All soils can age and their engineering properties also change with time. Such a time effect in sands, e.g., the shear modulus and the shear strength increasing with elapsing time, have been reported not only from the laboratory tests but also based on the field observations. Nevertheless, the associated underlying mechanisms remain unclear or even unexplored at present. For instance, the phenomena of structuration and aging in sands have been discussed for decades. However, there still lack of clear explanations on these phenomena, in particular, with micromechanical insights. It should be noted that sand is inherently a particulate medium, and relevant physical principles behind the macro-scale engineering properties originate from particle interactions. In view of this strong need, t...[ Read more ]

All soils can age and their engineering properties also change with time. Such a time effect in sands, e.g., the shear modulus and the shear strength increasing with elapsing time, have been reported not only from the laboratory tests but also based on the field observations. Nevertheless, the associated underlying mechanisms remain unclear or even unexplored at present. For instance, the phenomena of structuration and aging in sands have been discussed for decades. However, there still lack of clear explanations on these phenomena, in particular, with micromechanical insights. It should be noted that sand is inherently a particulate medium, and relevant physical principles behind the macro-scale engineering properties originate from particle interactions. In view of this strong need, the main objective of this research is to carry out a comprehensive study to better the fundamental understanding on time effects in sand, i.e., its aging behavior, and to recognize the involved micromechanics. It is expected that outcomes from this research ultimately can provide a general guideline about making use of these time effects, e.g., the increased strength with time, thereby improving and optimizing the current design to reduce the cost and time involved in the construction.

In the experiments, newly developed measurements are adopted to characterize aging behavior in sand, i.e., (1) use the innovative wave-based technique to measure the changes of stiffnesses in sand, and (2) utilize film-like tactile pressure sensors to record the process of contact forces homogenization. The corresponding DEM simulations can provide valuable visualization of particle interactions and micromechanical insights into the sand responses during aging; they can therefore greatly assist in identifying associated underlying mechanisms.

Results obtained from the true triaxial tests and the DEM simulations show that for the specimens with isotropic internal fabric, the shear moduli (G_xy, G_yz and G_zx) and associated aging rates are similar under isotropic loading. The results also demonstrate that some strong forces decrease while some weak forces increase. The forces are redistributed and become more homogeneous. The deformation becomes difficult. Therefore, a greater shear modulus can be obtained after aging. In the anisotropic loading where σ_z > σ_x = σ_y, the increment of measured shear modulus is greater in G_yz (or G_zx) than in G_xy during aging. This behavior can be attributed to the increase in both the strong and weak forces in the z direction, due to arching breakdowns. Under the K₀ condition, the shear modulus and the aging rate is higher in G_yz (or G_zx) as well. In addition, the structuration behavior is observed due to the enhanced structure during the secondary compression. The evolution of K₀ is also monitored. Moreover, it is found that K₀ keeps constant in response to elevated vertical loading. It is attributed to the proportionally increments of contact forces in both horizontal and vertical directions as σ′_v increases, as a result of the similar force transmission pattern. However, during secondary compression in the loading path, the K₀ value keeps increasing owing to the raising horizontal stress σ′_h. The continuous increase in σ′_h makes the specimen more stable and the associated stress path is gradually deviated from the critical-state line.

The inherent fabric anisotropy generates not only the stiffness anisotropy, i.e., G_xy > G_yz ≈ G_zx, but also results in a higher aging rate in G_xy than in G_yz (or G_zx) during aging, thereby increasing the stiffness anisotropy. As the fabric anisotropy increases, more large forces appear in the x and y directions than in the z direction, in particular for the contact tangential forces. It is the higher sliding creep induced by larger tangential forces, in x and y directions, can lead to the greater force redistribution during aging, which in turn induces a greater stiffness.