9.1. Change in pore water pressure

There is a change in pore water pressure in conditions of seepage flow within the ground. Consider seepage occurring between two points P and Q. The potential driving the water flow is the hydraulic gradient between the two points, which is equal to the head drop per unit length. In steady state seepage, the gradient remains constant. Hydraulic gradient from P to Q, i = ∆h/∆s.

Pore water pressure refers to the pressure exerted by water within the soil's pore spaces. When water seeps through soil, this pressure changes based on the hydraulic gradient, which is the difference in water head between two points divided by the distance between them. If the groundwater is moving in a steady state, this gradient does not change.

Think of it like a sloping water slide. The height difference between the top and bottom of the slide represents the head drop, while the slide's length represents the distance. Just as water flows more quickly down a steeper slide, water in the ground flows faster when there's a greater hydraulic gradient.

As water percolates through soil, it exerts a drag on soil particles it comes in contact with. Depending on the flow direction, either downward or upward, the drag either increases or decreases inter-particle contact forces. A downward flow increases effective stress. In contrast, an upward flow opposes the force of gravity and can even cause to counteract completely the contact forces. In such a situation, effective stress is reduced to zero and the soil behaves like a very viscous liquid. Such a state is known as quick sand condition.

When water flows through soil, it interacts with the particles that make up the soil. If the water flows downwards, it helps push the particles together, increasing the effective stress (the stress that affects the strength of the soil). On the other hand, if the water flows upwards, it can lift the particles apart, lowering the effective stress and potentially leading to a quick sand condition, where the soil loses its strength and can behave like a liquid.

Imagine pouring a thick mixture of sand and water into a container. As the water flows down, it packs the sand tighter together. However, if you were to blow air upwards through the mixture, you would see the sand lift and separate, similar to how upward water flow can turn stable soil into quicksand.

This shows that when water flows upward under a hydraulic gradient of about 1, it completely neutralizes the force on account of the weight of particles, and thus leaves the particles suspended in water.

The critical hydraulic gradient is the point at which the upward flow of water is strong enough to counteract the weight of the soil particles. This gradient is crucial in determining the stability of soil under water pressure. If the hydraulic gradient reaches approximately 1, the effective stress becomes zero, and the soil can no longer support its weight, leading to conditions like quicksand.

Think of a balloon filled with air. The air inside creates pressure that holds the balloon's shape. If you make a small hole in the balloon, the air can escape rapidly. If you push enough air in through the bottom, it could keep all the balloon's parts afloat. The critical hydraulic gradient is like the pressure you need to keep that balloon from collapsing.

At any point within the soil mass, the magnitudes of both total stress and pore water pressure are dependent on the ground water position. With a shift in the water table due to seasonal fluctuations, there is a resulting change in the distribution in pore water pressure with depth. Changes in water level below ground result in changes in effective stresses below the water table.

Effective stress is significant because it reflects the true stress that soil particles experience. As the groundwater level changes, the total stress (the weight of the soil above) and the pore water pressure (the pressure from water in the soil) also change. An increase in water level increases pore pressure and decreases effective stress, while a decrease in water level has the opposite effect.

Consider a sponge soaked in water. When the sponge is full of water (high pore water pressure), it squishes easily because the water makes it less cohesive. When you squeeze it and let some of the water out (dropping the water level), the sponge becomes firmer and harder to compress, similar to how effective stress changes with water level.

Changes in water level above ground do not cause changes in effective stresses in the ground below. A rise above ground surface increases both the total stress and the pore water pressure by the same amount, and consequently effective stress is not altered.

While groundwater levels influence effective stress, changes in surface water levels do not affect it. If the water level above ground rises, both the total stress and pore pressure increase equally, hence, the effective stress remains constant. This is crucial for understanding soil behavior during floods or heavy rains.

Imagine a pot filled with water and soil at the bottom. If you raise the pot without changing the water inside, the weight (total stress) and the pressure of the water on the soil will both increase equally, but the way the soil behaves (effective stress) remains the same.

In some analyses, it is better to work with the changes of quantity, rather than in absolute quantities. The effective stress expression then becomes: σ' = σ - u. If both total stress and pore water pressure change by the same amount, the effective stress remains constant.

The formula for effective stress highlights its dependence on total stress (σ) and pore water pressure (u). By focusing on changes rather than absolute values, it becomes easier to analyze how variations in loading or pore pressure will affect effective stress. If both the total stress and pore pressure change by the same amount, the effective stress is unchanged.

Consider weighing a bag of flour. If you add a weight equal to that of the bag itself, the net effect doesn't change. This is analogous to effective stress; if both total stress and pore pressure increase or decrease equally, the ‘weight’ felt by the soil structure remains unaffected.

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