Studies On The Consolidation Theory For Unsaturated Soft Soils Containing Biogas

The undersurface soils are usually regarded as saturated soils. However there are bubbles in these soils as is often the case, e.g., the soils containing biogas distribute widely along the coastal and inshore zones. Nowadays it is scarce for studies on the consolidation theory of unsaturated soft soils containing biogas. The research unfolded in the thesis began from the attempt to solve two difficulties in the consolidation theory of unsaturated soils-one was about continuity condition, the other was about the formula of effective stress. The consolidation theory for unsaturated soils containing biogas in this thesis has been studied systemically and basically.The main contents were as followed:1) Based on the stress-sharing principle, the formulae of effective stress for soils with different saturation degrees were derived and proposed;2) A new expression for the compressibility coefficient of pore fluid was put forward, and the factors which influenced mostly the compressibility were discussed;3) According to the principle of mass balance, the continuity equation for three-phase soils was created, though which was inapplicable to the case of evaporation or condensation of pore water in soils;4) One-dimensional consolidation equation for soils with high degree of saturation was derived, and with the help of Abaqus the consolidation characteristic was analyzed;5) The compression process of unsaturated soils with middle degree of saturation was divided into three stages:instantaneous compression stage, consolidation stage due to the dissipation of pore air pressure, and consolidation stage due to the dissipation of excess pore water pressure. Then a simple consolidation theory was proposed. The characteristic of consolidation relative to the dissipation of pore air pressure was studied by finite difference method.The main conclusions are as follow:1) For nearly-saturated soils, the "real" effective stress would be a little smaller than TERZAGHI's effective stress. And air bubbles suspended in pore water do not influence the formula's exactness. For soils in which air phase is discontinuous, a new concept of pore air elastic pressure is put forward. For soils in which air phase is continuous, effective stress is equal to the value of the total stress plus suction. Suction may also be regarded as a type of load to some extent.2) There are three factors which influence mostly the compressibility coefficient of pore mixture fluid. The factors are degree of saturation, pore water pressure and dispersion degree of air bubbles.3) It is complicated to found continuity equation for unsaturated soils because of the uncertain volume of air phase. But when according to the principle of mass balance, the mass of air phase can be neglected. So the continuity equation of three-phase soils can be created compactly.4) The consolidation coefficient of soils with high degree of saturation is influenced mainly by degree of saturation and the compressibility of pore fluid. The compression characteristic of this type soil is as follows:a) there is the instantaneous deformation when compressed; b) the instantaneous excess pore water pressure is smaller than the applied load; and c) the excess pore water pressure dissipates slowly, so that the time needed for this type soil to complete consolidation is extended comparing to the corresponding saturated soil. When studying the monitoring data of pore water pressure in a certain real engineering, the monitoring data fits the analytical solution nicely. It is proved that the consolidation theory of soils with high degree of saturation founded in the thesis is proper and applicable.5) For the consolidation stage due to the dissipation of pore air pressure, the presented equation, not similar to Terzaghi's consolidation equation, is a type of absolute nonlinear partial differential equation. For the consolidation stage due to the dissipation of excess pore water pressure, the mass of pore air can be neglected. And the consolidation equation is solely about the dissipation of excess pore water pressure. The numerical results from this simplified theory are compared with the experimental results from laboratory tests reported in the literature, and the agreement is good. It can be concluded that the hypothesis is rational and the simplified computation is practical in engineering.In this thesis, some basic and important problems in the consolidation research of unsaturated soils are discussed and carried out. The results obtained from the thesis would be a valuable theoretical foundation for subsequently researching the consolidation of unsaturated soft soils containing biogas and the application on the engineering.