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GEO5

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Water Flow

The program allows for performing either the steady state or the transient flow analysis in a soil body. The transient flow analysis allows for monitoring the evolution of pore pressure (pressure head) and the degree of saturation in time. Time after which the distribution of pores pressure no longer changes determines the time needed to reach the steady state conditions. This value depends both on the soil flow characteristics (coefficient of permeability, parameters of models describing the retention curve - dependence of the degree of saturation or water content on the negative pressure head or suction) and type of the analyzed problem (e.g. confined/unconfined flow). In case of steady state flow analysis, the individual stages of calculation are independent from each other. In case of transient flow analysis, the solution is performed similarly to standard stress analysis. Individual calculation stages then depend on each other. The first stage of construction stays independent and serves to set initial conditions, i.e. to assign initial pore pressures/pressure heads and degree of saturation at the onset of time dependent analysis in both a fully saturated (positive pore pressures) and a partially saturated (negative pore pressures - suction) soil. The subsequently defined stages require inputting the time duration of a given stage together with the load history (time history of hydraulic boundary conditions). The current version of the program allows us to either introduce the entire load at once at the beginning of the calculation stage or to assume that it linearly increases with time during the course of stage calculation.

In both cases (steady state/transient flow) the program describes in general the flow in an unsaturated or partially saturated medium. The flow in a fully saturated medium appears only below the ground water table. Above the ground water table (flow in a partially saturated medium) the flow is driven by a suitable material model. To analyze problems of unconfined flow the program introduces three material models: the Log-linear model, the Gardner model and the van Genuchten model. When performing the transient flow analysis we recommend adopting the van Genuchten model, because this model is capable of credibly representing the retention data of a soil. Since the choice of the material model influences the setting of initial conditions (initial value of degree of saturation) the program does not allow for changing material models in subsequent calculation stages. In the same spirit changing geometry in comparison to the initial stage is also not possible.

When performing the transient soil analysis it is first necessary to set in the 1st stage the initial values of the pore pressure/pressure head at time t = 0, particularly above the ground water table in the unsaturated or partially saturated soil (suction region). The program offers three options to introduce suction, either by performing the state analysis, or assuming an equilibrium distribution given by p = -γwz, where z is measured from the current location of the ground water table, or the initial values of suction can be specified directly by the user. When solving practical region we recommend not to specify values of the negative pressure head hp smaller than -10m (p > -100 kPa), especially in case of coarse texture soils. For example, for sands the retention curve is almost flat for the values of hp <-1m and for large changes in pressure head there is almost no change in the degree of saturation. This holds also for the coefficient of relative permeability Kr, which serves to reduce the fully saturated permeability in the unsaturated or partially saturated zones. A general recommendation for setting minimal values of negative pressure head is, however, rather complicated since for fine texture soils these values may reach several hundred and for clays even thousand meters.

The next step calls for defining the boundary conditions, either at a point or along a boundary line at the beginning of a new calculation stage.

Beam and contact elements can be introduced inside the soil body. The analysis results are presented in the form of pore pressure and total head distributions, suction, velocities and directions of flow and information about the total inflow/outflow into or out of the soil body. In case of transient flow it is also possible to plot the distribution of the degrese of saturation inside the soil body.