Seepage

If fluid pressures in a soil deposit are uniformly increasing with depth according to

is the depth below the water table, then hydrostatic conditions will prevail and the fluids will not be flowing through the soil.

If the water tables are changing levels with time, or if the soil is in the process of consolidation, then steady state conditions do not apply.

Darcy's law states that the volume of flow of the pore fluid through a porous medium per unit time is proportional to the rate of change of excess fluid pressure with distance.

The constant of proportionality includes the viscosity of the fluid and the intrinsic permeability of the soil.

For the simple case of a horizontal tube filled with soil The total discharge,

(having units of volume per time, e.g., ft3/s or m3/s), is proportional to the intrinsic permeability,

The above equation works well for a horizontal tube, but if the tube was inclined so that point b was a different elevation than point a, the equation would not work.

is the depth measured from an arbitrary elevation reference (datum).

, and expressing the rate of change of excess pore pressure as a derivative, we obtain a more general equation for the apparent velocity in the x-direction: where

[2]) Civil engineers predominantly work on problems that involve water and predominantly work on problems on earth (in earth's gravity).

For this class of problems, civil engineers will often write Darcy's law in a much simpler form:[3][4][5] where

The hydraulic gradient is the rate of change of total head with distance.

The total head is related to the excess water pressure by: and the

is zero if the datum for head measurement is chosen at the same elevation as the origin for the depth, z used to calculate

, can vary by many orders of magnitude depending on the soil type.

Layering and heterogeneity and disturbance during the sampling and testing process make the accurate measurement of soil hydraulic conductivity a very difficult problem.

[6] In two or three dimensions, steady state seepage is described by Laplace's equation.

But traditionally two-dimensional seepage problems were solved using a graphical procedure known as flownet.

Flownets may be used to estimate the quantity of seepage under dams and sheet piling.

When the seepage velocity is great enough, erosion can occur because of the frictional drag exerted on the soil particles.

Vertically upwards seepage is a source of danger on the downstream side of sheet piling and beneath the toe of a dam or levee.

Seeping water removes soil, starting from the exit point of the seepage, and erosion advances upgradient.

[8] The term "sand boil" is used to describe the appearance of the discharging end of an active soil pipe.

[9] Seepage in an upward direction reduces the effective stress within the soil.

When the water pressure at a point in the soil is equal to the total vertical stress at that point, the effective stress is zero and the soil has no frictional resistance to deformation.

For a surface layer, the vertical effective stress becomes zero within the layer when the upward hydraulic gradient is equal to the critical gradient.

[5] At zero effective stress soil has very little strength and layers of relatively impermeable soil may heave up due to the underlying water pressures.

The loss in strength due to upward seepage is a common contributor to levee failures.

Quicksand was so named because the soil particles move around and appear to be 'alive' (the biblical meaning of 'quick' – as opposed to 'dead').