Research of Microstructure

Abstract

As a matter of fact, the macroscopic deformation of saturated soft clay under subway loading is a reconstruction process of microstructure. All the structure elements including particles and pores change under subway loading. According to the above analysis, the rearrangement state of microstructure under subway loading is related to the vibration duration and stress level. Based on SEM and MIP tests, fractal theory analysis, and correlation analysis of pore microstructure parameters and macroscopic characteristics, some conclusions can be drawn: 1. The saturated mucky clay of the fourth layer in Shanghai mainly has flocculated structure and honeycomb-flocculation structure. Clay mineral particles are major illite, less chlorite, and tiny montmorillonite. The shape of structure elements are mainly laminar (see Fig. 4.11), while the aggregates are flocculated in flower or feather shape. The connections of structure elements are mainly edge to face and edge to edge which forms an open pore structure with high void ratio. 2. The saturated mucky clay of Shanghai has primary pore structure of large inter-aggregate pores (radiuses are over 50 nm); it has pore throat effect in the process of mercury intrusion. 3. The variation features of pore structure parameters with different depth can be indicated by the cyclic stress ratio (CSR). With the increase of depth, the CSR grows while the specific surface area decreases at first and slightly rebounds later. The mean distribution radius, retention factor, and porosity have the opposite trend. 4. The microscopic test results indicate that packing model is not suitable for explaining the mechanism of deformation in saturated soft clay under low stress conditions. 5. Based on the fractal theory, the study on clay pore distribution shows that Menger sponge random model is not appropriate to describe the fractal characteristic of mucky clay in the whole size range. But the model based on thermodynamic relations is suitable. The relationship between fractal dimension calculated by thermodynamic relations model and CSR was demonstrated as well as explanations of the variation features. 6. The correlation analysis of pore microstructure parameters and macroscopic deformation (macroscopic force) shows that a critical value of subway loading plays an important role in the microstructure variations of mucky clay near subway tunnel. 7. During a long time since the first operation of subway, soil structure elements of soil layers near subway tunnel and at the bottom of track tend to be compacted. However, the pressure applied on soil at the bottom of subway tunnel is quite large as the trend is alleviated. The rearrangement of soil elements is the main reason of deformation. Therefore, the accumulation of deformation will continue for a long time.