Measurement of Stresses and Strains in Soils
Eyes in the Soil
A bit about the difficulties of measuring soil pressure.
Measuring the pressure applied by soil or any particulate material in a reliable way is not a trivial task. This is true if the measurement is made at a structural boundary or within a soil mass.
Among the issue which make it so frustrating is the stiffness miss-match between the soil/particulate media and the sensor.
Since most soil pressure sensors are based on a deflecting membrane technologies this miss-match induces a non linearity and hysteresis which are both parasitic to the actual response of the media. The same is true for most fluid filled systems.
It turns out that when dealing with a particulate medium even tiny deflection as small as a few micrometers can have profound affect.
The result is that calibration of a particular sensor against/within a particular soil can not be applied to a second soil, or even the same soil in a different condition. See the figure below on the left.
The situation gets even messier in response to cycles of loading and unloading. See figure above on teh right. Note the non-linear hysteretic output in response to unloading. Because of the parasitic deflection the sensor responds differently during unloading and does not follow the same path as during loading.