Lessons Learned from the Performance of a Degradable Shale Embankment: Case Study

Degradable shales are sedimentary rocks with properties that can vary from those of solid rock to soil like materials if subjected to wetting and drying cycles. If the potential for degradation of the shale is not identified on time, rock like particles could be used as strong embankment material. After wetting and drying cycles, degradation occurs and the initial large voids formed between the generally uniform rock particles are filled with smaller fragments, resulting in significant settlements and slope instability. This paper present a case study of a 40 foot (12.2m) approach embankment having 1.5H:1V slopes that was unintentionally constructed with degradable shale. Preliminary testing showed that even though some samples were clearly degradable shale other samples exhibited durability indexes greater than the values generally accepted as durable rock. After embankment completion a settlement monitoring program was instituted for the prescribed quarantine period prior to construction of the bridge abutments. The embankment settled more than initially predicted and finally stabilized after more than 1 feet (0.305m) of internal deformation. A laboratory testing program was developed to investigate the causes of the observed degradation