Extrusion behavior of bentonite-based materials considering pore size and sand content effects

Abstract

• An extrusion model was developed and verified considering sidewall friction and sand content effects. • Sidewall friction factor increased with decreasing pore size and increasing sand content. • Sidewall friction or specimen mass loss plays dominant role in the extrusion, depending on pore size. • Diffusion coefficient of montmorillonite increased with increasing sand content. Extrusion of compacted bentonite or bentonite–sand materials into fractures and pores in the host-rock formations improves the buffering ability of the host-rock formations and simultaneously reduces the buffering capacity of the engineering barrier, and ultimately endangering the operation security and stability of the repository. In this work, a model was developed for describing extrusion of bentonite–sand mixture into pores considering pore size and sand content effects. Then, a test apparatus was designed and extrusion tests considering pore size and sand content effects were conducted. Images of the extrusion process were regularly captured and analyzed. Results show that according to the extrusion model, the diffusion coefficient increased with the increasing sand content. Moreover, the sidewall friction factor increased with decreasing pore size and increasing sand content. For samples tested with a sand content of 0, after 15 days test, the extrusion distance increased first and then decreased with increasing pore size. Moreover, the extrusion rate initially decreased with increasing sand content, while after about 42 days test, for tests conducted in a pore radius of 10 mm, the extrusion rate increased first and then decreased with increasing sand content. Finally, considering the influence of pore size on the anisotropy of swelling pressure, the model could well simulate extrusion tests with consideration of pore size and sand content effects.