08. Effect of pore water pressures on consolidation

Key Information
An interesting observation relating to the effect of pore water pressures (or in this case pore apple juice pressure) on consolidation.

Project Specification
The visualise the effects of pore water pressure (pwp) in relation to consolidation settlement.

Conducted by
Steve Rogers, Technical Director

One of the pleasures of autumn is pulping and pressing apples to make apple juice. During this process I noted that the mechanisms involved closely mimicked the consolidation characteristics of soil and the effects of pore water pressures in that process.

Having pulped the apples into the press the first thing that happens is that the pulp becomes fully saturated and gravitational juice is released. On pressing, the juice initially flows out rapidly, but the flow then reduces as consolidation of the pulp occurs. This can be directly related to the change in the permeability of the pulp during consolidation. It then becomes more difficult to impossible to press further, as the permeability is reduced resulting in high pore water (apple juice) pressures. However, if left until the very slow flow actually stops, the pore apple juice pressures dissipates, allowing the press to tighten more. The exact corollary of this is consolidation of a saturated clay soil under the action of a foundation. In this case the load is initially carried by the pore water pressures alone, but during consolidation there is a transfer of load from the pore water to the soil structure until finally the entire load is carried by the soil structure and none by the pore water.

Further, it also demonstrates why driven piles achieve a premature ‘set’ in some saturated fine grained soils. As the permeability of the soils decreases due to compaction beneath the pile toe, high pore water pressures are generated. As water is incompressible, the driven pile may be arrested in the same way as observed in the apple press.

What Steve says
The apple juice does taste fantastic!

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