Hydraulic design of geosynthetic and granular liquid collection layers comprising two different slopes

Liquid collection layers used in landfills often comprise two sections with different slopes. Typically, one of the slopes is much steeper than the ot her one. The steeper slope is generally the downstream slope in a landfill cover and the upstream slope in a leachate collection layer. The liquid col lection material is generally a geosynthetic (such as a geonet) on the stee p slope and is either a geosynthetic or a granular material (such as sand o r gravel) on the other slope. Design methods are available for the case whe re there is a drain that promptly removes the liquid at the toe of each of the two sections. This paper provides a method to design the liquid collect ion layer for the case where there is no drain at the connection between th e two sections, i.e. when the only drain is at the toe of the downstream se ction. The method consists of analytical expressions for calculating the ma ximum thickness of liquid under steady-state conditions in each of the two sections of the liquid collection layer. Design examples are presented and practical guidance is provided for the use of a transition zone between the two sections when needed.