Earth Pressure Cell Design; Figure 2: Stress Redistribution, Strong Soil With Stiff Cell; Figure 3: Stress Redistribution, Stiff Soil With Weak Cell - Geokon 4800 Series Instruction Manual

2 | INTRODUCTION | GEOKON
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 distressed zone around the edge of the cell is more
extensive; therefore, the degree of over registration of the mean stress is greater
at the center of the cell. This is represented schematically in Figure 2.
Mean
Stress
0

FIGURE 2: Stress Redistribution, Strong Soil with Stiff Cell

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 3.
Mean
Stress
0

FIGURE 3: Stress Redistribution, Stiff Soil with Weak Cell

Tests conducted at the University of Ohio (USA) with several different soil types
have shown that for
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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. In addition, the response of the cell to its
immediate surroundings depends mostly 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.

1.2 EARTH PRESSURE CELL DESIGN

Earth Pressure Cells are constructed from two stainless steel plates welded
together around the periphery to leave a narrow space between them. This
space is filled with de-aired hydraulic oil, which is connected hydraulically to a
pressure transducer. The pressure transducer converts the oil pressure into an
electrical signal, which is transmitted through a signal cable to the readout
location.
In general, Earth Pressure Cells use an all welded construction; this
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means the space confining the oil is entirely metal and does not require any o-
Cell
Cell
cells the maximum degree of over or under
2:
3: