Rational Basis for Evaluation and Design of Pavement Drainage Layers

A pavement drainage layer must have enough drainage capacity (that is, good drainage properties and adequate layer thickness) to discharge infiltrated water within a specified time, and it must be able to maintain sufficient drainage capacity throughout the entire design service life of the pavement (that is, have good anticlogging properties). A rational analytical engineering procedure for the evaluation and design of pavement drainage layers is lacking. Central to this problem is the need for a practical method with which to measure the fundamental drainage properties of the drainage layer materials and a suitable procedure with which to evaluate the clogging potential of the materials. Flow devices that measure empirical drainage or flow indices could not be used for mechanistic design of pavement drainage layers. An approach was adopted that is based on a falling-head apparatus similar in structure to conventional constant-head permeability test devices. Two procedures are proposed to meet the need for a rational design of pavement drainage layers based on engineering principles: (a) the use of an expedient laboratory falling-head test to determine the permeability of drainage materials; this enables the geometric design (layer thickness and cross slope) of a drainage layer to be performed on the basis of drainage capacity consideration; and (b) a laboratory clogging test to assess the clogging potential of the proposed material and thickness. A design framework is recommended to integrate the structural and drainage design of pavement drainage layers.