ABSTRACT
Typically the immersed pipe is buried in soil beneath a body of water. If the density of the soil exceeds critical density, failure is the formation of soil wedges uplifted as shown in Figure 21-2. Critical density occurs at that void ratio, greater than which the soil is so loose that it compresses when disturbed, and less than which the soil is so dense that it expands when disturbed. Critical void ratio is roughly 85% density AASHTO T-99 (ASTM 698). The specification for minimum
allowable density is usually set at 90% density. Granular soil looser than critical does not break out wedges, but flows around the pipe as liquid or plastic through which the buoyant pipe rises. This phenomenon can be investigated by approximate theory, but there are too many variables for a precise model. A simplified theoretical analysis follows. It is confirmed approximately by experimentation.
