Geotehnică - engleză

The permeability of a soil is a measure of its capacity to allow the flow of a fluid (a liquid or a gas

in general water) through it.

The principle is that soil consists of solid particles with voids between them. In general the voids

are interconnected, which enables water to pass through them. The degree of permeability is

determined by applying a hydraulic difference across a sample of soil, which is fully saturated and

measuring the consequent rate of flow of water. The "coefficient of permeability" in expressed in terms

of a velocity.

The flow of water through soils of all types, from gravel's and sands to clays, are governed by the

same physical laws. The difference between the permeability characteristics of extreme types of soil is

merely one of degree, even though clay can be ten million times less permeable than sand. Clays are not

completely impermeable, although they may appear to be so if the rate of low through them is not

greater than the rate of evaporation loss.

The method used for measuring permeability depends upon the characteristics of the material.

Permeability tests on natural disturbed soil are probably carried out more frequently in-situ than

in the laboratory, but field inspection and testing is beyond the scope of this laboratory guide.

There are two types of laboratory tests for the direct measurement of the permeability of soils:

Constant head test-for soils of high permeability, such as sands. The constant head test is a

permeability test in which water is made to flow through a soil sample under a constant difference in

head or hydraulic gradient.

Falling head test- for soils of intermediate and low permeability, such as silts and clays.

The falling head test is a permeability test in which the piezometer tube used for measuring the

head also provides the water, which passes through the sample, and therefore the level falls during the

test.

1. Darcy's Law

H. Darcy from France performed a classical experiment in 1856, using a set-up similar to that

shown in Fig. 1, in order to study the properties of the flow of water through a sand filter bed. By

measuring the value of the rate of flow or discharge, q for various values of the length of the sample, L,

and pressure of water at top and bottom of the sample, h1 and h2, Darcy found that q was proportional

to (h1 - h2)/L or the hydraulic gradient, i

Water Flow in Soil: "Permeability"