most finite element codes is based on Darcy's law.

Darcy's law states that the velocity of the fluid is

proportional to the hydraulic gradient. The constant of

Seepage problems involve the analysis of the flow of

proportionality in this relationship is termed the

groundwater through porous media such as soil and

coefficient of permeability. This coefficient is a

rock. Geotechnical engineers typically perform

parameter which is material dependent. Most finite

seepage analyses as part of the design process for

element codes are capable of handling materials having

embankment dams, levees, and river structures such as

anisotropic permeabilities.

locks and dams.

The coefficient of permeability is typically estimated in

three different ways:

(1) Sampling and laboratory testing. Sampling

The principal quantity solved for in a finite element

and laboratory testing where samples retrieved from a

solution of a seepage problem is the pressure head at

field exploration program are sent to the laboratory

each nodal point in the finite element mesh. All flow is

where permeability tests can be performed. The two

assumed to occur through the pore spaces of a rigid

most common types of laboratory permeability tests are

soil skeleton. From these heads, the quantity and

the constant head and falling head types of tests.

velocity of flow, and hydraulic gradients at any point

in the system can be determined. Flow nets can be

(2) Correlations with grain-size distribution. For

constructed from the finite element results to help the

example, the Lower Mississippi Valley Division of the

design engineer interpret the results.

Corps of Engineers has correlated the grain-size

distribution of sands in the Lower Mississippi Valley to

the coefficient of permeability. This relationship is

shown in Figure 33.

classified according the type of flow conditions

presumed to exist for the analysis. The analyses of

most seepage problems for engineering projects are

performed under the assumption of *steady-state *flow

conditions. This assumption implies that all conditions

affecting the flow of water through the system are the

same at all times, hence the solution is independent of

time. In contrast, *transient *solutions to seepage

problems, performed less frequently, are time-

dependent as factors such as changing headwater and

tailwater levels and the flow of water into partially

saturated soils can be accounted for in this type of

analysis.

Seepage problems can also be classified as confined or

unconfined depending on the boundary conditions

presumed to exist. In confined flow problems, the

locations of all boundaries are known and fixed.

Unconfined flow problems must have at least one

impervious boundary and a free surface boundary. The

location of the free surface boundary (phreatic surface)

is unknown and must be determined as part of the

solution. Boundary conditions are presumed to be

impervious to flow.

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