Fig. 12.7 Structural changes in a granular soil resulting from increases in applied stress and pressure deficiency (after Jennings and Burland, 1962).

Figure 12.7 is a reproduction of the diagram prepared by Jennings and Burland (1962) to help them to describe the structural changes that can occur in a non-uniform granular soil when it is subjected to water content and/or applied loading variations.

12.5.1 Granular soils

In a non-uniform saturated granular material the grains tend to form arches, as illustrated in Fig. 12.7a. The effect is, of course, three dimensional so that the soil has a honey-combed structure. The whole soil mass is obviously fairly compressible.

The application of an external stress system causes both shear and normal forces to develop at the grain contact points. If the soil is capable of drainage then there will be a reduction in volume of the soil mass, as the induced internal stress cause individual grains to slide or roll across each other, Fig. 12.7b.

The same effect is obtained if the compression is caused by surface tension effects instead of by applied loading. This situation can arise if water conditions alter so that the soil is no longer submerged but is still saturated, Fig. 12.7c. In this case the compressive forces are maintained by the action of the menisci that have formed around the edges of the soil.

If further loss of water occurs then air will enter the soil and the menisci will retreat into the inner voids of the soil mass.

The intergranular forces are now derived from the high curvature of the menisci at the particle contact points (Fig. 12.7d). These menisci induce only normal forces between the particles which, as there are now no shear forces present, tend to 'bond' together to form a strong and stable soil structure.

If, when in this state, the soil is subjected to externally applied loading it will offer considerably greater resistance to the shear forces induced at the particle contacts than it would have done had it been submerged.

12.5.2 Clayey soils

The shape of a clay particle hardly resembles that of a granular soil particle. Clay particles consist of combined minute thin flat sheets of silica and aluminium or other minerals. The surfaces of these sheets possess an electro negativity whilst their edges are electrically charged, either positive, neutral or negative, depending upon the mineral involved.

It is these electrical forces, acting on the particles at the time of its formation, which were responsible for the structure and therefore the strength of a clay soil.

12.5.3 Swelling and collapse phenomena of clayey soils

In the dry state the structure of a clay soil is somewhat analogous to that of a granular soil in that the clay tends to form itself into 'grains' or 'packets'. These grains consist of numerous clay particles tightly bonded together but, if the applied loading is increased, they will have little tendency to slip or roll and will only tend to distort, unlike granular soil particles.

If such a loaded soil is wetted the bonding between each 'packet' of clay particles is removed and the packets tend to be displaced relative to one another. At the same time each packet, as it takes in water, tends to expand.

The final behaviour of the soil is governed by the magnitude of both the applied loads and the change in the water content. With low loads the soil can be expected to expand whereas, at higher loads, it is more likely that it will decrease in volume.

In their paper, Jennings and Burland point out that, instead of a dry clay swelling as it becomes wet, as is predicted from Terzaghi's effective stress theory, it may first collapse:

'It is quite possible that soaking of the soil under a load will result first in a rapid reduction in volume due to removal of intergranular "bonds" and then a slow increase in volume due to particles taking up water.'

The possible catastrophic effects that can be caused when a dry compacted soil is flooded with water have been discussed by Charles and Burford (1987). They also give details of the complete failure of a block of eight two storey houses built on a stiff clay backfill, which had a maximum depth of 12 m. Soon after construction of the brickwork it was observed that settlement had occurred at the centre of the block after heavy rain. Tests carried out by the Building Research Establishment showed that the compacted fill was susceptible to collapse if flooded.

The block was never occupied and was finally demolished some 9 years later by which time the amount of settlement of the block had reached some 0.3 m.

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