1.8. Reloading Curves and Hysteresis Loop

During the initial stages (at low effective stress) sample follows recompression path (portion AB) and undergoes less compression.

In this chunk, we discuss what happens to the soil sample during the first phase of loading. At low effective stress, when the soil is initially compressed, it does not compress much, following a specific path known as the recompression path (segment AB). This behavior is crucial as it indicates that the soil is still in a relatively 'young' phase of compression where it retains much of its initial structure and is yet to settle into substantial compaction.

Think of this stage like a sponge that has just begun to be squeezed. If you apply a small amount of pressure, the sponge compresses slightly without losing too much of its original shape. Similarly, the soil, under low stress, shows minimal compression in this early stage.

Beyond this is the virgin compression line (portion BC) also called the normal compression line and the sample undergoes large compression.

As we increase the effective stress beyond a certain point (point C), the soil sample enters the virgin compression phase (segment BC). In this phase, the soil experiences significant compression compared to the initial stage. This line, referred to as the normal compression line, illustrates that the more stress applied, the more the soil continues to compress. This relationship highlights how soils behave differently under varying levels of stress, especially under substantial loading conditions.

Imagine pressing down on a thick book. Initially, you may not notice much change in its thickness. However, as you press harder, the book compresses significantly. This is analogous to how soil behaves as it receives increasing stress during loading.

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

Upon unloading the sample from point C, it does not return directly to its original state. Instead, the sample expands and traces the unloading curve (segment CD). This indicates that when the stress is removed, the soil tends to recover, but not completely, showing that some deformation remains due to its structural changes during loading.

This situation can be compared to a rubber band that has been stretched; when you release it, it doesn’t always return to its original size or shape. The soil behaves similarly—it expands back but not perfectly to its previous state.

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

This chunk highlights that during the loading and unloading cycles, the soil experiences permanent strain. This means that after removal of the load, the soil retains some deformations that cannot be reversed (irreversible soil structure). However, there is also a factor of elastic recovery where the soil can still return to its previous state partially, allowing for a small degree of rebound.

Consider a dense clay dough that has been pressed. Once you lift your hand, some dough will still be flatten due to the pressure applied, but if you look closely, you might see slight recovery, similar to how the soil behaves when the load is released.

When the sample is reloaded, the reloading curve lies above the rebound curve and makes an hysteresis loop between expansion and reloading curves.

In this portion, we focus on the behavior of the soil when it is reloaded after having been unloaded. The reloading curve lies above the recovery (rebound) curve, illustrating that it takes more stress to compress the soil again after the initial loading and unloading cycles. This behavior creates an area known as a hysteresis loop, which represents the differences in paths of loading and unloading, reflecting the energy dissipated in the soil due to internal friction and structural changes.

Think of this like pumping air into a tire that was previously deflated. The first time you pump it up, it holds air easily. However, after some deflation, you find that you need to put in more effort to inflate it back to the same pressure. This illustrates how the soil behaves—requiring more effort to reach the same compression level after having undergone loading and unloading.

The reloaded soils show less compression.

This indicates that upon reloading, the soil does not compress as much as it did during the initial loading. The reduction in compression reflects the fact that the soil has already undergone some structural adjustments from the previous loadings, making it more resistant to further compression under repetitive loads.

Imagine a sponge that has already been soaked with water. If you try to squeeze it again, it will not compress as much as when it was dry. Similarly, the soil, after being previously loaded, no longer compresses as easily when reloaded.

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

Here, we discuss the behavior of the soil when it is subjected to loading that exceeds the unloading point 'C'. In this case, the compression curve merges into a new portion (EF), indicating that the soil behaves as if it has not been unloaded at all. This suggests that at higher stress levels, the soil's structure can adapt under such loads, implying a change in its compressibility as it becomes more consolidated.

This is similar to a well-packed suitcase. If you keep adding weight to it, instead of going back to its original shape, it simply continues to compress more tightly. For soil, once the loading exceeds certain limits, it effectively ignores prior unloading behavior and acts as though it hasn't been stressed before.

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