# Mechanical Behavior of Weathered Granite Exposed to Water
## 1 Introduction  
Under the action of natural (i.e., physical, chemical, and biological) weathering, the original structure of rocks is destroyed and loosened, their chemical composition changes, new chemical minerals are formed, and their mechanical properties weaken. Due to the many factors controlling the weathering of rocks (i.e., rock characteristics and climatic and topographical conditions), weathered rocks possess different mineral compositions and structures. In most cases, the degree of weathering of a weathered rock diminishes from the surface to the bottom of the rock, and it is distributed in a zonal pattern. Weathered rock obviously differs from other rocks, but it is similar to soft rock. Water-rock interactions cause the microstructure of the rock to change and metasomatize [1]. The interactions between water and rock (body) cause the rock (body) to soften and disintegrate, resulting in the remarkable degradation of its engineering properties. Due to the convenience and low price of materials, weathered granite has been widely used as an emerging foundation filler for constructing transportation infrastructure [2, 3]. [PERSON] [4] found that weathered granite has strong shear cohesion after compaction by direct shear tests. [PERSON] et al. [5] conducted dynamic triaxial tests on five fully weathered granites, analyzed the effects of load ratio, critical dynamic stress, and loading times on fully weathered granite, and found that long-term cyclic loading has limited impact on the settlement of fully weathered rock subgrades. In addition, weathered rock has also been used as a foundation for the establishment of some reservoirs [6; 7]. Studying water-rock interactions is one of the topics at the frontier of geotechnical engineering. The physical, chemical and mechanical processes of rock under certain osmotic pressure or hydrodynamic conditions are the fundamental cause of deformation and failure of engineering rock mass [8; 9; 10]. Weathered granite widely distributed in the South China Sea (see Figure 1) belongs to types of special sedimentary rocks that are relatively complete and hard and have good mechanical properties in their natural state [11; 12]; nonetheless, they quickly swell, disintegrate, and soften after exposure to water, resulting in a significant decrease in their mechanical properties. Therefore, research on these rocks is of great significance. Figure 1: The distribution area of the studied weathered rock: (**a**) Sampling site; (**b**) Geological section of the sampling site (From the engineering geology report of the Shenzhen-Zhongshan Bridge). Early scholars have conducted ample research on the softening characteristics of mudstone, shale, and other soft rocks exposed to water and have achieved rich data on the mechanism for the softening of rocks in contact with water [13; 14; 15]. For instance, on the basis of experimental research, [PERSON] [16] found that the Pittsburgh red mudstone contains minerals that swell in contact with water. Thus, these mudstones decompose after exposure to water, and their strength continues to decrease under seasonal dry-wet and freeze-thaw cycles. [PERSON] [17] conducted a triaxial shear test on a disintegrable rock rich in hydrophilic clay minerals and found that the disintegration and siltation of the rock caused by the expansion due to water absorption and the uneven shrinkage fragmentation due to water loss are the main reason for the rock softening in the presence of water. In another work, [PERSON] et al. [18] discussed the softening mechanism of a shale in northwest Hubei through microscopic and macroscopic experiments and believed that the saturation time affected the compressive strength and elastic modulus of the rock. However, there are few works on special soft rocks such as weathered rock, and the research on the mechanical properties of weathered rocks softened by water is even scarcer.