# Effects of Water-Soaking Height on the Deformation and Failure of Coal in Uniaxial Compression
## 1 Introduction  
Water-conducting fissures will be formed in overburden strata during the coal mining process, which results in a large amount of mine water in gob area [1]. The coal and rock mass around gob will inevitably be in a certain mine water environment [2]. Due to the difference in water supply and drainage in the gob, the amount of water in the gob will be different. The water-preventing coal pillars may be soaked in water-accumulated gob with different water amounts, then the soaked height of the coal pillars in the gob water will be different. Coal is a porous medium [3], water will be absorbed from the bottom to the upper part of the coal pillar due to capillary force, while the gravity will hinderthe diffusion of water in the longitudinal direction [4]. Consequently, the diffusion of moisture in the coal pillar has a certain range [5]. Different soak heights result in a nonuniform distribution of water in the longitudinal direction of the coal pillar.  
Due to the water-rock interaction, deformation and failure characteristics of coal pillars soaked in water will change significantly. Water-rock interaction is a widespread and challenging issue in rock mechanics and engineering [6].  
A considerable number of studies have been carried out to investigate the influence of water content on mechanical properties and failure characteristics of coal and rock mass [7; 8; 9; 10; 11; 12]. According to former studies, the uniaxial compressive strength, elastic modulus, and tensile strength of the rock all decline as moisture content increases.  
Acoustic emission (AE) monitoring is a useful technique to capture the failure process of rocks [13; 14]. Many researchers [15; 16; 17; 18; 19] analyzing the mechanical properties of coal and rock with different water content combined acoustic emission data. Previous results have shown that there is a negative relation between the acoustic emission count and water content, and the increasing moisture content promotes creep damage in coal and rock.  
In terms of the damage model of coal and rock mass affected by water content, [PERSON] [15] applied statistical damage theory and strain equivalent hypothesis to derive a damage constitutive model of coal specimen which can reflect the influence of water content. [PERSON] et al. [20] prepared uniaxial compression tests of raw coal samples and briquette samples with different water contents, analyzed the relationship between mechanical parameters and water content of two coal samples, and established a segmental damage constitutive model considering moisture content. Based on the generalized strain equivalence principle and the statistical microscopic damage mechanics theory, [PERSON] [21] established a damage constitutive model considering the weakening effect of water and uniaxial load.  
Most of the former research focused on the influence of water content on the mechanical properties of coal and rock mass. However, such studies treated the water content as the only variable affecting the mechanical behaviors of the coal and rock specimens, neglecting the effects of the water distribution within them. [PERSON] [22] and [PERSON] [23] found that the distribution of water affects the mechanical properties of rock mass materials.  
The stability of coal pillars has an important impact on safe and efficient mining for coal mines [24]. Coal pillars in the gob water may be soaked in different heights due to different amounts of accumulated water, the water distribution within coal pillars may be different as a result. There are few studies on the deformation and failure of coal pillars with different soaking heights. Laboratory uniaxial compressive tests for different soaking heights of coal specimens are carried out to investigate the failure characteristics and mechanisms. It is helpful to gain a deeper understanding of the damage mechanism of water-bearing coal and rock mass and provide more scientific guidance for the retention of water-preventing coal and rock pillar.