Experimental Study On Soft Soil Deformation And Seepage Characteristics With Microscopic Parameter Analysis

Soft soil is a kind of common natural materials for civil engineering, which is widely distributed in coastal areas, middle-lower reaches of rivers and lacustrine delta areas. Slit and mucky soil are the most typical ones that characterized with complex physical-mechanical properties, such as high water content, large void ratio, high compressibility, low strength, weak permeability, remarkable structural and rheological properties and so on. Slit and mucky soil are mainly composed of solid phase matter including tiny clay particles, organics, oxides, and water. Correspondingly, soft soil is small in particle size and large in specific surface area, and the electric charges assemble densely on particle surface. Under the interaction of particle-water-electrolyte system, the adsorbed bound water film formed on particle surface is an important microscopic physical factor that remarkably influences the engineering characteristics of soft soil. The thickness of bound water changes with mineral composition, property and concentration of porewater, and temperature and so on, leading to the change of macroscopic engineering characteristics—plasticity, rheology, seepage and strength. The research of engineering characteristics of soft soil is still focus on macroscopic stage mainly, which is unable to reveal the mechanism and essential law of its inferior properties. Base on experimental study, this dissertation analyzes the deformation and seepage characteristics of tiny-particle soft soil and its corresponding microscopic mechanism.Deformation and seepage characteristic tests of soft soil were conducted with different combination of microscopic parameters such as mineral composition and proportion, specific surface area, cation exchange capacity, pore size and distribution and so on. Base on the theory of diffused double layer, the relationship between deformation and seepage parameters and microscopic parameters are shown quantificationally, the intrinsic relation of them is analyzed, and the micromechanism that influence the soft soil characteristics is explained. Finally, the microscopic seepage models of circular and parallel-plate pore are established, which are confirmed reasonably and effectively by test results.The outline of the research tasks and achievements of this dissertation are listed as follows:(1) Microscopic experiments—mineral composition and proportion, particle size, specific surface area(SSA), cation exchange capacity(CEC), micropore scale and distribution—were carried out, and the particle surface potential was figured out by surface charge density. The test results showed that the SSA and CEC vary significantly among different mineral particles, resulting in different surface charge density, and the mineral crystalline structure is the most important factor that leads to these distinctions. The particle surface potential decreases with the augmentation of ion concentration of pore water. The scale of pore in soft soil is generally ofμm level, and the characteristic of pore scale distribution relates to mineral composition, particle size and internal structure of soil.(2) Research of the strength and plastic deformation features of soft soil with different mineral composition, ion concentration of pore water and water content were carried out. According to the test results, the determinant of soft soil strength is frictional and cohesive effect among mineral particles, which can be changed by the variety of bound water film and results in the change of strength properties. Especially, the main micromechanism of low Atterberg limits and unusual strength properties about saline soil is the micro-electric field effect that changes the properties of bound water by soluble salt ions. (3) The influence of clay mineral, organics, oxides and water content on the rheology of soft soil was analyzed by the combination of direct shear creep test and microscopic parameter test. The results showed that the composition factor of soft soil rheology is presented by different influence degree of different ingredients, and the micromechanism that influence soft soil rheology is generated by the change of thickness and characteristics of bound water film, strong-bound water and weak-bound water are the main and secondary factors respectively that influence the soft soil rheology.(4) In order to research the micro-electric field effect and micro-scale effect on tiny-particle soil seepage, the permeability tests on artificial and natural soil were conducted with different electrolyte ion concentrations. And the mercury-intrusion tests were carried on to study the seepage-consolidation characteristics of soil during consolidation. The test results indicated that the micropore seepage is affected significantly by micro-electric field produced by particle surface charges, and the seepage characteristic is influence by the change of porewater viscosity and effective pore size, and'boundary slippage'of micropore seepage provides a reasonable explanation for the abnormal phenomena during the tests—the permeability coefficient increases with the reduction of hydraulic gradient, and the pore scale and distribution characteristics are important factors that affect the seepage-consolidation properties of soil.(5) The microscopic seepage models of circular and parallel-plate pore are established and confirmed reasonably and effectively by test results. The influence of micro-electric field on porewater viscosity and electric field force of moving ions are taken into consideration in the models, and the fluid motion equations are derived from circular-shaped and parallel-plate-shaped pore. The relationship of theoretical permeability coefficient and systematic microscopic parameters is verified by test results. In addition, the permeability equations prove that Darcy law is one of the special cases of the models.