A case study on utilisation of undrained strength analysis to design an upstream embankment on soft tailings

A case study is presented illustrating utilisation of Undrained Strength Analysis (USA) principles to design an upstream earth dam constructed on soft tailings to retain bauxite residue. A series of geotechnical investigations comprising cone penetrometer tests using piezocone with dissipation tests (CPTu) was carried out to ascertain the subsurface profile of the embankment footprint and to assess undrained strength ratios and permeability of soft soil layers. Targeted vane shear tests were carried out at selected CPTu locations as confirmatory tests. Additionally, laboratory tests including consolidated undrained (CU) triaxial tests and oedometer tests were performed on selected soil samples to further assess soil permeability and compressibility. Based on the outcomes of the investigation, a geotechnical model was developed for the proposed embankment which was then analysed for stability using both limit equilibrium and finite element methods. Staged construction of the embankment was modelled incorporating SHANSEP principles to account for strength gain in soft soil layers during staged construction due to consolidation. During the construction, subsequent geotechnical investigations and monitoring were carried out to assess the actual strength gain in soft soil layers and to refine stability models accordingly. At the conclusion of the project, a good agreement was observed between the predicted and observed behaviour of the embankment.