Effect of the existence of under drain on Uplift pressure and exit gradient


The effects of the existence of an under drain pipe beneath the foundation of a hydraulic structure, on each uplift pressure, and exit gradient are investigated. More specifically the effect of the horizontal and vertical locations and the diameter of this under drain pipe were investigated. The (Geo-Studio 2007) Seep/w software was used for simulating different geometrical configurations of a typical hydraulic structure model. Results indicate that the existence of this drain will reduce the pressure head beneath the foundation of the structure. The observed reduction in uplift pressure was found considerable. As the drain diameter increases the pressure head decrease more and more. This results in a decrease in the required volume of the structure to achieve the required factor of safety against uplift as a greater diameter for the drain pipe. A 100% decrease in this volume where observed where the under drain location is near the u/s sheet pile (location –D-). The exit gradient was found to decrease also in general when a drain pipe is exist near the upstream sheet pile. However, it was found to increase when the drain pipe was located below the downstream sheet pile especially (location-G-). For this case the downstream length of protection (L) increase from (2.837m compared to the case of no drain) to (4.70 m for with drain at location G) that means about 65.67% increases. This may be attributed to the densification of the stream lines near the downstream side when the drain was located in this position, the increase in the protection length required was found to be more as the diameter of the drain pipe increase.