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**Pressure, Elevation and Total Heads**

In soils, the interconnected pores provide passage for water. A large number of such flow paths act together, and the average rate of flow is termed the coefficient of permeability, or just permeability. It is a measure of the ease that the soil provides to the flow of water through its pores.

At point **A,** the pore water pressure (**u**) can be measured from the height of water in a standpipe located at that point.

The height of the water column is the **pressure head **(** h_{w}**).

To identify any difference in pore water pressure at different points, it is necessary to eliminate the effect of the points of measurement. With this in view, a datum is required from which locations are measured.

The **elevation head **(**h _{z}**) of any point is its height above the datum line. The height of water level in the standpipe above the datum is the

**h = h**** _{z}** +

**Darcy's Law**

**Darcy's law** states that there is a linear relationship between flow velocity (**v**) and hydraulic gradient (**i**) for any given saturated soil under steady laminar flow conditions.

If the rate of flow is **q** (volume/time) through cross-sectional area (**A**) of the soil mass, Darcy's Law can be expressed as**v = q/A = k.i**

where** k = **permeability of the soil

The flow velocity (**v**) is also called the Darcian velocity or the** superficial velocity**. It is different from the actual velocity inside the soil pores, which is known as the **seepage velocity,** **v _{S}**. At the particulate level, the water follows a tortuous path through the pores. Seepage velocity is always greater than the superficial velocity, and it is expressed as:

where **A_{V} **= Area of voids on a cross section normal to the direction of flow

**Permeability of Different Soils**

Permeability (**k**) is an engineering property of soils and is a function of the soil type. Its value depends on the average size of the pores and is related to the distribution of particle sizes, particle shape and soil structure. The ratio of permeabilities of typical sands/gravels to those of typical clays is of the order of **10 ^{6}**. A small proportion of fine material in a coarse-grained soil can lead to a significant reduction in permeability.

For different soil types as per grain size, the orders of magnitude for permeability are as follows:

**Factors affecting Permeability**

In soils, the permeant or pore fluid is mostly water whose variation in property is generally very less. Permeability of all soils is strongly influenced by the density of packing of the soil particles, which can be represented by void ratio (**e**) or porosity (**n**). **For Sands**

In sands, permeability can be empirically related to the square of some representative grain size from its grain-size distribution. For filter sands, Allen Hazen in 1911 found that **k » 100 (D _{10})^{2}** cm/s where

Different relationships have been attempted relating void ratio and permeability, such as

where **k _{o}** and

**For Silts and Clays**

For silts and clays, the Kozeny-Carman equation does not work well, and ** log k **versus

For clays, it is typically found that

where **C _{k}**is the permeability change index and

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