Long-term settlement behavior of peat after unloading and applicability of isotach law

Abstract

Preloading is commonly adopted to minimize the post-construction residual settlement in soft peat grounds. However, non-negligible long-term settlement can still occur during the service period of the overlying structures, posing a challenge to infrastructural maintenance. It is difficult to predict the settlement in a preloaded and then reloaded ground with a conventional creep analysis using the coefficient of secondary consolidation, C_αe. This is because the determination of its value can be subjective owing to the limited observation timescale in both the field and the laboratory, which reflects the lingering effects of the previous loading history. Thus, each value determined for C_αe is condition-specific and cannot be considered as an inherent material property. This paper investigates the viscosity characteristics of preloaded peat through long-term consolidation tests, with a particular focus on the validity of the isotach law in explaining the strain-rate effect. It is found that the peat behavior follows the isotach rule even during unloading when the overconsolidation ratio (OCR) is smaller than 1.5. The theory based on the isotach law can describe the general reloading behavior with a constant C_αe value unique to each peat, when C_αe is reappraised by applying the strain rate, rather than the conventional void ratio (e) – log time relationship. The paper also discusses how the preloading effect can be evaluated from the settlement time history during construction, giving adequate care to some important features of peat, such as the significant decrease in permeability upon consolidation.