Flexible Pavement Thickness (A Comparative Study Between Standard and Overloading Condition)

  • Krishna Nath Ojha Senior Divisional Engineer, Ministry of Physical Infrastructure and Transport, Department of Roads (DoR), Lalitpur, Nepal
Keywords: Standard and overloading vehicle, equivalent single axle load, pavement design, pavement service life, pavement layer thickness


The ability of a pavement structure in carrying out its function reduces in line with the increase of traffic load, especially if there are overloaded heavy vehicle passing through the road. Most of the road pavement is designed considering the legal load limit of the vehicles. But that may not be the actual condition and most of the transport enterprenures want to minimize the transport cost making the heavy vehicles overloaded. The impact of overloaded truck traffic includes economic, social and environmental losses. In this study, the effect of overloaded vehicles on the road pavement thickness was analyzed using the AASHTO 1993. Traffic load (ESAL) and Structural Number (SN) were calculated on standard and overloading conditions. The difference due to overloading condition was also presented. Study was done taking the traffic data on Narayanghat-Mugling road (AH42) segment which is a link between Prithvi Highway and East-West Highway, located in Chitawan district of Narayani zone, state 3, Nepal in which the composition of traffic seen to be 83.76% heavy vehicles, 9.18% Medium vehicles and 7.05% light vehicle. The presence of overloaded vehicles, particularly heavy vehicles resulted in traffic load (W18) value found to be greater than that of standard condition. The impact of overload conditions on the road pavement showed increase in the layer thickness than that of thickness at the legal axle load limit. From the results, it can be concluded that overloaded vehicles on the road are very influential to the reduction in pavement service life and require higher thickness. For S-N direction, the pavement thickness seems to be increased upto 22.81% due to overloading with respect to standard condition, while for the opposite direction; the thickness seems to be more than sufficient in standard condition. The total pavement thickness required for overloaded condition seemed to be 43.25 inch in which 30.735 inch subbase, 7.797 inch base and 4.718 inch bituminous wearing course but in standard condition total thickness required seemed to be 36.856 inch with 26.659 inch subbase, 6.355 inch base and 3.842 inch bituminous wearing course. Road infrastructure is used by various types of vehicles among which heavy vehicles imposes the most critical loading, causing damage in pavement structure, which ultimately leads to an increased maintenance and rehabilitation costs. Therefore, it is expected that road users to comply with existing regulations in the conduct of transportation.


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