1.9. Behavior Beyond Point C

1. BC – Virgin compression curve also called normal consolidation line

The virgin compression curve, also known as the normal consolidation line, represents the relationship between void ratio and effective stress for soil undergoing compression. This curve is critical in understanding how soil responds to loading, particularly beyond point C. The soil compresses along this line as effective stress increases, indicating a significant reduction in voids or pore spaces within the soil structure.

Imagine a sponge being squeezed. As you apply more pressure, the sponge minimizes its volume and the air voids inside decrease significantly. This is similar to how soil compresses along the virgin compression curve as pressure increases.

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

When the soil sample is unloaded from point C, it begins to expand. This expansion follows an unloading path, denoted as CD. The soil tends to recover some of its original volume, but this recovery is constrained by its structure and any permanent changes that occurred during loading.

Think of a balloon that has been inflated and then slowly deflates. Although it returns to a larger size compared to when it was fully deflated, it may not regain its original size completely due to stretched materials, akin to how soil may not revert entirely to its previous state after unloading.

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

During the compression process, soil can experience permanent strain, which means it doesn't return to its original state after the load is removed. However, a small part of the strain can be elastic, which is reversible. This phenomenon occurs because soil particles rearrange themselves under load, creating new positions that may not allow complete recovery.

Consider a play-dough model that you press down on. When you press it, it flattens out (permanent strain), but if you lift your hand quickly, it may bounce back slightly (elastic recovery) but not fully to its original shape.

4. The deformation recovered is due to elastic rebound

The deformation that is recovered during unloading is primarily attributed to elastic rebound. This implies that when the pressure is released, all or some of the elastic component of the deformation returns, but any permanent deformation remains. It highlights the ability for soils to slightly recover from compressive forces.

Imagine a slingshot that has been pulled back and then released. Initially, it snaps back to its position (elastic rebound), but some permanent stretching could remain if it was pulled back too hard.

5. When the sample is reloaded-reloading curve lies above the rebound curve and makes a hysteresis loop between expansion and reloading curves.

When the soil sample is reloaded after it has expanded, the path it takes does not overlap completely with the initial loading path. This creates a 'hysteresis loop.' The reloading curve lies above the initial rebound curve, indicating that the soil experiences a different response during reloading compared to the initial loading, demonstrating that the soil's behavior has changed due to previous stress.

Think of a rubber band that has been stretched multiple times. Each time it is stretched, it behaves slightly differently, and it may not fully return to its original state. The varied behavior creates a sort of loop effect, similar to the hysteresis seen in soil mechanics.

6. The reloaded soils shows less compression.

Soils that have been preloaded (loaded previously) typically exhibit less compressibility upon reloading. This means that they will compress less under the same applied stresses after being unloaded. The rearrangement of soil particles and changes in soil structure contribute to this reduction in compressibility.

Consider a suitcase that has been packed tightly. After you unpack it and press on the suitcase to repack, you'll notice it doesn't compress as much as it did the first time because the fabric has already been stretched out.

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

When loading continues beyond point C, the compression curve starts to merge into another section, known as EF. This suggests that the soil behaves as if it has not been unloaded, indicating a consistent response to stress subsequently—that once the soil reaches this point, changes in its state are not reflected in a simplistic manner as unloading would suggest.

Imagine a heavy box placed on a foam mattress. Once the box is removed, the foam does retain some shape from the box (unloading effect) but if you were to place a different, heavier object on it, it compresses again almost as if the mattress didn't have a chance to fully revert, illustrating how the soil behaves under continued loading.

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