Theory of Pipe Drainage Assisted by Mole Drainage

Abstract

This paper addresses the problem of draining a heavy soil of low hydraulic conductivity overlying an impermeable layer by means of a combined system of pipe and mole drains. The analysis uses the complex functions and the theory of images to develop formulas for pipe and mole drain discharges and for pipe drain spacing design. A flow pattern predicted on the basis of these formulas shows that boundary conditions are satisfied and the stream line distribution is logical. An equation for the unsteady vertical movement of the water table is presented, from which an approximate but satisfactory spacing equation is established. A numerical example is provided and it is shown that although the spacing formula is implicit, only a small number of trials are needed. It is also shown that introducing mole drains may allow the spacing of pipe drains to be increased several times. A technique is proposed for choosing the number of mole drains between two pipe drains such that the optimal spacing design is achieved.