Study on the evolution law of weathering characteristics of granite surrounding rock with different weathered degrees in subsea tunnels

Currently, the classification of weathered degrees in granite surrounding rock of subsea tunnels primarily relies on preliminary methods such as field characteristic observation, rock sample compressive strength test and wave velocity test. These approaches offer limited accuracy and yield relatively coarse classification results, which can lead to misjudgments regarding rock stability during tunnel design and construction. As a result, weak surrounding rock areas that should have been treated with grouting reinforcement may be overlooked, increasing the risk of tunnel face collapse or water inrush. Based on the weathering characteristics of granite surrounding rock of subsea tunnels with different weathered degrees along the southeastern coast of China, this study aims to provide a more reliable basis for the refined classification of surrounding rock, thereby helping to avoid the occurrence of such engineering accidents. The mechanical properties of weathered granite were evaluated through uniaxial compressive and triaxial compression tests. Then, the polarizing microscopy and the X-ray Computed Tomography (X-ray CT) were employed to obtain the variations in microstructural parameters of weathered granite. Subsequently, X-ray diffraction (XRD) analysis was performed to examine the mineral composition of granite with varying degrees of weathering. The results indicate that weathering not only reduces the mechanical strength of the granite surrounding rock but also increases its porosity. Additionally, the content of clay minerals within the granite surrounding rock continuously increases with the degree of weathering, particularly in completely weathered granite, where clay minerals dominate the composition. This further causes the surrounding rock structure to loosen, increasing the number of pores and ultimately reducing the overall strength and stability of the surrounding rock. Finally, the mathematical relationships between the mechanical parameters, pore fractal dimension, and clay mineral content were established to quantitatively characterize the weathering effect of granite surrounding rock. These results could serve as reference for the construction of subsea tunnels crossing weathered granite strata.