PERMEABILITY OF SOILS-Introduction

A material is said to be permeable if it contains continuous voids. Since such voids are contained in all soils including the stiffest clays, and in practically all nonmetallic construction materials including sound granite and neat cement , all these materials are permeable.

Furthermore, the flow of water through all of them obeys approximately the same laws. Hence the difference between the flow of the water through clean sand and through sound granite is merely one of degree.

The permeability of soils has a decisive on the cost and the difficulty of many construction operations, such as the excavation of open cuts in water-bearing sand, or on the rate at which a soft clay stratum consolidates under the influence of the weight of a superimposed fill. Even the permeability of dense concrete or rock may have important practical implications, because water exerts a pressure on the porous material through which it percolates. This pressure, which is known as seepage pressure, can be very high. The erroneous but widespread conception that stiff clay and dense concrete are impermeable is due to the fact that the entire quantity of water that percolates through such materials toward an exposed surface is likely to evaporate, even in a very humid atmosphere.   As a consequence, the surface appears to be dry. However, since the mechanical effects of seepage are entirely independent of the rate of percolation, the absence of visible discharge does not indicate the absence of seepage pressure.

Striking manifestations of this fact may be observed while an excavation is being made in a very fine rock flour. The permeability of this material is very low. Yet, a slight change in the pressure conditions in the porewater may suffice to transform a large quantity of the material into a semiliquid.

All soils are permeable materials, water being free to flow through the interconnected pores between the solid particles. The pressure of the pore water is measured relative to atmospheric pressure and the level at which the pressure is atmospheric (i.e. zero) is defined as the water table(WT) or the phreatic surface.

Below the water table the soil is assumed to be fully saturated although it is likely that, due to the presence of small volumes of entrapped air, the degree of saturation will be marginally below 100%. The level of the water table changes according to climatic conditions but the level can change also as  a consequence of constructional operations.

A perched water table can occur locally, contained by soil of low permeability, above the normal water table level.

Artesian conditions can exist if an inclined soil layer of high permeability is confined locally by an overlying layer of low permeability: the pressure in the artesian layer is governed not by the local water table level but by a higher water table level at a distant location where the layer is unconfined.

Below the water table the pore water may be static, the hydrostatic pressure depending on the depth below the water table, or may be seeping through the soil under hydraulic gradient: this chapter is concerned with the second case. Bernoulli’s theorem applies to the pore water but seepage velocities in soils are normally so small that velocity head can be neglected. Thus :

Above the water table, water can be held at negative pressure by CAPILLARY TENSION: the smaller the size of the pores the higher the water can rise above the water table.

The capillary rise tends to be irregular due to the random pore sizes occurring in a soil.

The soil can be almost completely saturated in the lower part of the capillary zone but in general the degree of saturation decreases with height.

When water percolates through the soil from the surface towards the water table some of this water can be held by surface tension around the points of contact between particles.

The negative pressure of water held above the water table result in attractive forces between the particles: this attraction is referred to as soil suction and is a function of pore size and water content.

PERMEABILITY

In one dimension, water flows through a fully saturated soil in accordance with Darcy’s empirical law:

In general the smaller the particle the smaller is the average size of the pores and the lower is the coefficient of permeability. The presence of a small percentage of fines in a coarse-grained soil results in a value of k significantly lower than the value for the same soil without fines.For a given soil the coefficient of permeability is a function of void ratio.If a soil deposit is stratified the permeability for flow parallel to the direction of stratification is higher than that for flow perpendicular to the direction of stratification. The presence of fissures in a clay results in a much higher value of permeability compared with that of the unfissured material.

The coefficient of permeability also varies with temperature, upon which the viscosity of the water depends. If the value of k measured at 20C is taken as 100% then the values at 10C and 0C are 77% and 56% respectively .

The coefficient of permeability can also be represented by the equation:

 

A material is porous if it contains interstices. The porous material is permeable if the interstices are interconnected or continuous. A liquid can flow through a permeable material. Electron photomicrographs of even very fine clays indicate that the interstices are interconnected. However, the size, cross-section, and orientation of the interstices in different soils are highly variable. In general, all the soils are permeable.

The property of a soil which permits flow of water (or any other liquid) through it, is called the permeability. In other words, the permeability is the ease with which water can flow through it, A soil is highly pervious when water can flow through easily. In an impervious soil, the permeability is very low and water cannot easily flow through it. A completely impervious soil does not permit the water to flow through it. however , such completely impervious soils do not exist in nature, as all the soils are pervious to some degree. A soil is termed impervious when the permeability is extremely low.

Permeability is a very important engineering property of soils. A knowledge of permeability is essential in a number of soils engineering problem, such as settlement of building, yield of wells seepage through and below the earth structure, it controls the hydraulic stability of the soils masses. The permeability of soils is also required in the design of filters used to prevent piping in hydraulic structure.

As mentioned in chapter 7, free water or gravitational water flows through soils under the influence of gravity . flow of free water depends upon the permeability of the soil and the head causing flow. This chapter deals with Darcy’s law for flow of water. The method for the determination of permeability and the factors affecting the permeability of soils. Further details of flow of water and seepage problems are discussed in the next chapter.  

As a result of the hydrological cycle(rainfall, infiltration) it is inevitable that the voids between soil particles will fill with water until they become fully saturated. There then exists a zone of saturation below ground level the upper surface of which is called the water table.

The water table generally follows the shape of the ground surface topography but in a subdued manner. A sloping water table surface is an indication of the flow of groundwater or seepage in the direction of the fall. Water tables change with varying rates of infiltration so that in winter they can be expected at high levels and at lower levels in summer.

GROUNDWATER

The voids of permeable deposits such as sands will fill up easily and also allow this water to flow out easily , so they are called aquifers (bearing water). The void spaces in a clay will also contain water but these void spaces are so small that flow of water is significantly impeded, making a clay impermeable. Clay deposits will then act as aquicludes (confining water).The location and state of groundwater in soil deposits is often determined by the stratification of sand-clay or permeable-impermeable sequences. Some commonly used terms are described in Fig. 3.2 .

FLOW PROBLEMS

In nature groundwater may be flowing through the ground but this flow will not normally be large enough to cause instability, so the ground is stable or in equilibrium. Ground engineering works, particularly excavations, will disturb this equilibrium and alter the pattern of flow. It is the responsibility of the geotechnical engineer to identify where a problem may be encountered and how to ensure that stability of the works is maintained

   

Reference

• Soil Mechanics - Principle and Practise

• Introductory soil mechanics  

• Soil Mechanics - R. F. Craig
• Soil Mechanics in Engineering Practice - Karl Terzaghi, Ralph B. Peck