Projected Flexible Road Pavement Performance of Nairobi - Thika Superhighway in Kenya

Abstract

The Nairobi – Thika Superhighway has been open to traffic for five years and yet sections of the road pavement have already deteriorated substantially and require reconstruction. That was established for the section between Kenyatta University and Thika Town despite the fact that the road was designed to last twenty years with periodic and routine maintenance. The research was based on the following objectives: (i) to establish the pavement surface condition by conducting roughness survey, (ii) to evaluate the structural performance of the pavement based on the deflection measurements under the prevailing and projected traffic loading conditions (iii) to evaluate the remaining life of the pavement. Traffic survey was conducted at the Nairobi – Thika Superhighway at the Eastern Bypass from Tuesday 7 May 2013 to Monday 13 May and the Average Daily Traffic (ADT) computed. The as-built records of the flexible pavement were reviewed and the properties of the pavement layers obtained from the design records. The road condition survey and the deflection measurements were conducted by the Kenya Ministry of Roads in the year 2012. Pavement analysis was done to establish the performance of each lane. The back-calculated values of the Residual Modulus of Subgrade Reaction and the Equivalent Pavement Moduli of the pavement layers were compared with the as-built records of each lane. The remaining or residual life of the pavement in years and the corresponding overlay requirements were also established for each homogeneous section as an indicator of the performance. Analysis of the roughness survey established that the road performance corresponded to that of a new pavement. However the deflection analysis revealed the road had started deteriorating early and the sections that were most affected were those where speed bumps had been placed. The last one kilometer of the road from Kenyatta University to Thika Town on all lanes had failed and reconstruction was recommended after 5years of opening the road to traffic. The rest of the sections also deteriorated substantially and an overlay of 55mm on all lanes was recommended after 5years of projected traffic use. Similarly, the first one kilometer from Thika Town to Kenyatta University had also failed and required reconstruction after 5years of projected traffic use. The deterioration of the rest of the sections also indicated that the road required an overlay of 80mm after 10years of projected traffic.