Development of a Precision Hydraulic Head Difference Generator

Abstract

In order to obtain accurate value of coefficient of permeability of soil in laboratory, it is necessary to measure precisely the hydraulic gradient and the flow rate. A new precision hydraulic head difference generator was developed for this purpose. The new generator is divided into two parts. One is a water supply part which is composed of an outer tank and an inner tank floating on the water of outer tank. The other is a drain tank. When the volume of supply water stored in the inner tank is decreased as a result of percolation, the inner tank becomes light and is elevated by a buoyant force. The dimensions of these tanks were determined in such a way that the hydraulic head difference between the supply and the drain water is kept constant during percolation. The hydraulic head difference and the flow rate are measured by a precision differential pressure transducer and a load cell, respectively. These data are recorded by a computer which also control electric valves so as to enable the automatic operation of percolation test. In order to evaluate the performance of this apparatus, the change in hydraulic head difference was observed during percolation test. A similar test with an ordinary Mariotto bottle was also carried out. The results showed that the hydraulic head difference of the new apparatus was kept in a range of 0.02 cm, whereas that of the Mariotto type fluctuated in a range of 0.5 cm. The principle of the new generator can be applied to other kinds of apparatus which require constant hydraulic head difference.