Study On The Secondary Consolidation Creep Properties And Microscopic Characteristics Of Tianjin Coastal Soft Soil

In recent years, with the rapid development of economy in our country, there are more and more civil constructions on a national scale, such as highways, railways and high-rise buildings and other projects. Therefore, the demand for land is increasing, especially in coastal cities where the soft soil is widely distributed. A lot of engineering construction would be built on the soft soil layer. Tianjin coastal soft soil has many characteristics such as high void ratio, high compressibility, high moisture content, low permeability and low strength, etc. For this reason, the soft soil foundation in this area has many poor engineering geological properties, such as large settlement deformation, low strength, and so on. When the projects are built on the soft soil foundation, the poor nature of the soft soil will have a very negative impact on the construction of these projects.In view of the problem of poor engineering properties of the soft soil in Tianjin Binhai New District, the present study selects the Tianjin coastal silt clay as the research object, investigating and analyzing the consolidation creep deformation of soft soil from both macroscopic and microscopic views. The main research work and results of the study are presented as follows:(1) According to the analysis of the indoor test of basic physical and chemical properties of the soil sample, the organic matter content of the soft soil is high with strong activity, strong hydrophilicity and high water content. In addition, the difference of the pH value between the soil sample solution and the mineral is large and the thermodynamic potential is high. Moreover, the content of Na+ was higher in the soil sample, which leads to the thick diffusion layer of the soil. As a result, the hydrated film on the surface of the soil particle is thick, which accounts for the high moisture content of the soil sample.(2) Through one-dimensional consolidation creep tests of soil samples by using single lever high-pressure consolidation apparatus, it is concluded that the soil sample belongs to under-consolidated soil but it has a constitutive property with the structural yield strength of 141 kPa.(3) Through the pre-compression consolidation test, the preloading can effectively reduce the primary consolidation and secondary consolidation deformation of the soil. When the consolidation pressure is less than the preloading load, this effect is more obvious. When the consolidation pressure is greater than the preloading load, the secondary consolidation coefficient increases first and then decreases with the increase of the consolidation pressure, but the peak of consolidation pressure will move back.(4) Through the investigation of the change of micro pore scale distribution characteristics of soil samples before and after consolidation by means of mercury injection test, it is concluded that the porosity of the soil decreases with the increase of consolidation pressure, but gradually it becomes gentler. When in the natural state, the pores in the soil are mainly mesopores. When the consolidation pressure gradually increases, the mesopores in the soil are gradually compressed into small pores and micro pores, and then the pores in the soil are mainly small pores and micro pores.(5) Through the qualitative research on the microstructure characteristics of the soil before and after the process of consolidation by scanning electron microscope, it is concluded that when the soil is in the natural state, the contact ways between the particles in the soil are mainly based on the slice-to-slice contact between the film and the slice-to-surface. The main structures of the soil sample are honeycomb structure and flocculent structure. When the consolidation pressure gradually increases, the large pores in the soil are compacted with particles contacting closer. The ways of contact between the particles in the soil are mainly surface-to-surface contact, and its microstructure becomes mainly the clotted structure.(6) On the basis of the quantitative analysis of SEM image, namely, the quantitative study on consolidation deformation from the abundance of pore and structural units, the equivalent pore diameter and the fractal dimension, it is concluded that the shapes of the pores and the structural elements are mainly in the presence of elliptical shape. With the increase of the consolidation pressure, the shape of the pores in the soil change little. The shape of structural units develops toward flattening, but it mainly includes oval shaped ellipse and flat ellipse. The fractal dimension can be used to distinguish the degree of separation of soil. The fractal dimension of the soil studied in this study is in the range of 1~1.5, with good sorting characteristics. With the increase of consolidation pressure, the fractal dimensions of pore and structural element in the soil are increased gradually, and the fractal dimension of the structural unit is larger than that of the pore.(7) The mechanism of consolidation and creep deformation of soft soil are investigated and the structural formation mechanism of soft soil and the effect of the consolidation pressure on the consolidation creep are analyzed in the present study. The internal factors affecting the soft soil structure mainly include the ionic composition and mineral composition of the soft soil, and the external factors include temperature, humidity, weathering, time and load. The consolidation and creep of soil is a new process of dynamic equilibrium. From a macro point of view, the soil mass is consolidated and creep deformed after the external force and he internal structure of soil mass will be adjusted accordingly. The soil porosity is compressed(the amount of consolidation and creep deformation increase in the macro performance) in order to increase the supporting force of the internal structure of the soil to keep a balance of the force under the condition of external force. From the microscopic point of view, the consolidation and creep of the soil is in fact, the process that the pores in the soil are pressed gradually or even disappear, and the arrangement of particles in the soil and the soil structure are transformed from the loose state to the tight state.