Evaluation of Creep in a Normally Consolidated Glacio-Lacustrine Clay

ABSTRACT: It is recognized that glacio-lacustrine varved clay in Abitibi region of Quebec, Canada presents a moderate level of strain-weakening that needs to be considered in the design of embankments over these clays. Moreover, failure under sustained shear load, i.e., creep-induced failure, has been identified as a potential failure mechanism. The literature suggests that for a material that is susceptible to strain weakening, any mechanism that causes deformations large enough to bring the material to instability could lead to a failure. In this paper, it is suggested that advanced laboratory testing, field monitoring data, and stress-deformation modeling could provide a framework to judge whether creep in the normally consolidated clayey foundations of embankments could lead to a failure. In this study, the containing structures of a tailings storage facility (TSF) of a mine site located in Abitibi region were studied and this framework was examined. For the studied dikes and their upstream raises, undrained shear tests in the field and laboratory on the normally consolidated clay showed some level of brittleness, which implied that the clay under the dikes and the tailings pond is susceptible to strain weakening under undrained loading. The monitoring data along with the construction and deposition stages suggests that creep and consolidation were taking place at the same time. The coupled stress-flow numerical models using FLAC software also indicate that the clay behavior under construction was partially drained. Therefore, clay is expected to also behave partially drained during creep. Thus, both field data and numerical models indicate that in the stress path space, the clay should not approach the instability line while undergoing creep. Therefore, creep failure was judged to be unlikely.