Effect of gradation on in-plane permeability of geocomposite-embedded subbase layers

The gradation characteristics of the granular subbase (GSB) layers play a crucial role in highway subsurface drainage. This study investigates the gradation effects on in-plane permeability of GSB layers (k_GSB) with and without drainage geocomposites (GC). Large-scale permeameter experiments were performed, using 12 different GSB gradations and two types of GCs (Biaxial Geonet Geocomposite, BGG and Triaxial Geonet Geocomposite, TGG) under standard traffic load. Densely-graded GSB materials exhibited a lower in-plane permeability (k_GSB ∼ 5 m/day), which increased by ten-fold and 15-fold with BGG and TGG embedment, respectively. Openly-graded GSB materials (k_GSB > 750 m/day) showed a 18% and 35% enhancement with the embedment of BGG and TGG, respectively. Traffic loading caused a 15% reduction in in-plane permeability. A suitability chart based on the time-to-drain approach was developed to ensure effective subsurface drainage. Furthermore, globally applicable condition-based empirical models, considering subbase gradations proposed by various transportation agencies, were proposed to estimate the permeability of GSBs with and without GCs. The findings highlighted that the GCs significantly enhanced permeability, particularly of densely graded GSB materials. Additionally, a design example is presented to illustrate the effectiveness of highway subsurface drainage with GC installation.