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1.
INTRODUCTION
1.1 THEORY OF OPERATION
Earth Pressure Cells, sometimes called Total Pressure Cells or Total Stress Cells
are designed to measure stresses in soil or the pressure of soil on structures.
Cells will respond not only to soil pressures but also to ground water pressures
or to pore water pressure, hence the term total pressure or total stress. A
simultaneous measurement of pore water pressure (μ), using a piezometer, is
necessary to separate the effective stress (σ') from the total stress (σ) as
defined by Terzaghi's principle of effective stress:
σ' = σ - μ
EQUATION 1: Terzaghi's Principle of Effective Stress
These parameters coupled with the soil strength characteristics will determine
soil behavior under loads.
Earth pressure cells of the type described here are the hydraulic type; two flat
plates are welded together at their periphery and are separated by a small gap
filled with a hydraulic fluid. The earth pressure acts to squeeze the two plates
together thus building up a pressure inside the fluid. If the plates are flexible
enough (i.e., if they are thin enough relative to their lateral extent), then at the
center of the plate the supporting effect of the welded periphery is negligible,
and it can be stated that at the center of the cell the external soil pressure is
exactly balanced by the internal fluid pressure.
This is true only if the deflection of the plates is kept to a minimum and thus it is
important that the cell be stiff. This in a practical sense means that the fluid
inside the cell should be as incompressible as possible and that the pressure
transducer required to measure the fluid pressure should also be stiff having
very little volume change under increasing pressure.
Tests conducted by various researchers (as reported by Dunnicliff, 1988) have
shown that the introduction of a flat stress cell into a soil mass will alter the
stress field in a way dependent on the relative stiffness of the cell, with respect
to the soil, and also with respect to the aspect ratio of the cell, i.e., the ratio of
the width of the cell to its thickness. A thick cell will alter the stress more than a
thin cell. For these reasons, a thin, stiff cell is best, and studies have shown an
aspect ratio of at least 20 to 1 to be desirable.
Ideally, the cell ought to be as stiff (compressible) as the soil, but in practice this
is difficult to achieve. If the cell is stiffer (less compressible) than the soil, then it
will over register the soil pressure because of a zone of soil immediately around
the cell which is "sheltered" by the cell and therefore does not experience the full
soil pressure. This can be represented schematically as shown in Figure 1.
1:
Mean
Stress
0
Cell
FIGURE 1: Stress Redistribution, Weak Soil with Stiff Cell
MODEL 4800 SERIES VW EARTH PRESSURE CELLS | INTRODUCTION | 1
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