Abstract: To prevent a tunnel drainage pipe from crystal clogging, the primary technical indices of a permanent magnet drainage pipe are proposed. Then, an indoor and field test is conducted on a selfmade permanent magnet drainage pipe to examine the influence of magnetic field intensity, flow rate, and magnet spacing on the crystallization of a permanent magnet drainage pipe. Finally, experimental analysis is conducted according to the mass variation of crystals under various conditions and using scanning electron microscopic images. The results demonstrate the following: (1) When the magnetic field intensity ranges from 0.1 to 0.3 T, the permanent magnet drainage pipe has a good anticrystallization effect. When the magnetic field intensity exceeds 0.2 T, the anticrystallization effect gradually decreases. When the magnetic field intensity exceeds 0.4 T, the crystallization effect on the permanent magnet drainage pipe is greater than that in an ordinary drainage pipe. (2) With increasing solution flow rate in the test pipe section, the anticrystallization effect of the permanent magnet drainage pipe will be enhanced. When the flow rate exceeds 450 mL/s, the anticrystallization effect will not be enhanced with an increasing flow rate. (3) After the permanent magnetic treatment, the crystallites in the pipeline change from regular cubeshaped objects to rough and loose irregular objects, and subsequently, from hard blocks to powder, with a tendency to transform from calcite to aragonite and bulgonepheline. (4) As the placement spacing of permanent magnets decreases, the final crystallization amount in the pipeline shows a downward trend. Among placement spacings of 10, 40, and 70 cm for permanent magnets, 40 cm is the optimal placement spacing.

Key words: tunnel engineering, drainage pipe crystallization, magnetic field anticrystallization, pipe flow rate