Evaluation of pavement design parameters for national highways of Bangladesh

Abstract

This study is performed to evaluate the pavement design parameters in different National Highways of Bangladesh. National Highways like N-1, N-2, N-5, and N-8 are selected for this purpose based on availability of daily flow and axle load data. Traffic flow related parameters such as daily, weekly, seasonal variations, directional distribution, traffic composition, traffic growth pattern, expansion factors and regression models are extensively analyzed to ascertain if any commonality exists among different design flow parameters in these selected National Highways as well as to assess if there is any need for improvement of the existing pavement design parameters. Besides, the axle load characteristics are also evaluated to observe the actual wheel load damaged factor against the current pavement design practices. From the time series analyses of pavement design parameters, it is found that like any other stabilized corridors, the basic traffic flow patterns of these highways are repetitive in nature. It is observed that there are significant differences in various traffic flow parameters among the four studied highways, viz. N-1 corridors carries almost double traffic (190.50%) than N-5 corridors and almost two and a half times traffic (265.46%) than N-2 corridors. With regards to annual average traffic growth rate, a wide range of variations (11.17 to 21.03%) are also observed along the selected corridors. These values are found to be much higher than the standard growth factor of 10% as considered to the RHD pavement design guide (PDG, 2005). Similarly, comparison of other important traffic parameters viz. DD, truck percentage and expansion factors among different corridors also revealed dissimilarities and wide range of variations. This essentially implies that updating of existing pavement design parameters are essential and for rational pavement design, corridor specific parameters should be used instead of using common parametric values. Moreover, detailed vehicle overloading investigations disclosed that there is a strong need for immediate revision of existing pavement design parameters like 85th percentile design truck weight, equivalent ESAL value, axle damage factor, etc. due to presence of significant proportion of exceptionally heavily laden trucks in the RHD network. For instance, the 85th percentile design truck weight and the average ESAL per six wheeler trucks are found 26.50 tons and 32.41 tons respectively along Dhaka-Aricha corridor which are much higher than the current legal gross vehicle weight of 15.5 tons. Similarly, the presence of significant number of grossly overloaded vehicles, particularly 2-axle medium trucks, disclosed much higher equivalency factor (>200) as compared to PGD recommended design value of 4.62. Thicker asphalt concrete layers can resist excessive deformation under heavily loaded traffic volumes and prolong the life of the pavement. But, permitting the uncontrolled plying of grossly overloaded vehicles must be economically unsustainable since it would result either in high capital costs for heavily over designed pavements to cater for the illegal overloads or early deterioration of pavements designed for a normal/legal range of vehicle loading leading to heavy premature periodic maintenance and/or rehabilitation costs. As such, it is essential that effective monitoring and control measures be introduced and implemented, as soon as possible, to curb the ever increasing axle load along with some structural design improvements for all National Highways to meet the present need for carrying bulk freights.