Evaluating the Effect of Fiber Reinforcement on the Anisotropic Undrained Stiffness and Strength of Peat

This paper presents the results of an investigation into the role of peat fibers in the development of anisotropic stiffness and strength within peat samples taken from below three Canadian National Railway embankments across Canada. These sites include two from northern Alberta on the Edson and Lac-La-Biche subdivisions and one from the Lévis subdivision in southern Quebec. The peat samples were collected in Shelby tubes and were subjected to a program of consolidated undrained triaxial testing. These samples represented a wide variation in fiber content and degree of humification. The measured pore-pressure response during undrained loading indicated that the peat samples were undergoing an anisotropic elastic response to loading. This pore-pressure response was similar in the peat from all three sites, with an anisotropic pore-pressure parameter, a, of approximately 0.3. The stiffness and strength of the different peats were also similar, with an elastic modulus of approximately 1.7–2.2 MPa and an effective shear strength of 38°. The yielding behavior of the laboratory specimens suggested that plastic yielding occurs without the development of distinct failure planes and is accompanied by strain hardening. The combined effect of the effective strength parameters and the anisotropic pore-pressure response on the reinforcement provided by the peat fibers is also presented. This analysis showed that the net effect is dependent on the initial stress state of the peat. Under elevated mean effective stresses and low deviatoric stresses, the fibers may have a detrimental effect on strength.