Geotechnical basis for failure of sections of the Lagos–Ibadan expressway, south western Nigeria

A number of geotechnical properties of the subgrade soils in some sections of the Ibadan end of the Lagos–Ibadan expressway were investigated with a view to elucidating the geotechnical basis, if any, for the instability of sections of the flexible highway pavement. The results of the investigations show that the subgrade soils below the stable sections have a higher maximum dry density, unsoaked California bearing ratio (CBR) and uncured, unconfined compressive strength than those below unstable sections. In addition, the soils below stable sections have both a lower proportion of fines and clay-sized fraction and a lower optimum moisture content and linear shrinkage than the material below the unstable sections. Surprisingly, the soils below the unstable pavements not only have a lower plasticity index and higher soaked CBRs than those below the stable pavements but also are more mechanically stable. The investigations have shown that significant differences need not exist between the geotechnical properties of soils below stable zones and unstable sections before such parameters can serve as bases for predicting the stability of flexible highway pavements in the tropics.