Effects of Preloading and Prestressing on the Strength and Stiffness of Geosynthetic Reinforced Clay in Plane Strain Compression

A series of plane strain compression (PSC) tests was performed to evaluate effects of preloading and prestressing on the stress-strain properties of reinforced saturated relatively soft clay. Saturated specimens of compacted clay, either unreinforced or reinforced with two horizontal layers of a composite consisting of woven and nonwoven geotextiles, were consolidated anisotropically simulating typical field stress conditions. Without drained preloading, the undrained strength of reinforced clay specimens did not noticeably increase when compared to unreinforced specimens, because a high positive excess water pressure developed in the reinforced clay due to the original high contractibility of the clay, which was not eliminated by reinforcement alone. The drained compressive strength increased substantially with reinforcement, while the peak strength was attained only at a large strain and the initial stiffness did not increase. By drained preloading to a load level that was much higher than the drained strength of unreinforced clay, the overall pre-peak stiffness, yield strength, and peak strength of reinforced clay under undrained conditions became substantially larger than those of unreinforced clay that had not been preloaded. The initial stiffness of preloaded clay at small strains became larger with increases in the prestress level as long as the prestress level was not too close to the preload level.