Numerical Simulation And Analysis Of Unsaturated Seepage Drainage In Pavement Structures

According to a lot of investigations on pavement service condition, inadequate subsurface drainage of pavement has been identified as a primary cause of unsatisfactory pavement performance and premature failures. Most research on the effects of moisture in pavement has been based on conditions of total saturation with loss of pavement strength calculated using saturated flow assumptions. Yet most dense graded pavement base layers tend to be unsaturated most of the rain time. The flow of water through unsaturated soil is primary a function of matric suction and gravity. Based on unsaturated flow theory, a finite element software named SEEP/W is used to simulate saturated-unsaturated pavement seepage and subsurface drainage system performance. The primary contents are as follows:(1)Based on the fundamental seepage physical equation, the basic differential equation of saturated-unsaturated seepage in porous mediums has been introduced. Saturated flow theory and unsaturated flow theory are compared.(2)This pape focuses on modeling unsaturated flow through typical secondary road system. The initial conditions, boundary conditions and geometries are relatively simplified for analyzing the role of rainfall infiltration. Volumetric moisture content curves are plotted to analyze subsurface drainage perfomace. Sections that have low permeability base courses and long horizontal drainage path with edgedrains are not designed to effectively drain infiltrating water. Permeable bases have significantly reduced moisture in the sections. Saturated and unsaturated flow theory are compared by canculating t50(50% of drainage). The time to drain calculated based on unsaturated flow theory will be generally longer than that evaluated under saturated flow assumption. Water table, rainfall duration and density are considered in the sensitivity analysis and those parameters influence infiltration results greatly. It is suggested that field measured parameters be considered in the numerical simulation.(3) Unsaturated flow in silty clay and silty sand median are modeling under different rainfall event. When the rainfall density is stronger, the saturation in the surface layer will be higher. More water will infiltrate in the deep layer when the rainfall duration is longer. Rainfall data and backfilled soil type should be considered in whether or not the underdrainages should be installed. Underdrainages have significantly reduced moisture under the median. Mass water have Infiltrated into sections when existing cracks and joints. Permeable bases have significantly reduced moisture in the sections.