What is meant by boiling or piping in soil mechanics?

The term "boiling" in soil mechanics refers to the phenomenon where the upward movement of soil occurs due to the increase in pore water pressure, typically related to saturated soil conditions. When water is pressurized—such as during excavation or loading—it can lead to conditions where the effective stress in the soil may become significantly reduced. This pressure can then cause soil particles to be lifted or "boiled" upwards, creating a condition similar to that of a boiling liquid.

This process is particularly relevant in the context of soil that is saturated and under significant loading conditions. As the pore water pressure increases and approaches a point where it can no longer maintain the structure and stability of the soil, the soil may lose its load-bearing capacity and begin to move upwards.

Other options relate to soil behavior but do not capture the specific mechanics of boiling effectively. The movement of water from the surface to subsurface typically describes infiltration, while the escape of water from soil under pressure often pertains to drainage rather than the upward movement of soil. Consolidation involves the process where soil particles rearrange and settle under pressure, which differs from the behavior described by boiling.