This paper presents an analytical method for surrounding rocks in symmetrically shaped tunnels or roadways, with the symmetrical rise and fall of groundwater over a certain period. The influence of reservoir water level on wet-dry cycles were studied. The changes in the microstructure and strength of altered granite and its evolution were explored using mechanical tests and scanning electron microscopy (SEM). The results showed that: (1) the wet-dry cycles weakened the strength of altered granite. Furthermore, the uniaxial compressive strength, elastic modulus, cohesion, and internal friction angle decreased with the increase of the number of cycles, while the maximum reduction in these parameters reached 50.22%, 63.84%, 93.76%, and 53.90%, respectively. (2) The wet-dry cycles damaged the microstructure of altered granite. The SEM analysis showed that, under wet-dry cycles, the structure of altered granite changed from a smooth and integrated internal structure to the initiation, development, and expansion of pores and cracks. The porosity and fractal dimension of rock were determined using the SEM results. The degree of damage to altered granite under wet-dry cycles was quantitatively analyzed. (3) According to the rock mechanics strength tests and SEM and X-ray diffraction analyses, the damage mechanism of altered granite subjected to wet-dry cycles was discussed. The results provide the basis for a stability analysis of symmetrically shaped tunnels, especially symmetrical tunnels constructed in water-rich areas such as symmetric circular tunnels and symmetric horseshoe tunnels.
