Overturning Instability

This paper shows how the safety factor against a flexible structure overturning on compressible ground depends not only on the weights and lever arms involved but also on the height of the weights above the ground. A rigid structure on rigid ground (without piles) resists overturning under applied loads by the restoring moment of the structure's weight acting about an axis of rotation at the bearing edge. In contrast a top-heavy structure on soft ground can overturn like a boat without any applied loads or warning, because any sway of the elevated weight creates a over-turning moment that causes further rotation and sway on the soft ground, and so on uncontrollably. These two conditions represent the extremes of a single pattern of structure-soil interaction. Worked examples illustrate reductions of an apparent safety factor of about 20% for a crane and an offshore jack-up platform on soft ground.