Study On One-Dimensional Consolidation Characteristics Of Unsaturated Soil

The study of mechanical behavior of unsaturated soil is a popular topic in geotechnical engineering.Since the unsaturated soil contains solid,liquid and gas three phases,the existence of the gas phase and the mutual coupling relationship between the liquid phase and the gas phase make the consolidation mechanism of the unsaturated soil complicated,which cause a lot of engineering problems to the design and construction of geotechnical engineering.The application of saturated soil consolidation theory directly to pratical engineering will cause more serious errors.The problem of multiphase coupling in porous elastic materi als has important significance in many fields such as geotechnical engineering,petroleum engineering,acoustics and materials science,involving many subjects such as physics,unsaturated soil mechanics,fluid thermodynamics and mechanics.Since the development of Biot theory,most scholars have only studied two-phase media for porous materials,that is,there is only one fluid in the pores.Nevertheless,in practical engineering materials,especially natural materials,the pores of the porous medium are mostly filled with two or more fluids,that is,unsaturated.For instance,a petroleum layer containing natural gas,a backfill of a building foundation,etc.Elastic mechanics of porous media is a method that can be used to solve the time-dependent coupling between the stress generated by the pressure applied on deformable porous media and the pore fluid pressure.The classical Biot model applies this concept to the consolidation of saturated soils,but does not generalize and extend its rationality to the consolidation settlement theory of unsaturated soils.According to the theory of porous media elasticity and the principle of effective stress expressed by intergranular absorption stress,a class of partial differential control equations for volumetric st rain,pore gas pressure and pore water pressure coupling with respect to time are established by theoretical deduction.Based on this,the one-dimensional consolidation mechanisms of unsaturated soil are researched.The main research work is as follows:(1)On the basis of referring to a large number of papers,according to the basic theory of unsaturated soil mechanics,combined with the elastic theory of porous media,consider the compressibility of the solid soil skeleton and the viscosity of the pore fluid,a set of partial differential equations for three-dimensional consolidation of unsaturated soils is established by calculating the divergence of the momentum balance equation of homogeneous porous elastic media containing two kinds of immiscible,compressible and viscous fluids at the same time,combining the stress and strain relations between pore pressure,pore water pressure and the deformation of solid soil skeleton and pore fluid.The governing equation under unsaturated condition is degenerated into a three-dimensional consolidation governing equation under saturated condition.Compared with the governing equation of saturated soil established by Biot,it is found that the two governing equations have the same form and meaning,which proved that the governing equation of unsaturated consolidation is corret to some extent.(2)Based on the set of partial differential equations for three-dimensional consolidation of unsaturated soils established in this paper,the analytical solution of one-dimensional consolidation under two-sided drainage and one-sided drainage under fixed loads is derived by using mathematical methods and introducing the initial conditions obtained by calculating at the moment of loading.Taking clay as an example,the effects of initial saturation on pore water pressure,pore gas pressure and total settlement in unsaturated soil are analyzed by numerical examples,and the variation of pore water pressure on time scale is also discussed.It is found that the dissipation rate of pore water pressure increases with the increase of initial saturation in consolidation process.The dissipation rate of pore pressure is faster than that of pore water pressure.When the pore pressure dissipates,the pore water pressure will continue to dissipate.At the same time,the distribution of pore water pressure in unsaturated soils under unilateral and bilateral drainage conditions is compared by the obtained one-dimensional consolidation analytical solution,and the differences and relations between them are explored.The results show that when the soil conditions and thickness are the same,and the permeable surface can drain and exhaust without the permeable surface being unable to drain and exhaust.The permeability path of double-sided drainage in unsaturated soil is one-half of that in single-sided drainage,which is consistent with that in saturated soil.At the same time,the correctness of the analytical solution in this paper is illustrated.(3)Based on the elastic theory of porous media,a s et of macro-coupled partial differential equations is established by extending the governing equation of one-dimensional consolidation,which can be used to solve the one-dimensional consolidation problem of locally saturated porous media under cyclic load ing.Fourier series representation in space domain and Laplace transformation in time domain are used to transform the boundary value problem of coupled model equation with periodic loads into two coupled inhomogeneous ordinary differential equations,which correspond physically to the forced vibration of coupled vibration subsystem under viscous damping.Then,the complete analytical solutions including transient and steady components are derived by using the formulas.Furthermore,the effects of excitation frequency and initial saturation on one-dimensional consolidation characteristics of unsaturated soils are analyzed by numerical examples.