Pore Pressure Generation

Hodge was having difficulty practicing as a Geotechnical consultant, especially when it came to dynamic soil-structure interaction. His problem was in trying to rationalize how water in the spaces between soil particles gets pressurized. Excess pore pressure, as this condition is called, was at that time considered to be of prime importance, since it was believed water pressure increases diminished the soil's shear strength. And, despite its crucial importance, there was no good explanation: Only an on/off assumption which flipped from zero to 100% as the designer saw fit. For safety sake they naturally opt for "full on", and that costs society an awful lot of money.

He believed the quandary could be resolved, at least in his mind, if an answer could be found to what he called the "3-beaker" question. Really, there is only one beaker, it been assessed at three subsequent times. The first beaker (left) depicts solid spheres submerged in water, and in their loosest packing arrangement. The third beaker (right) has the very same spheres in their densest packing. The middle beaker has the spheres in a state of collapse, that is, from loose to dense. The question is: ¿ What is the weight of this second beaker ?

The test setup to model this situation was beyond his resources so he distilled the conceptual question even further, to the case of a single ball falling through a column of water. So, on a Saturday morning in early March, 1998 Hodge went to the kitchen to find and then assemble the bits and pieces he needed to carry out a "laboratory" test.

If you want to run this "laboratory" test yourself, here's what to do: Arrange things as shown in the sketch to the left. Fill the tall jar almost to the top with water and place it on the scales. Take a hard-boiled egg, and suspend it steady just below water, then tare (zero) the scales' readout. When you let is go watch the weight reading on the scales.

The weight the scale "feels" will be shown on the readout display to continually climb higher as the egg falls, that is, until it reaches the full buoyant weight of the egg, at which stage the reading becomes constant. And if the jar is tall enough, that fixed weight will show up before the egg hits the bottom. This shows that as the egg fell it transferred more and more of its weight to the scales, and that can only have been through the water beneath it, thus proving that the water column under the egg was pressurized because of the egg's movement overhead. Furthermore, the increasing weight shown on the scales may be seen to be related to the speed the egg is falling.

Try a new potato and a golf ball too. Also, see how differently the results turn out when something like a brass cupboard knob or a stone is dropped.

What Hodge gleaned from this is: Pressure is generated in the water ahead of a solid moving relative to it. Or, stated more generally, during relative motion between solids and liquids there is a transference of kinetic energy between the phases. Game over.

Sometime later this "kitchen" experiment was reproduced by Professor Emeritus Yoginder Vaid in his fundamental Soil Mechanics laboratory at the University of British Columbia, giving the same results, but now, with more dignified apparatus. (see GN Parts 1 & 2).

And, by the way, the middle beaker is lighter than its neighbours.