Solution to incessant road failures in Nigeria

Road construction

Sir: Most Nigerian roads are in deplorable state. This makes travelling along such roads very unpleasant, while failed sections are zones of operation by armed robbers and kidnappers. Flexible highway pavements in Nigeria often show signs of failure shortly after their construction. Such failures often manifest in form of large potholes and pronounced waves. Reconstruction of failed roads in Nigeria often proves to be ineffective, for such roads often start to fail after a few days or months.

Many reasons can be responsible for road failure.These include poor construction, use of sub-standard construction materials and over-usage. Other important factors are geological and geotechnical. Preconstruction sub-soil investigations are seldom executed for roads in Nigeria. When they are done, they are rarely comprehensively carried out. This is rather unfortunate for stability of flexible highway pavement depends to a large extent on the integrity of the foundation soils and / or rocks.

The proper approach is to carry out detailed geological, geophysical and geotechnical investigations. Field sampling of subgrade (foundation) and sub-base soils must take cognisance of locations that require special treatment. Researches have shown that tropical soils exhibit wide variability in geological and geotechnical properties even within a restricted area. Designs based on results of tests executed on few samples are thus likely to be faulty. Laboratory testing of soils must be thorough with modifications of temperate zone methods. Modifications (such as soaking of air-dried soil samples in weak calgon solution prior to wet sieving) will expose the true nature of sub-base soils. Determination of both unsoaked and soaked California Bearing Ratio (CBR) can be used to simulate the influence of water on both sub-base and sub-grade soils on highways. Results of laboratory investigations should be utilised to identify difficult locations, appropriate soils for construction and treatment to which they must be subjected.

Experiences over the years have shown that most lateritic soils are not only too impervious but also have tendency to lose strength when in contact with water.

Adequate drainage facility must thus be provided in order to prevent contact between such soils and water, if they are to be used for road construction.

In conclusion, geoscientists must be made to work hand in hand with highway geotechnical engineers in order to prevent incessant failures of Nigerian roads and wastage of money on road rehabilitation.

• Prof. Gabriel Oladapo Adeyemi, Department of Geology, University of Ibadan.

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Gabriel Oladapo Adeyemi

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