Soil Pressure Measurement, Forgotten Facts about Compliance.

Abstract

Reliable measurement of soil pressure has been a frustrating task for experimentalists in the field of soil mechanics and soil structure interaction for many years. The difficulty stems from the fact that most sensing systems involve compliance in the measurement concept. This compliance induces a local redistribution of pressure which in turn imparts a parasitic output to the measurement. Even the smallest deflections introduce errors into the ambient pressure field. In the past researchers have attempted to calibrate this effect. Over the years it has been demonstrated that calibrations of this sort are dependent upon soil type, particle size, density and stress history dependent; the response upon unloading is highly non-linear resulting in a high degree of hysteresis. Because of these difficulties data collected by commercial soil pressure transducers may be considered somewhat unreliable. The paper utilizes a concept called the Null Soil Pressure System. The system is an active sensing system based on the requirement that the sensing element is continuously and stringently maintained in an undeflected state. Sensors of this type were embedded and tested a series of uniformly graded soils with particle diameter ranging from less than 0.15 mm to 13 mm. Results presented in this paper illustrate that the pressure required to maintain the undeflected state consistently over registers by a value of 4%±3%, independent of soil type, particle size, density. No hysteresis is noted between loading and unloading and additional loading cycles align with that of the virgin loading segment. This level of certainty is far better than that of standard commercial systems.