Performance, Problems and Remedial Measures for Roads Constructed on Expansive Soil in Ethiopia – A Review

Abstract

Construction of pavement on weak or soft soil is highly risky because such soil is susceptible to differential settlements, poor shear strength, and high compressibility.  In developing countries like Ethiopia transportation facilities are very important for sustainable development.  However, a better performance of the agricultural sector in particular as Ethiopia’s economic growth is highly dependent on it, and the sustainable economic growth of the country at large would be achieved through an improvement of the basic infrastructure. Consequently, the road network has been identified as a serious bottleneck for the economic development of the country. An appreciable part of Ethiopia is covered by expansive soil. Most of the roads constructed and proposed as well as substantial amount of the newly planned railway routes in the country pass through in the heart of expansive soils. The roads on this type of soils fail before their expected design life, in some cases after few months of completion. It has been reported in 2004 that Addis Ababa City Roads Authority had annual expenditure of around 300 million Ethiopian Birr for road construction and maintenance out of which more than 30 million Ethiopian Birr was expended for routine maintenances which is too big and require special attention. The current maintenance and rehabilitation practice also depends more on visual observation and functional evaluations such as surface roughness and visual survey at network level rather than detail pavement evaluation at project level. Major trunk and lower class universal rural access roads failed with in liability period where subgrade soil is black clay soil, but researches show that various treatments such as mechanical, lime and chemical stabilization has been implemented. Moreover, based on the pavement survey, the CBR criteria couldn’t result in reliable solution in case of pavement on expansive subgrade. Thus additional Stability Criteria should be adopted to resist the heaving condition. This paper aims to present a review on the performance, problems and possible remedial measures practices for roads constructed on these problematic soils in the country. Finally, emphasis should be given to the importance in construction in this kind of soil of strictly applying engineered design of geometric, drainage, pavement thickness, material selection and proportioning. Thus, those concerned bodies shall decide to accept and control the risk associated with construction on this soil or not, or to decide that more detail study is required to allow for extra design and construction pre-emptive measures once the potential problem has been identified and the end user convinced of the cost-savings in adopting a pro-active approach.

Keywords: Expansive soil, Subgrade, Stabilization, Pavement, Road construction