Environmentally optimised design of road surfacing alternatives for steep slope sections on rural roads

Abstract

Like other sub-Saharan African countries in the tropics, the middle-belt of Ghana has rolling and mountainous landscapes with road problems usually associated with drainage and erosion control. Undulating terrains characterised by steep slopes pose road access challenges. Conventional rural roads wearing course materials such as gravel or single bituminous seals have proven inadequate to address the issues, especially on steep hill sections. To address the problem, the Environmentally Optimized Design (EOD) concept was employed to select resources, including construction materials, to determine the suitability of optimum road surfacing alternatives for steep slope sections (gradients of 12% or more) of rural roads in Ghana. Subsequently, based on assumptions of nominal subgrade (CBR=10%) and low traffic volume (<300 vehicles per day), three alternative derivatives of concrete, bituminous and stone setts/cobbles surfacing were proposed. Further, a tentative pavement structure design based on Transport Research Laboratory’s (TRL) estimator for gravel surface design was used to present different road layer combinations. For construction demonstration and monitoring, the three surfacing alternatives are to be placed over different road base materials; i.e., lateritic gravel stabilised with cement, pozzolana, and quarry dust as additives. This paper focuses on three surfacings on stabilised lateritic base/subbase layer materials. The outcomes of the study will contribute to the development of construction guidelines and specifications for rural roads in Ghana, which could serve as a model for other sub-African countries.