Construction of Crystallization Risk Control Index and Evaluation System of Tunnel Drainage System

Crystal disease is very common in mountain tunnel drainage system, and the establishment of quantitative evaluation method of crystal risk has great practical engineering significance. Based on the analysis of the chemical crystallization process and physical plugging process in the tunnel drainage tube, seven solution parameters were identified as the key judgment indicators for identifying crystallization risk. The influence tests of single factor crystallization and orthogonal crystallization were designed and carried out, and the influence rules and degree ranking of each judgment index on crystallization were determined respectively. The index grading criteria were determined according to the change rate of calcium ion concentration. The index weights were calculated by the improved analytic hierarchy process (AHP), the crystallization risk evaluation system was established, and the comparison with the field situation was verified. The results show that the crystallization plugging process of mountain tunnel includes three stages: calcium osmosis in the first branch, drainage crystallization reaction and physical precipitation in the tube. In the drainage solution parameters, Ca²⁺, HCO₃^-, pH, SO₄^2-, flow rate, Mg²⁺ and temperature can be used as indicators to judge the crystallization risk. The first five have a positive promoting effect on crystallization, and the last two have a negative inhibiting effect. The relative influence degree of crystallization reaction is pH > Ca²⁺ > flow rate > HCO₃^- > SO₄^2- > temperature > Mg²⁺ from large to small, and the corresponding weight varies between 0.319 and 0.034. Based on the quantitative curve of calcium ion concentration change rate, the scoring criteria can be divided into 4 ~ 6 intervals, and the scoring criteria can be determined by curve slope interpolation. The engineering application shows that the crystallization risk grade evaluation system of tunnel drainage system can objectively reflect the crystallization risk grade of target tunnel.