Highway Research Record

Development of Design Methodology for Rural Roads

Date of Start:July 2004
Civil Engineering Department, Indian Institute of Technology, Roorkee (R, I)

Scope and Objective
(i) Review available relevant worldwide studies and compare a comprehensive literature review summarizing the state of      knowledge on the subject.
(ii) Develop design categories for low volume roads, relating to needs, region, topography and expected traffic.

Findings and Conclusions
(i) The design approach developed by IRC, New Delhi is economical as it gives the soil parameters with help of the nomographs     developed. This saves time and money both.
(ii) The drawback of the IRC method is that it does not take into account the traffic growth and the vehicle load. All the     commercial vehicles have been clubbed in same category.
(iii) The proposed approach to calculate the traffic index is more rational approach as it accounts the load carried by vehicle. (iv) Much variation in the thickness obtained by IRC method and proposed method has not been observed since the range of       the Traffic Index curves is very wide 0-150 and 150-500 although there is considerable difference in the Traffic Index.
(v) The method suggested by Rural Roads Manual does not consider the traffic loading and all vehicles weighing more than 3      tonnes are clubbed in same category of commercial vehicles.
(vi) The Rural Roads Manual suggests four categories of the traffic, which are very wide. This results in over design and under       design of pavement. For instance for CBR of 4% the thickness can vary between 275 mm to 465 mm for different levels       of traffic.
(vii) The design specification as laid in rural road manual are uneconomical to fulfill the specifications of WBM as suggested by        Rural Roads Manual.The cost has touched more than Rs 5.00 lakh per kilometer per layer in many states.
(viii) The proposed design methodology has converted the traffic into equivalent traffic and then into cumulative standard        axles. This is more rational approach as it has accounted vehicle weight in terms of vehicle damage factor.
(ix) In the proposed approach the anomaly of having same thickness over a range of traffic has been removed. The traffic has      been converted into cumulative standard axles which makes the thickness a continuous function of the traffic.
(x) The design obtained by the suggested method is neither under design nor over design as it varies with the traffic. This      approach is similar to the approaches adopted in the other countries.
(xi) The method gives economical design.
(xii) Analytical design methodology is a more realistic approach as it uses the fundamental mechanical properties of the materials       that are used in the road construction.
(xiii) The thickness comparison reveals that IRC is giving the least thickness. The thickness obtained by RRM method, an       analytical method, is comparable.