Abstract
Loading/unloading tests and field emission scanning electron microscope (FESEM) tests were performed on undisturbed soft clay specimens to study the pore evolution under the loading/unloading process. The results showed that small pores (<0.2 μm) had intrinsic characteristics, and the distribution and the fragmentation fractal dimension of small pores were basically unchanged with pressure, while large pores (>0.6 μm) changed greatly under loading/unloading. The pore-size distribution was mainly influenced by large pores. The microstructure of soft clay before unloading has an influence on the change of the swelling index (Cs) and the pores evolution under unloading. Cs increased as the surface fractal dimension of the pores and the area of large pores decreased, and the fragmentation fractal dimension of the pores increased under the loading process. The variations in fractal dimensions and large pore area increased under unloading. Moreover, the compression index (Cc) changed nonlinearly with the pore evolution under loading. Below 100 kPa, Cc increased slightly with a small increase of the fractal dimensions and large pores area under loading. From 100 kPa to 400 kPa, Cc increased to a peak value of 0.484, and the fractal dimensions and large pore area were the greatest under loading. Above 400 kPa, all of them changed slowly. Based on the evolution of pore fractal characteristics, the loading/unloading process could be divided into three stages: the natural structural stage, the structural adjustment stage, and the new equilibrium stage, which was important to study the loading/unloading properties of soft clay.
Highlights
Soft clays are widely distributed, with low strength, high sensitivity, and high compressibility [1,2].Due to the rapid development of urbanization and the increase of population, it is inevitable to carry out a significant number of foundation pit engineering projects, including preloading treatments, and the excavation of foundations
D f of pores, the loading/unloading process could be divided into three stages: theevolution natural of the D stage and D
The high compressibility of soft clay The increased value of the compression index of Shanghai soft clay was at kPa, which agreed with the previous before the failure of the soil microstructure, and maintained high compressibility [35,36]
Summary
Soft clays are widely distributed, with low strength, high sensitivity, and high compressibility [1,2].Due to the rapid development of urbanization and the increase of population, it is inevitable to carry out a significant number of foundation pit engineering projects, including preloading treatments, and the excavation of foundations. The loading/unloading process may produce large deformation of soil, leading to safety problems. The loading/unloading process was analyzed quantitatively by the finite element method [3] and constant rate of strain tests [4], showing that soil characteristics under the loading/unloading process behave elastically. The loading/unloading test can be exerted by an oedometer or apparatus to obtain the void ratio–stress curves with loading and unloading loops, which reflect the change of macroscopic mechanical properties of soil. Macro parameters like the compression and swelling indices of soil can be collected [5], reflecting the change of soil structural properties. The loading/unloading process is controlled by the microstructure, which provides another method for studying the mechanical properties of clay [6]
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