Influence of a localized plastic layer on embankment stability

Conventional stability analyses of a 47 ft (14.3 m) high embankment constructed of clayey silt fill indicated a satisfactory design with 2:1 slopes. However, cracking of the fill and movements of the embankment occurred when its height reached 32 ft (9.8 m). Investigation revealed that, in general terms, the geotechnical profile employed for the stability analysis was satisfactory. There was a localized layer of firm clayey soil at the interface between the fill and natural soil, which coincided with the observed cracks and the zone of high pore pressure.Construction scheduling was critical, and an initial wedge analysis showed that a 17 ft (5.2 m) high berm would ensure adequate safety during completion of the fill. A detailed investigation followed to determine the actual deformation mechanism responsible for the cracking. This included plane strain finite element runs using estimated moduli values. It was concluded that the cracking was caused by ‘spreading’ of plastic material at or near the base of the embankment. This case history illustrates that localized layers of weaker soil can be critical even when construction has been carefully controlled.