Abstract: A composite sealing layer structure of “bentonite-thin steel plate” has been constructed in the lined rock caverns (LRC) for compressed air energy storage (CAES). In this sealing layer, bentonite absorbs water and expands, thereby squeezing the thin steel plate. The thin steel plate generates an elastic reaction force under pressure, which in turn restrains the expansion of bentonite. Therefore, the swelling behavior of bentonite falls under the category of expansion under constant stiffness constraint. To simulate this behavior, a swelling apparatus with comprehensive functions, convenient operation and low cost was developed. The instrument mainly consists of the instrument base, sample chamber, piston, constant stiffness constraint reaction frame, water injection controller, data acquisition box, and so on. With granular bentonite as the research object, verification tests were conducted. The findings indicate that the instrument is capable of accurately replicating the swelling behavior of bentonite under the constant stiffness constraint. Compared with the results obtained by the staged loading equilibrium method, the swelling pressure of the former is generally higher than that of the latter at the same swelling ratio. Moreover, the results of the former are more consistent with the real situation. By changing the constraint combination of the reaction frame, the instrument can also be used to simulate swelling behaviors under constant volume constraint and zero-loading constraint. Meanwhile, in conjunction with the specially designed high- pressure loading system (with a maximum loading capacity of 20 MPa), the loading and unloading tests of bentonite after expansion can be carried out to investigate the deformation behavior of bentonite during the charging and discharging processes of the CAES. The preliminary demonstration of the rationality of the “bentonite-thin steel plate” composite sealing layer structure fully underscores the reliability and scalability of the instrument, highlighting its functional advantage of “one machine serving multiple purposes”

Key words: swelling apparatus, constant stiffness constraint, bentonite, compressed air energy storage (CAES)