## 1 Introduction  
The presence of water can markedly alter the mechanical behavior of rock materials [1; 2; 3; 4; 5; 6; 7], which are responsible for a variety of geological disasters such as landslides [8], goat collapse [9; 10] and fault activation [11; 12; 13]. In many engineering applications such as tunnels, mines, basements and radioactive waste storage, rocks are commonly subjected to water erosion resulting from construction beneath the groundwater table. The properties of constructional materials under such conditions are expected to be significantly different from those in a dry state. Therefore, it is crucial to investigate the water saturation effects on the mechanical behavior of rocks for the design and construction of underground rock engineering. Massive efforts have been made to explore the water-induced changes in rock properties [1; 2; 3; 4; 5; 6; 14; 15; 7; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Among them, the effects of water on the uniaxial compressive behavior, such as the uniaxial compressive strength (UCS) and elastic modulus, are more commonly studied than other mechanical parameters due to their most direct, practical relevance to rock engineering applications [7]. Over several decades, uniaxial compression tests have been conducted on a large variety of rock types under different moisture conditions, involving igneous rocks [25; 26], sedimentary rocks [1; 3; 4; 14; 15; 16; 27; 28] and metamorphic rocks [2; 29; 30]. For instance, [PERSON] and [PERSON] [15] in 1991 reported that an increase in water content tends to reduce rock strength. They suggested that the interaction between water and silicate minerals converts strong silicon-oxygen bonds to weaker hydrogen bonds, decreasing the strength of quartz-rich sandstones. Similarly, [PERSON] and [PERSON] [27] investigated the water-weakening effects on the UCS and stiffness of the Pennant sandstone and the Oughbitridge ganister. They speculated that the intrinsic microfabric of rock (e.g., porosity, geometry of grain boundary and grain-matrix relationships) significantly affects the water sensitivity of rock strength. [PERSON] and [PERSON] [1] further investigated the sensitivity of strength and deformability of 35 British sandstone types to variable water content. They observed that the UCS of sandstones follows a negative exponential relationship with water content. They also found that the degree of sensitivity to water content is primarily controlled by the mineral composition, particularly the abundance of quartz and clay minerals, and to a lesser extent by the rock fabric. [PERSON] and [PERSON] [2] showed that the UCS and elastic modulus of clay-bearing rocks can decrease up to 90% and 93%, respectively, with the increasing water content. They also established empirical models to estimate the water sensitivity of clay-bearing rocks using coefficients relevant to physical properties of the rocks such as porosity, dry unit weight and water absorption. [PERSON] et al. [16] discovered that the UCS of mudstone decreases to only 5 MPa after immersed in water for one month, from 45 MPa under a dry condition. Additionally, they employed X-ray diffraction and scanning electron microscopy to examine the real-time mudstone-water interaction. They concluded that water-induced deterioration processes for mudstone can be divided into three stages: water firstly invades into the rock specimen from initial defects; then water gives rise to the dissolution of carbonate cement and the volumetric swelling of clay minerals by physical and chemical reactions; finally, these effects facilitate the propagation and interconnection of microcracks within rocks. It is universally acknowledged that the presence of water weakens the strength and stiffness of rock materials under quasi-static compression. To further reveal the loss degree of UCS induced by water saturation, the test data on the UCS of rocks in dry and saturated states were compiled and analyzed from hundreds of studies according to the calculating method proposed by [PERSON] et al. [7].