9.4.2. Impacts of water table changes

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. A rise increases the pore water pressure at all elevations thus causing a decrease in effective stress. In contrast, a fall in the water table produces an increase in the effective stress.

When the water table rises, it causes the pore water pressure in the soil to increase. This, in turn, reduces the effective stress, which is the pressure that contributes to the soil's ability to support loads. Conversely, if the water table falls, the pore water pressure decreases, resulting in increased effective stress. Effective stress is essential for soil stability and is a key factor when considering ground support for structures.

Imagine a sponge soaked in water. When you press down on it, the sponge can deform and absorb some of that pressure. If you keep adding water (raising the water table), the sponge becomes swollen and less able to support weight. Now, if you remove some water (lowering the water table), it becomes firmer and can support more weight again.

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.

When water levels rise above ground, both the total stress (overall pressure from the weight of soil and water) and pore water pressure increase by the same amount, keeping effective stress constant. This means that the changes in surface water level do not impact the soil's ability to support structures as the balance between total and pore pressure is maintained.

Think of a balloon filled with air and water. If you increase the amount of air or water but maintain their ratio, the balloon's structure remains the same. Similarly, when surface water level rises, the relationship between total pressure and pore water pressure stays balanced, leaving effective stress unchanged.

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 indicates how to calculate it using total stress (σ) and pore water pressure (u). If both increase or decrease equally, the effective stress will not change. This helps engineers understand soil behavior under load and predict how it will react to different conditions.

Consider a scale where you place a weight on one side and water on the other. If both sides increase equally—like adding the same amount of weight and water—the balance stays the same. This is similar to how effective stress works when total stress and pore water pressure both change together.

Total and effective stresses must be distinguishable in all calculations. Ground movements and instabilities can be caused by changes in total stress, such as caused by loading by foundations and unloading due to excavations. They can also be caused by changes in pore water pressures, such as failure of slopes after rainfall.

It's crucial to differentiate between total stress and effective stress when analyzing soil and ground behavior. Changes in total stress, such as from added building loads or excavation, can lead to instability. Similarly, changes in pore water pressure, like after heavy rains, can also cause slope failures. Recognizing these factors allows for better prediction and management of potential issues.

If you've ever built a sandcastle near the water on a beach, you’ve seen how water affects structure. If you add too much wet sand on top (increases total stress) without enough drainage (pore water pressure), the castle might collapse. It’s important to balance both factors to keep it standing.

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