## 1 Introduction  
Water has a great impact on the safety of the rock engineering. For example, seawater intrusion will threaten the construction and operation of subsea tunnels [1], damage the structure of underground sealed oil storage or reservoir [2; 3] or cause collapse due to coastline erosion [4]. Moreover, water has a significant impact on the construction and operation of urban rock tunnels [5; 6]. Combined with the influence of rock weathering and tunnel construction blasting, water is easier to enter the tunnel through some channels [7; 8; 9; 10; 11; 12]. Water also affects the safety of slopes and chambers. For example, water will weaken the stability of red bed soft rock bank slope [13]. Water intrusion or erosion mainly occurs in areas with poor rock mass quality, which mainly depends on the mineral composition (especially clay minerals) and porosity of rock mass, solute composition and high salinity of water solutions, and the action of external environment [14; 15]. The prediction of water absorption of soft rock is very important for the failure prediction of rock engineering. If the critical time point of rock damage due to the change in water absorption cannot be judged, the prevention and control of rock engineering disasters will be too conservative or too risky [16]. However, existing research studies mainly focus on the weakening effect of water on rock physical and mechanical properties [17; 18], or natural rock samples and natural water solutions are used to study the water absorption of soft rock. Under the influence of anisotropic factors, the influence of composition, porosity and solute on the water absorption of soft rock cannot be quantitatively analyzed [19]. Therefore, this paper needs to study the test method and prediction model of water absorption of soft rock considering the effects of composition, porosity and solute quantitatively. At present, there are many studies on rock water absorption. Some scholars have studied the effect of solute composition of water on rock water absorption. For example, [PERSON] et al. studied the adsorption of rocks rich in clay minerals in water with different mineralization [20]. [PERSON] et al. studied the effects of solute type and solution concentration in solution on chemical permeability, diffusion and hydraulic parameters of mudstone, and found that solute type has little effect on rock water absorption, while solution concentration has a great effect on rock water absorption [21]. [PERSON] et al. studied the water absorption characteristics of rocks in solutions with different chemical properties and obtained the effects of different chemical paths on the water absorption process of clay rocks [22]. Some scholars mainly pay attention to the influence of rock porosity on its water absorption. For example, [PERSON] and [PERSON] et al. studied the relationship between physical properties such as porosity and water absorption of rocks and mechanical properties, and found that the greater the porosity, the greater the water absorption of rocks [23; 24]. [PERSON] et al. studied the water (solution) absorption characteristics of natural clay-based materials with different porosities and found that osmotic swelling and its influence on pore structure are the key influencing factors of the rock water absorption process [25]. [PERSON] studied the effect of alteration on rock water absorption and found that the formation of secondary facies changed the rock pore space (effective porosity) and played a leading role in controlling rock water absorption [26]. Moreover, the rock porosity can be obtained by ultrasonic velocity, image processing technology, neural network method, fractional flow theory, etc. [27; 28; 29; 30; 31; 32; 33]. Some scholars also mainly study the influence of clay minerals on rock water absorption. For example, [PERSON] et al. studied the change of water absorption over time of rocks containing different types and contents of clay minerals (silststone, mudstone, marl, tuff) [34]. [PERSON] et al.