Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-9

As can be seen there is a stress concentration at the rigid rim but in the center of the cell the soil stress is only slightly higher than the mean soil stress, i.e. only slightly higher than the stress which would obtain were the cell not present.

In a stronger soil the de-stressed zone around the edge of the cell is more extensive and hence at the center of the cell the degree of over-registration of the mean stress is greater. This is represented schematically in Figure 2.2

Fig .2.2: Stress Redistribution, Strong Soil with Stiff Celll

In a stiff soil the cell may be less stiff (more compressible) than the soil, in which case the cell will under-register the mean soil stress as the stresses in the soil tend to "bridge" around the cell.

This is represented schematically in Figure 2.3

Fig.2.3: Stress Redistribution, Stiff Soil with Weak Cell

With several different soil types have shown that for cells the maximum degree of over or under-registration amounts to 15% of the mean soil stress. Other factors should be kept in mind; the inherent variability of soil properties which give rise to varying soil stresses at different locations and a corresponding difficulty in getting a good sample of the mean stress from a limited number of cell locations. Also, the response of the cell to its immediate surroundings depends very largely on how closely the soil mass immediately around the cell has the same stiffness or compressibility or the same degree of compaction as the undisturbed soil mass. Installation methods will need to pay particular attention to this detail.