Modeling of liquefaction mitigation using bentonite

The phenomenon of liquefaction, the loss of effective confining stress in a saturated soil deposit, is a potential cause of severe damage to structures. Liquefaction, particularly that occurring under cyclic or dynamic loading conditions, is generally attributable to a combination of (1) a tendency for volumetric contraction of the soil matrix and (2) the negligible compressibility of the pore fluid relative to that of the soil matrix. Several schemes for liquefaction mitigation have been put forth which propose to affect one of these two factors. One such approach, the replacement of the pore fluid with (thixotropic) bentonite slurry is particularly relevant to this study. Previous work has shown this treatment to be an effective means of increasing liquefaction resistance in undrained cyclic triaxial tests. However, no experimental study to date has demonstrated the effectiveness of bentonite treatment at the field scale. This study employed a numerical approach to investigate the effectiveness of bentonite treatment at the field scale. A bounding surface-based constitutive model was selected from the literature and implemented in the finite difference software FLAC. The model parameters were calibrated both for clean (i.e. untreated) sand and for sand treated with bentonite. The implemented model was used to analyze a dynamic problem: a hypothetical case of a vertically-vibrating footing founded on a saturated sand. Three indicators were used to measure the performance of bentonite treatment: (1) settlement of the footing, (2) shear strain in the soil layer, and (3) reduction of effective confining stress. It was shown that, for this hypothetical case, the application of bentonite treatment dramatically improved all three aspects of performance. These results, combined with the previous laboratory-scale experimental work, suggest that bentonite treatment may be a viable option for liquefaction mitigation at the field scale