Analysis of long-term settlement of road embankment on peat ground during its service period

Roads on peat grounds are subject to stringent requirements in terms of controlling the residual settlement during their service period in order to minimize the post-construction maintenance work. This paper addresses the applicability of the Finite Element Method (FEM) by incorporating a time-dependent model for peats to an evaluation of the residual settlement. The applicability of isotach viscoplasticity, used to describe the peats’ behavior under variable-rate compression, was confirmed in laboratory tests. Long-term consolidation test results and a model simulation revealed the particular importance of two mechanisms in the residual settlement in peat grounds, namely, the stress-dependency of the coefficient of consolidation, c_v, and the viscosity-induced secondary consolidation. A model peat ground, capturing these mechanisms, was implemented for the FEM and applied to a case history of a high-standard road construction in Hokkaido, Japan. The viscoplastic parameters determined from standard oedometer tests successfully reproduced the observed long-term settlement in a peat-dominated section of the expressway. The analysis was extended to address the settlement in the peat ground with the installation of prefabricated vertical drains (PVDs). The results indicate that the delayed primary consolidation due to the stress-induced c_v reduction can be significantly alleviated, while the settlement arising from the secondary consolidation over a longer term is little affected by the PVDs. These insights point to the importance of untangling the two mechanisms of long-term settlement for the sake of making accurate predictions of the residual settlement in different time scales.