Smith's Elements of Soil Mechanics. Ian Smith

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Fig. 2.16 Example wetting and drying water retention curves.

The amount of water in the pores for a particular value of suction will depend on whether the soil is wetting or drying. This gives rise to the phenomenon known as hysteresis, and the shape of the water retention curve for each process is shown in Fig. 2.16. A full descriptive text on water retention curves and hysteresis is given by Fredlund et al. (2012).

      2.13.5 Measurement of soil suction

      From a geotechnical point of view, there are two components of soil suction as follows:

      It should be noted that these two forms of soil suction are completely independent and have no effect on each other.

      The total suction exhibited by a soil is obviously the summation of the matric and the osmotic suctions.

      If a soil is granular and free of salt, there is no osmotic suction and the matric and total suctions are equal. However, clays contain salts and these salts cause a reduction in the vapour pressure. This results in an increase in the total suction, and this increase is the energy needed to transfer water into the vapour phase (i.e. the osmotic suction).

      There are several types of equipment available which can be used to measure soil suction values. Amongst them are psychrometers, porous blocks, filter papers, suction plates, pressure plates, and tensiometers, the last being the most popular for in situ measurements. A useful survey was prepared by Ridley (2015).

       The psychrometer method

      A psychrometer is used to measure humidity and is therefore suitable to measure total soil suction, i.e. the summation of the matric and the osmotic components. The equipment and its operation have been described by Fredlund et al. (2012).

      A sample of the soil to be tested is placed in a plastic container. A hole is then drilled to the centre of the specimen, a calibrated psychrometer inserted and the drilled hole is backfilled with extra soil material. The whole unit is finally sealed with plastic sheeting and placed in an airtight container, where it is left for three days with its temperature maintained at 25 °C. After this time, the soil sample is deemed to have achieved both thermal and vapour pressure equilibrium and relative humidity measurements can be taken.

       The filter paper method

With this technique, described by Campbell and Gee (1986), both total and matric suctions can be measured. In a typical test, the soil specimen is prepared in a cylindrical plastic container and a dry filter paper disc is placed over its upper surface (Fig. 2.17). This filter will measure the matric suction.

      A perforated glass disc is placed over the filter paper and a further filter paper is then placed over the glass. As this top filter paper is not in actual contact with the soil sample, it can only measure the total suction.

The assembled specimen/filter paper is left for at least a month, at a temperature of 25 °C, to obtain thermal and vapour equilibrium. At the end of this time, the assembly is dismantled, and the water contents of the specimen and the two filter papers are determined. The water contents obtained for the filter papers can be converted into the required suction values using a suction/water content curve for the filter paper material.

Fig. 2.17 Soil suction measurement – an arrangement for the filter paper method.

       The tensiometer

      Stannard (1992) presented a review of the standard tensiometer and covered the relevant theory, its construction, and possible uses. The apparatus is mainly used for in situ measurements and consists of a porous ceramic cup placed in contact with the soil to be tested.

      A borehole is put down to the required depth and the ceramic filter lowered into position. Water is then allowed to exit from a water reservoir within the tensiometer and to enter the soil. The operation continues until the tensile stress holding the water in the tensiometer equals the stress holding the water in the soil (i.e. the total soil suction).

      The tensile stress in the water in the tensiometer is measured by a pressure measuring transducer and is taken to be the value of the total soil suction.

      The tensiometer must be fully de‐aired during installation if accurate results are to be obtained. The response time of the type of apparatus described is only a few minutes but it has the disadvantage, until recently, that it could only be used to measure suctions up to about 100 kPa.

      

2.14 Earth dams

      2.14.1 Seepage patterns through an earth dam

      As the upper flow line is subjected to atmospheric pressure, the boundary conditions are not completely defined, and it is consequently difficult to sketch a flow net until this line has been located.

Part of such a flow net is shown in Fig. 2.18. It has already been shown that the hydraulic head at a point is the summation of velocity, pressure, and elevation heads. As the top flow line is at atmospheric pressure, the only type of head that can exist along it is elevational, so that between each successive point where an equipotential cuts an upper flow line, there must be equal drops in elevation. This is the first of three conditions that must be satisfied by the upper flow line.

Fig. 2.18 Part of a flow net for an earth dam.

Fig. 2.19 Conditions at the start of an upper flow line.

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