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.
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