9.4.1. Ground water position effects

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 in soil is influenced by the pressure of water present in the soil pores, known as pore water pressure. When the water table rises or falls, it changes the amount of pore water pressure acting on the soil. When the water table rises, it increases the pore water pressure throughout the soil mass, which reduces the effective stress, as effective stress is defined as total stress minus pore water pressure. Conversely, if the water table falls, the pore water pressure decreases, leading to an increase in effective stress. This relationship is crucial for understanding soil stability and behavior.

Imagine a sponge submerged in water. When you push down on the sponge (which represents total stress), the water within it (pore water pressure) pushes back. If you remove some of the water, the sponge behaves differently under pressure because the resistance it faces (effective stress) changes. This principle is similar to how shifts in the water table affect the soil's ability to support structures.

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 level below the ground increases, it applies pressure to the soil, which raises the pore water pressure and ultimately decreases the effective stress. This reduction in effective stress can compromise the stability of soil, potentially leading to problems like the soil becoming more prone to sliding or failing. On the other hand, when the water level decreases, the pore water pressure is reduced, which increases the effective stress, enhancing the soil's ability to support loads. This effect is particularly important in civil engineering, where understanding soil behavior is essential for building safe structures.

Think of a small hill made of sand. When it rains heavily, the water table rises, soaking the sand and reducing its ability to hold together. As the water saturates the sand, it becomes loose and may slide down. If the rainfall stops and the water table drops, the sand becomes firmer and able to support more weight, similar to how a dry sponge can hold more pressure when less water is within it.

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.

Actions or events that cause water levels to rise above the ground, such as flooding, can increase both the total stress and the pore water pressure exerted on the soil underneath. However, since both pressures increase by the same amount, the effective stress theoretically remains the same. This principle suggests that while there may be more water present, the balance of forces on the soil particles remains unchanged, preserving its stability.

Imagine a heavy weight placed on a balloon filled with water. If you increase the water level in the balloon, thus increasing the pressure from both the balloon's weight and internal water pressure, the balloon doesn't collapse further because both forces increase proportionately. This illustrates how changes above ground don’t necessarily alter the balance of stress below ground.

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.

In engineering and soil mechanics, it's often useful to examine how effective stress changes in relation to total stress (σ) and pore water pressure (u). The equation σ' = σ - u defines effective stress (σ'). This means if both total stress and pore water pressure change evenly—say due to a constant rise in water level—the effective stress remains unchanged. This understanding helps engineers analyze potential shifts in soil behavior under varying conditions without needing precise absolute values.

Think of a scale with weights on both sides that are perfectly balanced. If you add more weight to both sides equally, the balance doesn't tip; the relationship remains the same. This symbolizes how effective stress behaves when both total stress and pore water pressure increase by the same amount—balance is maintained and the soil's reliability remains intact.

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 calculating soil behavior in engineering and construction. Total stress relates to the overall load on soil, while effective stress is what influences how the soil particles interact and hold stability. If total stress increases—like when heavy machinery is placed on soil—effective stress calculations also need to consider any changes in pore water pressure. Similarly, after heavy rainfall, if pore water pressure increases, it can destabilize slopes even if total stress remains constant.

Consider a balancing act on a tightrope. The total weight of the performer (total stress) affects balance, but without proper footing (representing effective stress), they can easily fall. This highlights the importance of recognizing the difference between total and effective stress—both need to be managed to maintain stability and prevent accidents in construction and earthworks.

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