9.4. Importance of Effective Stress

At any point within the soil mass, the magnitudes of both total stress and pore water pressure are dependent on the ground water position.

This concept highlights how both the total stress (the pressure exerted by the weight of the soil above) and the pore water pressure (the pressure of water within the soil pores) change depending on how high or low the water table is. When the water table shifts, the balance between these pressures also changes, significantly affecting the effective stress.

Imagine a sponge that you press in water. If the sponge is submerged, it feels heavier due to water filling its pores, increasing pore water pressure. When you lift it out of the water, the sponge loses some of that weight (effective stress increases) due to the water leaving the pores.

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 raises the pore water pressure in the soil, which can lead to a decrease in effective stress. This is because the effective stress is essentially the total stress minus the pore water pressure (σ' = σ - u). When the water table falls, total stress remains the same while the pore water pressure decreases, causing effective stress to increase, making the soil more stable.

Consider a balloon filled with water. If you squeeze the balloon (increase the pore water pressure), the balloon becomes softer and more flexible (decreased effective stress). If you release pressure, the balloon regains shape and stability (increased effective stress).

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.

This point underlines that fluctuations in water levels above ground don’t affect the effective stress in the soil below because both total stress and pore water pressure change equally. Therefore, the net effect on effective stress stays constant, maintaining the soil's stability despite surface water changes.

Think of a swimming pool. If the water level rises evenly across the entire pool due to addition of water, the pressure at the bottom remains unchanged because both the total weight and the pressure of water exerted by the rising water level are in balance.

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.

This equation simplifies understanding effective stress as it allows us to see the relationship between the total stress and pore water pressure. If both change equally, their difference (effective stress) stays the same. This is critical in engineering calculations as it helps assess soil strength and stability under varying conditions.

Imagine you have a seesaw with equal weights on both sides. If you add or remove the same weight from both sides, the seesaw remains balanced. Similarly, if total stress and pore water pressure shift together, the effective stress remains unchanged.

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.

This emphasizes the importance of differentiating total stress from effective stress in engineering practices. Ground movements, such as landslides or subsidence, often occur due to miscalculations in these stresses. For instance, adding weight (total stress) without accurately accounting for pore water pressure can lead to stability issues.

Consider a stack of books on a table. If you add a heavier book on top (increasing total stress), the bottom books (the effective stress acting on them) may start to bend or compress if their weight isn’t properly calculated against the support of the table.

--