Experimental investigation of unconsolidated undrained shear behaviour of peat

This study conducted a series of standard classifications to determine the physical properties of peat, followed by 28 unconsolidated undrained triaxial tests to investigate undrained peat shear behaviour. Results show that the undrained deviatoric shear resistance of peat grows continuously with increasing shear strain, but ends up with no obvious peak strength even at very large strain (i.e. 25%). The shear stiffness of peat specimens degrades substantially with increasing shear strain. To make the stiffness of peat specimens comparable, the secant shear modulus G is then normalized with the shear modulus G_{0.1 %} at relatively small strain of 0.1%. Furthermore, results from 16 previous unconsolidated undrained tests and 66 consolidated undrained tests available in literature were taken into account together with 28 lab test data obtained in this study, aiming to reveal the general behaviour of peat shear stiffness degradation. For all the test data, the shear strain γ is normalized with a reference strain γ_{ref , 0.1 %}, at which the stiffness G/G_{0.1 %} = 0.5. A hyperbolic function is then adopted to fit the relationship between the normalised stiffness versus the normalised shear strain. Over 78% of the data falls within a ± 30% margin within 5% shear strain, while the predicted stiffness out of the margin is usually underestimated.