For noncrushable sand, this paper describes the experimental phenomenon of the opposite turning directions of stress-dilatancy curves between sands before and after cementation. Then, based on the thermomechanical framework and Legendre transformation, the stress-dilatancy model is obtained from the dissipation function. This stress-dilatancy model considers the coupled effect of bond breakage and rearrangement energy. This model also incorporates the mechanism that cementation-improved strength leads to the opposite turns of sands before and after cementation. Compared with the other four existing stress-dilatancy models, this paper's model can depict the opposite turning directions of stress-dilatancy curves between uncemented and cemented sands. This stress-dilatancy model is also verified through five types of cementation: colloidal-silica-cemented sand, (CaCl2+Na2SiO3) cemented sand, naturally bonded sand, microbially induced carbonate precipitation (MICP)-cemented sand, and portland cement-treated sand. The broader application of the model is that it can also be used for crushable sand with particle breakage, as well as artificially cemented sand after freeze-thaw damage.

The degradation of soil bonding, which can be described by the evolution of bond degradation variables, is essential in the constitutive modeling of cemented soils. A degradation variable with a value of 0/1.0 indicates that the applied stress is completely sustained by bonded particles/unbounded grains. The discrete element method (DEM) was used for cemented soils to analyze the bond degradation evolution and to evaluate the degradation variables at the contact scale. Numerical cemented soil samples with different bonding strengths were first prepared using an advanced contact model (CM). Constant stress ratio compression, one-dimensional compression, conventional triaxial tests (CTTs), and true triaxial tests (TTTs) were then implemented for the numerical samples. After that, the numerical results were adopted to investigate the evolution of the bond degradation variables BN and B0. In the triaxial tests, B0 evolves to be near to or larger than BN due to shearing, which indicates that shearing increases the bearing rate of bond contacts. Finally, an approximate stress-path-independent bond degradation variable B sigma was developed. The evolution of B sigma with the equivalent plastic strain can be effectively described by an exponential function and a hyperbolic function.