PERFORMANCE OF TWO LAYER ASPHALT STABILIZED DRAINAGE BLANKETS FOR HIGHWAY SUBDRAINAGE

The performance of two layer asphalt stabilized drainage systems is reported. The two layer drainage system consists of a filter layer of sand-gravel blend filter material, aggregate base or similar material which will satisfy the filter criteria overlain by an open graded drainage layer stabilized with 1.5 to 2 percent paving grade asphalt. The low asphalt content bonds the open graded material into a stable mass without reducing permeability. Asphalt treated permeable material has a drainage capacity up to 100 times greater than a California standard Class 2 permeable material. One installation after six winter seasons had an efficiency 12 times greater than a single layer system under the same service conditions. The two layer system is no more difficult to construct than normal permeable blankets and normal construction equipment and methods can be used. Direct cost comparisons indicate that two layer drainage systems can be constructed at a cost competitive with or lower than conventional single layer drainage systems.