Undrained failure mechanisms of slopes in random soil

•We examined the failure mechanisms of slopes in random undrained soil.•Relatively flat slopes tend to display shallow failure mechanisms.•The proportion of toe failure mechanisms increases with increasing slope angle.•The more variable the shear strength, the less likely a toe failure mechanism is.

Since the charts of Taylor (1937), it has been well known in engineering geological investigations that the location of the critical failure mechanism in a homogeneous undrained clay slope goes either deep (tangent to a firm base) or shallow (through the toe) depending on whether the slope angle is less than or greater than about 53°. In reality, natural soils always exhibit spatial variability and the above expectations no longer hold true. The objective of this note is to investigate the failure mechanisms and probability of failure of slopes in random undrained soil over a wide range of slope angles. An elastic–perfectly plastic finite element method in combination with random field generation, well known as the random finite element method (RFEM), is employed. RFEM represents a powerful tool for slope stability analyses that allow mechanisms to develop naturally within soil masses. It is found that, for certain combinations of random field properties, relatively flat slopes may display a significant number of shallow mechanisms and steeper slopes may display a significant number of deep mechanisms. For a steep slope, the more variable the undrained shear strength, the less likely the slope is to display a toe mechanism. Understanding the uncertainty of failure mechanisms is important because the consequences may be more serious in a deep failure as it involves a greater volume of soil.