Causes and Triggers of Mass-Movements: Overloading

Overloading is defined as the addition onto a slope of excessive surcharge load, which either directly triggers landsliding or brings the slope so close to failure that any small accidental trigger will imminently cause it to fail. Static surcharge loading may occur in various natural settings under the weight of groundwater, growing forest, volcanic fallout, displaced soil or rock masses, but is now also more and more frequently related to human activities concerned with large construction works, solid waste disposal, and urban development on steep slopes. Dynamic loading may be considered another kind of transient surcharge, broadening the scope of overloading issues to earthquake-triggered landslides, coastal cliff collapse under extreme storm wave action, and human-induced dynamic loading, e.g., from traffic vibrations or mine blasting. However, although (over)loading is undoubtedly involved in many mass-movement events, its definite contribution to the rupture process remains often elusive. This is either because, as a primary driver, it is more or less overshadowed by a more apparent trigger, such as the loss of strength caused by pore pressure increase, or it acts as part of an intricate set of causative factors, which is for instance the case when tree loading interferes with vegetation-dependent changes in pore pressure and the effect of the root mat on soil cohesion. The difficulty in assessing the influence of a surcharge load derives also from its twofold effect of simultaneously altering the slip driving and resisting forces, which stresses the importance of slope gradient for modulating its net impact on stability. Examination of the theoretical changes brought by an added load to the formulation of the factor of safety and up-to-date stability analyses help to unravel the issue.