Rock cavern compressed air energy storage (CAES) represents a promising solution for large-scale physical energy storage. Yet

it faces a critical challenge in ensuring the long-term airtightness of underground storage reservoirs. This paper introduces a composite sealing layer consisting of granular bentonite and a thin steel plate to address this issue. Granular bentonite is fabricated by compacting bentonite powder and subsequently crushing it into particles of varying sizes. Vibration tests and constant stiffness swelling tests were conducted on granular bentonite mixtures to evaluate its feasibility for constructing a sealing layer. Results indicated that

for granular bentonite mixtures

the packing dry density stabilizes after 210 seconds of vibration. For multi-size mixtures adhering to the Andreasen equation with a maximum granular size of 5 mm

the peak packing density is attained at n=0.5

corresponding to a value of 1.35 g/cm3. All binary-size mixtures achieved their peak packing densities at an optimal coarse content of 70%. This phenomenon can be explained by the theory of equal-size particle stacking states of granule. During vibration

granular bentonite tends to segregate

compromising its uniformity. However

upon hydration and subsequent expansion

bentonite compacts sufficiently to fill the interstitial voids between the original granules

thereby restoring overall uniformity. This process is crucial for ensuring uniform force distribution across the thin steel plate and the surrounding rock. Under constant stiffness constraints

the swelling deformation of bentonite partially releases its swelling potential

while the residual swelling potential manifests as residual swelling pressure. The relationship between residual swelling pressure and swelling deformation rate

under constant stiffness constraint

has been established. This relationship can serve as a design reference for constructing a composite sealing layer to prevent damage to the thin steel plate due to buckling deformation.