Laboratory Investigation of the Mechanical Stability of Unbound Permeable Aggregate Base Materials: Preliminary Direct Shear Test Results

Unbound permeable aggregate base (UPAB), due to its relatively high porosity and permeability, is considered as an alternative to traditional impervious (e.g., dense-graded) bases for controlling subsurface drainage and pavement longevity in an economical and environment-friendly way. The UPAB layer is normally made of open-graded unbound aggregates and thus tends to suffer from insufficient structural strength (or stability). Aimed at further validating a new gradation concept for enhanced structural strength, this paper presented preliminary findings from a series of laboratory tests conducted to evaluate various gradation designs of UPAB materials in relation to mechanical stability. Five representative gradations were selected according to the current UPAB gradation band, and the effects of different UPAB gradation designs on the shear strength properties were investigated using a large-scale direct shear test device. Based on the experimental results, the optimum aggregate gradation design recommended by the new gradation concept for enhanced stability was consequently validated. The use of such optimal gradation is expected to achieve cost-effective UPAB design for long-life concrete pavements.