Transient unsaturated flow in the drainage layer of a highway: solution and drainage performance

In order to quickly remove water entering pavement layers so as to mitigate water-induced damage, drainage layers are widely used in highway pavements. This paper introduces a 1-D mathematical model in order to quantify the transient behaviour of the groundwater flow in the drainage layer of highway pavements. The model takes into account the capillary effect and the nonlinear relationship between hydraulic conductivity and saturation. The analytical solution is obtained and validated against the 2-D Richards equation-based model. Then, sensitivity analysis is carried out and the results show that the drainage time is reduced and drainage efficiency is enhanced by capillarity. The drainage capacity (the total water drained from the drainage layer) depends on the water retention curve (e.g. alpha(G) in the Gardner equation). In view of material selection, a relatively low value of alpha(G) helps improve the drainage capacity of the drainage layer, whereas a suitable amount of fine particles also leads to a better drainage performance. Furthermore, the drainage material with hydraulic properties is recommended in terms of drainage performance improvement and strength requirement. The analytical solution, which is simple and easy to use, provides guidance for assessing the capillary effects on the effectiveness and efficiency of subsurface drainage systems of a highway.