Predicted and experimental water table drawdown during tile drainage

Abstract

Comparisons were made between experimental and predicted positions of the water table during drawdown by drains. Experimental positions were obtained from tank drainage data. The predicted water table positions were determined with a modified form of the Kirkham-Gaskell equation for drawdown. An electrical resistance network was used to obtain the components of the potential gradient which are required by this equation. Two modifications were made in the Kirkham-Gaskell equation: The capillary fringe replaced the water table as the upper boundary of the region in which flow occurred. Instead of using a constant drainable porosity, one was used which related drainable pore space to the water table depth. The agreement between experimental and predicted positions of the water table was good. When either of the aforementioned modifications was not made, significant discrepancies were noted. There was considerable deviation in both shape and mean position of the water table when a constant porosity was used in the theory. The proposed equation for drawdown offers a sound basis for studying drawdown by drains. An electrical resistance network as used in this study is convenient for eliminating the tedious calculations which are required by the equation. Because of extensive calculations, the theory is still not suited for design purposes. It will serve its greatest role in evaluating more practical drawdown equations.