Pavement and Subgrade Drainage Design

This chapter presents a systematic overview of pavement and subgrade drainage design, focusing on its theoretical basis, structural components, and practical applications in highway and urban road engineering. It examines the adverse effects of surface water and groundwater on subgrade and pavement performance, including strength reduction, erosion, frost action, mud pumping, and long-term structural deterioration. Emphasis is placed on the fundamental objective of drainage design: maintaining the subgrade in a stable and relatively dry condition through the effective interception, collection, conveyance, and discharge of water. The chapter reviews major surface and subsurface drainage facilities, such as side ditches, intercepting ditches, drainage ditches, blind drains, seepage trenches, and seepage wells, as well as integrated drainage measures including separator layers and edge drains. In addition, it highlights the growing role of hydrological simulation tools, particularly SWMM, in evaluating runoff control, pollutant reduction, and the hydraulic effectiveness of permeable pavement systems under different design conditions.