Back-analysis approach for the design of drainage systems

The geotechnical design of drainage and well point systems involves the adequate evaluation of the water flux necessary to reach the sought lowering of the water table. This in turn requires the solution of a free surface seepage problem. It is observed in practice that sometimes the calculated flux is not sufficient to reach the predicted result, even when the hydraulic conductivity of the soil mass has been properly determined. This prevents the initiation of the in situ works e.g., excavation with a relevant economical loss. The possible causes of this problem are investigated through a series of two- and three-dimensional unconfined seepage analyses. They show that the numerical results strongly depend on the characteristics of the numerical model and that, consequently, the computed fluxes and lowering of the water table do not necessarily correspond to the field conditions. To overcome this drawback a back-analysis approach is suggested for calibrating the numerical model adopted in design. An application of this technique to an actual field problem is finally discussed.