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After a period of using high-grade highways, there can be emerged various diseases inevitably, in which water is the main natural factor to endanger highways. The majority of subgrade subsidence, erosion, collapse, pumping, asphalt pavement ravelling, peeling, cracking and so on, are relevant to the erosion of surface water and underground water at varying degrees. The role of water aggravates the damage of subgrade and pavement structure, speeds up the deterioration of the pavement performance, and shortens the life of pavement structure. As a result, whether the road drainage is clear is an important factor to pavement performance and service life.There inevitably exits moisture in pavement due to nature factors. This paper explored the effects of subsurface drainage conditions on the performance of asphalt pavements. Analyses and discussions show that the drainage conditions have significant influences on the loading capability and serving duration of asphalt pavements, particularly those for heavy traffic highways. Compared to standard good quality drainage, fair, poor and very poor drainages may result in a reduction in cumulative BZZ-100 axle loads of a pavement more than 60%, 80% and maximum 96%, respectively. On the other hand, excellent drainage may result in an increase in cumulative BZZ-100 axle loads of a pavement by at least 110% with potential up to 480%. In view of the significance of the effect, research and development of subsurface drainage design, and incorporation of the drainage design into the pavement design method as an integrated part, should be in the agenda.In order to optimize the structural design of asphalt pavement, this paper adopted the whole life circle cost analysis in order to get a combination of the best drainage quality and drainage layer thickness. This design adopted pavement condition index PCI as the performance indicators, in order to control the minimum level which the road users can accept at the end of performance. The paper used the pavement structure life-cycle behavior model, through the ESAL and flexure value which vary because of the drainage coefficient and subbase thickness, then set up the ralationship among pavement damage condition index PCId, pavement performance index PCIt, and the subsurface drainage coefficient, subbase thickness, at last we could get the best subbase thicknesses under different drainage conditions when the road reached certain performance standards.
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