To understand the effect of drying-wetting cycles on the expansion characteristic of confined compressive clay-sulfate rock

a series of cyclic drying-wetting tests was carried out by a self-designed instrument. The test samples were special clay-sulfate rock collected from the Tianshui No. 2 tunnel of the Pingliang (Gansu) to Mianyang (Sichuan) expressway. The macroscopic expansion regulation and mesoscopic expansive mechanism of the samples during the drying-wetting cycles were analyzed. The results show that: (1) The self-designed instrument can couple the drying-wetting cycle in a confined compressive stress state in the whole process of the test. Meanwhile

there is no free surface of the specimens

which accords with the real occurrence of the underground rock mass. (2) When the normal pressure is less than the expansive stress of clay-sulfate rock

the volume of the sample increase significantly after the first wetting process

and the proportion of recoverable expansive deformation is small in the subsequent drying process. (3) After the first drying-wetting cycle

the mineral particles of specimens degenerate from cementation to flocculent accumulation

which is an argillation process

with a proportion of decrease in expansion stress exceeding 80%. (4) Normal pressure can reduce the expansion deformation of clay-sulfate rock

however

it cannot inhibit the degeneration of mesostructured of clay-sulfate rock. (5) The loaded drying-wetting cycles may eliminate the expansibility of clay-sulfate rock

at the expense of some unrecoverable deformation. However

it maybe reduce the shear strength of the surrounding rock at the same time. The main conclusions of this study play a guiding role in tunnel construction and geological hazard prevention in clay-sulfate rock areas.