The causes and mechanisms of rain-induced highway and pavement collapse in Obolo-eke, Southeast Nigeria

摘要

Majority of highway failures in Nigeria occur during or immediately after heavy rainfall. Although rapid rise in pore water pressure is usually adduced as the major factor leading to these failures, the mechanisms that initiate or propagate the failures have not been adequately clarified. To investigate these mechanisms, a series of laboratory experiments, detailed field survey and digital elevation model produced from Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models (DEM) were integrated to investigate the failure processes on the unconsolidated sandy terrains where failures were common. The study area is underlain by loosely consolidated, moderately sorted sandstone sequence and shale intercalations. Results of the ASTER DEM clearly revealed the topographic, geomorphic, and hydrologic features associated with instability and showed that streams played key roles in the failure processes. While many collapsed portions were observed where rivers were close to the highway, there were no failures on the parts far from the streams. Field analysis showed that the failures were restricted to only the portions of the highway with damaged man-made drainage systems. The broken drainages probably facilitated saturation and erosion of the underlying sediments. The liquefaction resistance of the saturated sediments was weakened and transient surface flows arising from prolonged rainfall initiated failure. The rapid post-failure strength reduction and the low steady-state strength suggested the sediments were liquefiable. The results showed that a good drainage system can serve as an effective preventive measure for protecting engineering structures on collapsible sands.