Relief Wells for Dams and Levees Considering Landward Head

Abstract

Relief well systems for dams and levees are often designed and analyzed as an infinite line using average and midwell heads. Well discharge and excess head in the foundation are calculated by adjusting the solution of a continuous slot to account for additional head losses as flow lines converge to wells. The average and midwell uplift factors allow this by representing the effects of well system geometry and foundation depth in discharge and excess head equations. For partial penetration systems, upwelling may occur landward of the well line with head exceeding that in between the wells. Therefore, without a method for estimating landward head, the average value in the plane of the wells has historically been used as a conservative design value. This approach is better than considering the midwell uplift alone, but in some cases, it can lead to costly overdesign when the landward head is substantially less than the average. Uplift beyond an infinite well line is complicated as its behavior also depends on the ratio of the effective seepage exit distance to well spacing. A third so-called landward uplift factor was developed through finite-element modeling to improve the accuracy of the design process. It allows for a more comprehensive understanding of excess head distribution and for certain scenarios, such as low well penetration and high pressure confinement, improves design head accuracy by one-third to 1 m compared to the average head-based design approach. New charts and equations for each of the uplift factors are provided, along with an example problem to aid the practitioner in efficient graphical problem solution and finite-element model verification.