Time dependent behaviour of reinforced embankments on soft foundations

The time dependent behaviour of reinforced and unreinforced embankments constructed over soft cohesive foundations is studied using a finite element program modified to be capable of modelling the behaviour of soft cohesive soils, geosynthetic reinforcement and prefabricated vertical drains, and the soil-reinforcement interactions. An elasto-plastic and elasto-viscoplastic elliptical cap model coupled with Biot consolidation theory is adopted for rate insensitive and rate sensitive soft clays respectively. Both undrained and partially drained conditions are considered. The influence of the rate sensitive and consolidation behaviour of soft foundation soils, and viscoelastic behaviour of geosynthetic reinforcement on both the short-term and long-term performance of embankments is examined. The combined effect of soil improvement using both reinforcement and prefabricated vertical drains (PVDs) on the embankment construction is investigated. Factors examined include different foundation soil properties, different types of geosynthetic reinforcement, construction rates, stage construction methods, the presence or absence of prefabricated vertical drains, spacing and well resistance of PVDs. The time dependent response of excess pore pressures, reinforcement strains and foundation deformations are evaluated. It is shown that the use of reinforcement can effectively reduce foundation shear and creep deformations, and increase embankment failure height. The partial consolidation during the early stage of construction when the foundation soil is in its overconsolidated state may be significant especially when PVDs are installed. A relatively simple method is developed to estimate the consolidation during construction allowing for the change in consolidation coefficient of the soil in its overconsolidated state and normally consolidated state. The shear strength gain of foundation soils with PVDs due to partial consolidation during construction can significantly increase embankment stability and the effects of consolidation of the foundations are more pronounced and beneficial for reinforced embankments than unreinforced embankments. The use of PVDs combined with reinforcement can potentially allow for a more rapid construction of higher embankments than can be achieved with either method of soil improvement alone. A design procedure is proposed to allow for combined effects of reinforcement and PVDs. For embankments constructed over typical rate sensitive soils, it is shown that creep and stress-relaxation of the foundation soil after the end of embankment construction can be significant. In terms of stability, the critical stage for embankments occurs during creep and stress-relaxation of rate sensitive foundation soils after construction. It is shown that the strain rate corresponding to the critical stage controls the operational shear strength of rate sensitive foundation soils. A correction factor to measured undrained shear strength is proposed based on the critical stage and critical strain rate. For embankments with reinforcement that exhibits significant viscous behaviour, the mobilized reinforcement stiffness at the end of construction is significantly lower than the stiffness measured in standard tensile tests. The viscous nature of geosynthetic reinforcement can decrease the embankment failure height and may magnify the shear deformations of the embankment and foundation soils during a long-term period. Design recommendations are made for reinforced embankments on soft clayey foundations.