|M.Sc Student||Dabush Razi|
|Subject||Development of a Hybrid Soil Pressure Transducer to Reduce|
the Effect of Arching
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Mark Lloyd Talesnick|
|Full Thesis text - in Hebrew|
The purpose of this study was the development of earth pressure measurement instrument, whereby characteristic measuring errors of such devices are reduced. The development is based on monitoring the measurement errors in oil-filled earth pressure gauge (assumed error origins were proven in this research), and correcting them based on the monitored results.
Earth pressure measurements can be divided into two categories - within an earth mass, and flush with a structural surface. For these measurement types there are two largely used earth pressure gauge configurations. The first is based on membrane bending, made of a chamber (usually cylinder shaped), with hard bottom and sides, which has a thin membrane as a top side with strain gauges attached to it. This membrane bends under applied pressure, and the voltage change in the strain gauge is measured. The second sensor type is oil-filled earth pressure gauge, which is widely used as it claims to restrain the membrane bending. The sensor is made of two plates welded together, with an oil-filled cavity in between; the external plates transfer the applied pressure to the oil within, and the oil pressure is measured by a probe on the device's edge. The measured oil pressure might reflect the outside earth pressure. This type of device is based on the assumptions that (1) the general structure (external membrane) does not bend, (2) the oil filling the device is incompressible, (3) the cavity is completely filled with oil (no air in the system), and (4) the applied pressure on the probe does not evacuate a significant volume of oil, which in turn would result in deflection. In practice, however, each of these assumptions are violated in a way, and the question is how and to what extent does it affect the measurement. A third type of gauge was developed at the Technion based on "the nulling technique". The technique is based on the principle of using air pressure on the backside of the membrane to eliminate its bending.
In this study all three sensor types were used to perform both in-mass and flush measurements. This was done to better understand the problem when using advanced instruments, and to receive a comparable basis for further studies. The same sensors were used in both free bending and nulling configurations. Oil-filled sensors of similar geometries were fabricated and tested as well. The experiments were repeated with various granular materials - sand, rice grains, glass balls and corn kernels.
The most important contribution of this study is exposing the oil-filled pressure gauge's weaknesses, which have never been presented or addressed before. These weaknesses undermine the credibility of the oil-filled pressure gauge configuration. In addition, a new pressure gauge geometry is introduced, improving strain gauge sensing beyond current standards, with only mild modification of the membrane’s fabrication. Finally, the study presents the arching properties of different materials.