9.4.4. Distinguishing stresses

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 of pore water pressure with depth.

Effective stress is a key concept in understanding how soil behaves under various conditions. Total stress is the force applied on the soil, and pore water pressure is the pressure of water within the soil's pores. The position of the groundwater affects both total stress and pore water pressure. When the water table rises or falls, it changes the pore water pressure, which in turn affects the effective stress. Specifically, if the water table rises, pore water pressure increases, resulting in lower effective stress and potentially causing the soil to behave differently. Conversely, a drop in the water table leads to a decrease in pore water pressure, which increases effective stress.

Imagine a sponge immersed in water. When the sponge is fully submerged, it feels heavier because of the water pressing on it. If you lift the sponge so part of it is above water, it feels lighter. This corresponds to the idea that when water pressure in soil changes, it influences how "heavy" the soil feels, or its effective stress.

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.

The relationship between water level and effective stress is critical for geotechnical engineering. When the groundwater level rises, the pressure exerted by the water increases, which decreases the effective stress acting on the soil particles. This lower effective stress can lead to instability in the soil, making it more prone to failure, such as landslides. On the other hand, when the groundwater level falls, effective stress increases since there is less pore water pressure acting against the weight of the soil. This is generally beneficial for soil stability.

Think of a balloon filled with air. If you squeeze the balloon (similar to increasing the water table), the pressure inside decreases, causing the balloon to lose its shape. When you stop squeezing (like the water table dropping), the balloon regains its shape, just as soil becomes more stable when the water levels drop.

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 it might seem intuitive that changes in surface water would affect underground soil stress, this is not the case. For example, if there is a heavy rainfall that raises the surface water level, both the total stress and pore water pressure would increase proportionally. Since effective stress depends on the difference between total stress and pore water pressure, this increase leaves effective stress unchanged. Thus, effective stress is only influenced by changes in groundwater levels below the surface.

Consider a large bowl filled with water and some floating balls. If you add more water to the bowl, the buoyancy on the balls increases, but their relative positions don’t change. In terms of soil mechanics, adding surface water does not change the internal forces acting on the soil particles beneath.

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.

In geotechnical engineering, distinguishing between total stress and effective stress is vital for ensuring the stability of soil structures. For instance, increases in total stress due to the weight of buildings (loading) can result in changes in effective stress, potentially leading to ground failure if not properly accounted for. Similarly, if there is a significant increase in pore water pressure due to heavy rain, it can reduce effective stress and lead to slope failures.

Think of a stack of books on a table. If you add more books (increasing total stress), the books at the bottom feel more pressure. If the table starts to sweat due to humidity (increasing pore water pressure), the bottom books might slip. This illustrates how both total and effective stresses interact to affect stability.

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