Engineering classification of tunnels in altered strata: A case study from the Luding-Shimian expressway on the Western Margin of the Yangtze Block

The extensive rock mass alteration caused by the tectonic-magmatic activities triggered by plate movement in the continental margin regions has led to numerous underground engineering problems. This paper takes the Luding-Shimian expressway on the western edge of the Yangtze Block as the research object. Through geological surveys, rock thin section analysis, and geochemical analysis, this area is identified as a composite alteration system of hydrothermal alteration, tectonic alteration, and weathering, resulting in deteriorated rock masses. The alteration types causing the deterioration of the rock masses are represented by sericite-chlorite metasomatism and epidotization metasomatism in granite, and sericite-chlorite metasomatism in diabase dykes. Comprehensively considering the mineral strength deterioration caused by hydrothermal alteration and the rock mass fragmentation caused by tectonic activity and weathering, the engineering properties of the altered rocks are classified into four classes, from Class I to Class IV, indicating a significant decrease in rock mass strength, enhanced water absorption characteristics, and a tendency towards looser structure. Based on the engineering property classification of altered rocks, a tunnel engineering classification model for altered strata is established using a nonlinear fuzzy comprehensive evaluation system that combines combination weighting and a trapezoidal cloud model. The model considers three key factors: the alteration degree of granite, the structure and weathering development degree of rock mass, and the influence degree of diabase dyke intrusion. It enables stability evaluation of altered strata with strong spatial heterogeneity at the tunnel engineering scale. This study provides a theoretical basis and evaluation method for the safe design and risk prevention of tunnel engineering in altered regions with similar magmatic–hydrothermal evolutionary processes.