|THE PROBLEM |
Soft clay is widely distributed in southeast coastal cities of China, such as Shanghai, Hangzhou and Wenzhou. It has been recognized as one of the most problematic soils in China because of its high water content, high compressibility, low permeability and low bearing capacity. In the past decades, a large number of infrastructure projects have been constructed on this kind of soil. After years of operation, large settlements of the subgrade have been observed. Therefore, it is essential to study the deformation behaviour of soft clay under traffic loading to prevent this. As the subgrade soils are subjected to large numbers of load applications (105 – 106 cycles) at a stress level below their shear strength, the deformation behaviour of soft clay under a large number of load cycles (i.e., long-term cyclic loading) is of particular importance.
According to figure 1, the dynamic stress field induced by moving loads is three-dimensional, involving the cyclic variation of major, intermediate and minor principal stresses (σ11, σ12, σ22) however, limited laboratory studies have actually been undertaken on the one-way deformation behaviour of saturated clays in three-dimensional stress state.
GDS Instruments Advanced True Triaxial device was chosen to carry out the research. It applies the confining pressure by water pressure, which is further provided by the compressed air. The cyclic vertical stress (Δσ1) and horizontal stress (Δσ2) in the direction of σ2 are provided by two horizontal metal plates with length=75mm and width=75mm and two vertical metal plates with width=75mm and height=140mm, respectively. Every metal plate is driven by a high-precision servo motor.
Fig.1. Stress components of the soil element in soft subsoil induced by vehicle loading
Fig.2. (a) The overall schematic, (b) the detailed schematic, (c) photo of the true triaxial apparatus and (d) photo of the pressure chamber.