STUDY ON ROADWAY SUBSURFACE DRAINAGE SYSTEM AND RELATED PERFORMANCE USING FEM

Poor subsurface drainage is one of the factors that causes pavement distress and reduces pavement service life. The evaluation of roadway subsurface drainage system required good knowledge in groundwater flow especially the unsaturated water flow through pavement layers and related properties that affect the ability of drainage system to remove moisture helds in base, subbase and subgrade layers. This paper uses the finite element method (FEM) to solve the well-known unsaturated flow equation called 'Richards Equation' that applied to simulate water flow in roadway subsurface drainage layers with the presence of edge drain. Three granular bases, five subgrade soils and one type of trench have been used to study the effect of pavement layers properties on the performance of subsurface drainage system. Current study reveals that the performance of drainage system not only depends on edge drain design but it also heavily depends on subgrade layer properties. It is found that residual degree of saturation after long time not highly affected by pressure heads at air entry but this suction pressure affects the time required to reach the FHWA indicator and the total moisture that drained by edge drain decreases with the increase of hydraulic conductivity of subgrade soil.