation and shear zone distribution. [PERSON] et al. studied the shear mechanisms of red-bed soft rock through shear test and CT analysis [22]. On the other hand, basing on the image technology, many researches of SEM [23; 24] (Scanning Electron Microscope), AE [25; 26] (Acoustic Emission), CT [2; 27] (Computed Tomography) are carried out. [PERSON] et al. [28] found that the red-bed soft rocks generally develop various micro-defects around the clay mineral aggregations, and studied the expansion mechanism during the water immersion process. [PERSON] et al. [29] analyzed the disintegration mechanisms of mudstone from the perspective of mineral composition, and proposed a discontinuous network model. [PERSON] et al. [30] studied the effects of water corrosion mechanisms on the micro-scale of soft rocks. In general, these researches are limited in macro-mechanical deterioration or meso-structure damage analysis. However, we may want to know more about the relation between meso-damage and macro-failure in the damage evolution process. In addition, the critical point that separate the stable state and unstable state is also necessary to be predicted which is quite important to the engineering guidance. Note that, the water induced deterioration is a phase transition process (from stable phase to unstable phase).