Study On Saturated-Unsaturated Seepage And Its Application

Saturation-unsaturated seepage is the widespread and objective regularity of groundwater movement. Because its significant influence on rock and soil slope stability, the use of groundwater resources, pollution control, the exploitation of petroleum and hydrocarbon gases, as well as last ten years domestic and international engineering demands increase greatly in the above realms, the saturation-unsaturated seepage research becomes a hot-spot problem of geotechnical engineering. At first the saturation-unsaturated seepage, coupling problem between stress field and seepage field, the stability analysis of slope under the condition of rain infiltration etc are introduced in the paper. Base on the former research, the theory of saturation-unsaturated seepage about porous media and rock, the test of single fracture unsaturated penetration, FEM simulation of saturation-unsaturated seepage, rain infiltration and its influence on the stability of slope and coupling interaction of seepage field and stress field are analyzed. Its major contents are as follows:1. The theory of saturated seepage of porous media and its differential equation are introduced. The unsaturated soil-water potential theory, the soil-water characteristic curve of porous media, various empirical equations and the influencing factor of unsaturated permeability coefficient are formulated. The permeability characteristic of various fractures are analyzed and jointed rock mass also are presented. The conception of REV about fractured rock is also introduced and expression of REV is deduced. Based on the laboratory measurement data of capillary pressure-saturation relationships in rock fracture, the distribution function of fracture apertures is studied. The imitation of fracture unsaturated permeability parameters and theoretics analysis are carried out; the mechanism of unsaturated flow in fracture is discussed.2. Basing on basic seepage theory, the differential equation of the 3D saturated-unsaturated seepage of continuous medium and the basic boundary conditions are deduced. The Galerkin FEM formation regarding pressure water head as basic unknown quantity is presented. Unifying the saturated-unsaturated, stable-unstable seepage problems and saturated-unsaturated seepage field, a uniform governing equations is applied in the whole seepage field. The free surface is only the interface of positive water pressure andnegative water pressure, so it is unnecessary to adjust seepage free surface in computation. But there are some disadvantages in saturated-unsaturated Galerkin FEM. For example, the unsaturated permeability parameters are not easy to confirm, it is not easy to control calculating precision and hard to gain the solution and convergence because the type of different control equations is not unified in the plane between positive water pressure and negative water pressure. The selection of the unsaturated permeability parameters, the disposal of the overflow surface, the nonlinear iteration technology and the convergence standard and to confirm the initial pressure waterhead conditions are discussed.Based on the verification by the 3D saturated-unsaturated seepage FEM program, simulation computation on seepage field is done as the water level lowering and rising. The results show that negative pressure zone become reduced and pressure head in positive pressure zone become raised and partial rock become saturated from unsaturated. The rising of the zone-pressure line lags behind the rising of the reservoir level can also be seen. Owing to the differences of initial pressure head in slope and the unsaturated permeability coefficient of rock, the negative head pressure and the zone-pressure plane vary complicated. As the water level lowering, the zone-pressure plane fall rapidly and the negative zone become lager and the pressure head in positive and negative zone fall gradually. The vector of groundwater velocity directs the dip direction of the slope as a whole. The velocity adjacent to the zone-pressure is greatest. The rapider the water level fall, the greater the hydraulic slope become, the greater the flowing pressure along slope becomes. So the stability of the slope becomes worse. The computing results show the regularity of the pressure head and velocity during the variation of the water level are reasonable.3. According to the principle of soil hydro-dynamics, the general mechanism of rain infiltration is presented. It includes that the flow regulation of groundwater at the condition of rain infiltration, the boundary condition of ground surface infiltration and one dimension infiltration model such as experience formation, Green-Ampt model and Richards equation.The simulating method is presented base on saturated-unsaturated FEM calculation and the methods of handling boundary of rain infiltration and calculating seepage flux are analysed. At the same time, some examples about rain infiltration are calculated, including the regulation of infiltration, redistribution and drainage of groundwater at rain infiltration condition in one dimension sand column. The affect of the intensities, duration of rainfall, the change of initial pressure waterhead and the permeability coefficient on the saturated-unsaturated seepage field is analyzed. The variation process of thesaturated-unsaturated seepage field of one slope is simulated at the conditions of rain infiltration.The seepage field considering the atomizing rainfall is simulated aiming at Jinping first hydropower project high slope on the side of left bank. The results show that the pressure head is raised at superficial place in the high slope by atomizing rainfall and temporary saturated zone can exist at gentle base of slope. Without regard to drainage facility, the pressure head raise about 140m at the base of the slope because of the atomizing rainfall, the other way round, about 60m. The moisture content and the increase of pressure head in the slope surface are affected by the strength of rainfall. Without regard to drainage facility, the cumulative infiltration rate in the atomizing region is 27.62%, the other way round, 5.00%. The waterproof and drainage work can suppress effectively the increase of pressure head and the appearance of temporary saturated zone because of the surface infiltration. So slope protection and the effective drainage for the surface flow and the rational layout of drainage outlet in superficial layer can enhance markedly the stability of the slope.Base on unsaturated material strength theory and the limited balance analysis method, the slope stability analysis method considering the material strength variance in the unsaturated region is studied according to the variance of saturated-unsaturated seepage field during rainfall, taking the Jinping first hydropower project high slope on the side of left bank as the example. The computation results of VI-VI section show the stability factor decrease as the infiltration of the surface water reduce and when rain is over, after a period of time the stability factor recover. The atomizing rainfalls have a great effect on superficial layer slide (the stability factor decrease at most 14%) and a slight effect on deep layer slide (the stability factor decrease at most 14%). The waterproof measure in the slope surface and the drainage measure in the slope body play an important role on the control of infiltration and improvement of slope stability.4.The FEM equation of unstable seepage considering the variety of porosity ratio is deduced base on the differential equation of saturated unstable seepage field and the stress-strain FEM method is discussed. The coupling mathematical model and FEM calculating formation are presented considering the interaction between seepage field and stress field. And the coupling mechanism of the interaction between seepage field and stress field is studied and computation method to calculate seepage force in FEM is analyzed. The influence of the stress field of the two kinds of material on permeability character is discussed.Taking Maopingxi earth dam project in TGP as an example, utilizing the mathematical model of seepage field coupling with stress field and its FEM equation, the deformation field during construction period and the coupling of steady seepage field and stress field at 135.0m and 175.0m water level are calculated. The result shows that the deformation of dam body by computation can be uniform with the measured deformation and the whole regularity of stress field is rational. At 135.00m water level, the deformation field considering the seepage have some obvious characteristic. For example, both the horizontal displacement and vertical displacement of dam increase, but the vertical displacement increase more, the vertical displacement increases along gravity direction. The deformation and seepage regularity of earth dam gained by computation accord with the factual regularity, so the selection of computation model and parameters are reasonable. At 175.00m water level, the computation result considering the coupling shows the water head of the rock mass of upstream core wall at base of dam lower slightly, and that the water head of bottom and downstream core wall rise slightly, the total leakage increase slightly, both the horizontal displacement and vertical displacement increase respectively, the influence of coupling is limited.5. At last, conclusions and prospects are drawn from the studies of saturated-unsaturated seepage and the tentative ideas about this domain are presented.