Experimental Study On Soft Soil Micro-pore Structure Characteristics And Permeability Properties

Soft soil,which is widely distributed in coastal areas, middle-lower reaches of rivers and lacustrine delta areas,has the poor and complex engineering properties including high water content, high porosity, low strength, high compressibility, low permeability, structural rheology and so on. The study on consolidation and permeability properties of soft soil is important and unavoidable for foundation treatment and design in geotechnical engineering.In recent years, it has been realized that the micro-structure of soil account for many aspects of mechanical behavior of soil through the long-term and substantive researches and tests. Pore characteristics are one of the fundamental elements of micro-structure of soil. Pore characteristics such as pore size distribution, pore shape, connectivity and tortuosity affect the permeability of soft soil directly. Based on experiments, this dissertation study the pore characteristics of soft soil and their relationship with the consolidation permeability properties from micro perspective.The tested soft soil includes the artificial tiny-particle soft soil and the natural soft soil. The macro-tests include consolidation and permeability test, constant head permeability test and falling head permeability. The micro-tests include Mercury intrusion porosimetry(MIP), scanning electron microscope(SEM), Laser diffraction technique(LD), X-ray diffraction(XRD). Nitrogen Adsorption(BET), Ethylene Glycol Monoethyl Ether adsorption( EGME) and so on.The outline of the research tasks and achievements of this dissertation are listed as follows:(1)The effects of the dehydration methods such as freeze-drying, air-drying and oven-drying on the micro-pore structure characteristics of soil were analyzed. It is suggested that the freeze-drying should be used for the dehydration of soft soil with high moisture content and plastic.(2)It is proved through tests and statistical analysis that the specific surface area of soil by Mercury Intrusion Porosimetry(MIP) is effective and reliable. The specific surface area and the pore size distribution of soil are gained at the same time, so the test process is reduced.(3)The pore size distribution(PSD) evolution of soft soil during the process of permeability and consolidation was quantitatively analyzed. It is founded that the seepage and consolidation have obvious affect on the pore size distribution of the middle pores with diameter between 100 nm and 10,000 nm and little on the small pores with diameter smaller than 100 nm and its corresponding microscopic mechanism was discussed.(4)The pore size distribution characteristics of the substantive artificial tiny-particle and natural soft soil with different mineral ingredients, void ratios, moisture contents and saturations were tested by MIP. It is proved that the pore characteristics of soft soil are various by test results, so the void ratio which is a macroscope parameter can not represent the pore characteristics of soft soil perfectly and it is necessary to introduce other microscope parameters to describe the pore characteristics of soft soil.(5)The relationships between the soil pore structure characteristics parameters and the permeability coefficient were statistically analyzed. The pores of the soil are divided into the large pores(D>10000nm), the middle pores(100nm<D≤10000nm) and the small pores(D≤100nm) according to the test results. The statistical results show that: the permeability coefficient have no or weak relationships with void ratio by MIP and the absolute volume content of the middle pores; the permeability coefficient have medium relationships with the absolute volume content of the small pores; the permeability coefficient have strong relationships with the specific surface area, the average pore diameter and the absolute volume content of the large pores(6)The micromechanism of the permeability and consolidation in soil was discussed from micro-pore perspective. The conceptions of the large pores void ratio, the middle pores void ratio, the small pores void ratio and the effective void ratio were put forward. Then the Kozeny-Carman model was modified according to the effective void ratio that was proposed basing on test results. In comparison with the K-C model, the permeability coefficient predicted by the improved K-C model is closer to the measured for soft soil.