ABSTRACT
Paris Plastic Clay of the Sparnacian age (Lower Eocene) constitutes the first Tertiary deposits of the Paris basin. Because of its complex hydro-mechanical behaviour, it represents a major geotechnical challenge for the Grand Paris Express (GPE) project, the largest transportation project underway in Europe. In this paper, the hydro-mechanical properties of this plastic clay encountered along the metro lines 14 and 15 were firstly investigated through field investigations and laboratory testing. It appears that Paris Plastic Clay is a stiff fine-grained soil with a high plasticity index saturated in its natural state, and acting as an impermeable horizon for different water tables. It is either normally consolidated or overconsolidated. Furthermore, a large variability of this clay was identified. In addition, this plastic clay also exhibits a certain swelling capacity, which should be considered in the design of underground structures in contact with it. In the GPE project, the diaphragm walls can reach a depth of up to 75 m, and the average excavation depth of stations is about 30 m. The Paris Plastic Clay is thus frequently encountered during the construction of shafts, diaphragm walls, stations, and tunnels. Based on the characterization results of Paris Plastic Clay, several engineering considerations and practices in the construction of the underground structures were proposed.
