Stress Release–Induced Suction in Unsaturated MX80 Bentonite Pellet and Powder Mixtures: Fid-Bau Portal

Stress Release–Induced Suction in Unsaturated MX80 Bentonite Pellet and Powder Mixtures

Yang, Jin-Wen / Cui, Yu-Jun / Mokni, Naida / Wang, Hao

In this study, a series of suction-control oedometer tests were conducted on unsaturated and saturated MX80 bentonite pellet/powder mixtures. The mixture samples were imposed with various suctions from 0 to 113 MPa under a low vertical stress (0.1 MPa). Then, the mixture samples were loaded in steps with controlled suction until reaching three different target stresses $σ_{v}$ (0.1, 3.2, and 12.8 MPa) and instantaneously unloaded. The microstructure of unloaded samples was observed with mercury intrusion porosimetry (MIP) and microcomputed tomography ( $μCT$ ), together with the determination of suction, water content, void ratio, and saturation degree. Results showed that the water-retention behavior of bentonite pellet/powder mixture was rather characterized by pellets instead of powder. Due to the stress release, the measured suction after unloading $s_{m e}$ was larger than the imposed suction $s_{i m}$ . The suction difference $Δ s$ between $s_{m e}$ and $s_{i m}$ depended on both suction and load. Under a given $σ_{v}$ , the suction effect was characterized by an increase of $Δ s$ when $s_{i m}$ decreased from 113 to 4.2 MPa, but by a noticeable decrease of $Δ s$ when $s_{i m}$ further decreased to zero. This was found to be related to the changes of the slopes of soil-water retention curves (SWRCs): the slope was decreasing with suction increases after 4.2 MPa suction, but was increasing with suction increases before 4.2 MPa suction. As far as the stress effect was concerned, a larger $Δ s$ was identified at a higher $σ_{v}$ for a given $s_{i m}$ . The narrowing difference between $Δ s$ and the reduction of mean stress of pellet $Δ p^{pellet}$ with increasing load was because of the decrease of larger pores due to loading. The total mixture void ratio $e_{T}^{mixture}$ was highly dependent on suction and vertical stress, whereas the total pellet void ratio $e_{T}^{pellet}$ was only dependent on suction in the stress range considered.