1.6. Unloading and Expansion Curves

BC – Virgin compression curve also called normal consolidation line.

The virgin compression curve, also known as the normal consolidation line, represents the relationship between the void ratio and effective stress during the initial and main compression stages of soil. It illustrates how a soil sample compresses when subjected to increasing effective stress for the first time.

Think of this as a sponge. When you press down on a dry sponge (representing the soil), it compresses easily at first. The more you apply pressure, the more it compresses until it reaches a point where it becomes firmer and harder to compress further. This behavior reflects the virgin compression curve of soil.

From ‘C’ when the sample is unloaded, sample expands and traces path CD (expansion curve unloading).

Once you reach point 'C' on the compression curve and start unloading the soil, it begins to expand and traces out a new path known as the expansion curve (CD). This path follows a distinct path because soil does not return to its original state entirely due to some permanent changes that occurred during loading.

Imagine blowing up a balloon. When you release some air, the balloon doesn't return to its original size; it shrinks but still retains some stretch. Similarly, when soil is unloaded, it expands but remains altered from its previous state.

Sample undergoes Permanent strain due to irreversible soil structure and there is a small elastic recovery.

When the soil is unloaded, it experiences permanent strain, which means that some deformation remains even after unloading. This is because certain changes in the soil's structure are irreversible. However, there is also a small amount of elastic recovery, where the soil can slightly return to its original form.

Consider an elastic band. If you stretch it beyond its elastic limit, it becomes permanently stretched out. However, if you slightly release the stretch, you may notice it regains a small part of its original shape, just like how soil behaves during unloading.

The reloading curve lies above the rebound curve and makes a hysteresis loop between expansion and reloading curves.

When the soil is reloaded after unloading, the reloading curve will lie above the expansion or rebound curve, creating what is called a hysteresis loop. This indicates that the behavior of the soil changes depending on whether it is being loaded or unloaded, reflecting the energy loss and changes in the soil structure over cycles of loading and unloading.

Think of this as walking on a carpet. When you walk on it (loading), it compresses under your weight. When you step off (unloading), it doesn't return to its exact original thickness immediately. When you walk back on it (reloading), it feels different, perhaps firmer, highlighting the hysteresis effect.

Loading beyond ‘C’ makes the curve to merge smoothly into portion EF as if the soil is not unloaded.

If loading continues beyond point 'C', the behavior of the soil appears as if the unloading phase never happened, merging smoothly into the next portion of the compression curve (EF). This indicates that the stress applied continues to affect the soil's characteristics even though it was unloaded initially.

Consider molding clay. If you apply pressure to shape it, then briefly release, then apply pressure again, the clay seems to remember the shape you just pulled it into. Similarly, soil retains memory characteristics even when subjected to unloading and later reloading.

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