Back-Analysis approach for the design of drainage systems

The geotechnical design of drainage or dewatering systems requires a free surface seepage analysis to evaluate the water flux that corresponds to the sought lowering of the water table. 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. © 2007 ASCE.