• New analytical formulas are derived for drain spacing under non-steady flow conditions. • Asymptotic analysis is employed to enhance the accuracy of Glover’s classical equation. • Drain spacing is accurately computed while accounting for higher-order terms in the Fourier series solution. • Highly accurate solutions are achieved at t = 3 days, with relative errors consistently below 3.4%. Glover’s equation is used to describe the drawdown of the maximum height of the groundwater table to a desired depth within a specified time interval between two drainage pipes, following a sudden rise of the groundwater table, in the case where the drains are installed in a homogeneous soil over an impermeable layer. The determination of the appropriate drain spacing ( L ) under non-steady-state drainage conditions, assuming that the groundwater table is initially horizontal and parallel to the plane of the drains, is obtained from the solution of the linearized Boussinesq equation when all terms of the exponential series except the first are neglected for all times except very small ones. In the present study, a methodology is proposed in which additional terms of the exponential series are incorporated in a simple manner for the calculation of the drain spacing. This improvement enables the determination of drain spacing independently of time t. Results from a numerical example indicate that solutions obtained using three to six terms of the series provide highly accurate and reliable results for t = 3 days, with relative errors below 3.4%, in contrast to the classical Glover equation, which exhibits significant prediction errors for drain spacing over the same time period.
Mindrinos et al. (Fri,) studied this question.