1.3. Recompression and Compression Curves

The compressibility of fine grained soils can be described in terms of voids ratio versus effective stress.

Compressibility refers to how much a soil will compress under load. In finer-grained soils, this behavior can be analyzed using the relationship between the voids ratio (the ratio of the volume of voids to the volume of solids in a soil sample) and effective stress (the stress carried by the soil skeleton). This relationship helps us understand how the soil will behave under different loads.

Imagine a sponge. When you press it, the air pockets inside (voids) compress, and the sponge becomes denser. As you release the pressure, some of the air may return, but not all. Similarly, fine-grained soils react to loads in a particular way that can be captured in a graph, allowing engineers to predict how much the soil will settle.

A laboratory soil specimen of dia 60mm and height 20mm is extracted from the undisturbed soil sample obtained from the field. This sample is subjected to 1D consolidation in the lab under various pressure increments. Each pressure increment is maintained for 24 hrs and equilibrium void ratio is recorded before the application of the next pressure increment.

To assess how a soil sample consolidates under pressure, a specific size specimen (60mm in diameter and 20mm high) is taken from its natural environment. In the lab, the sample is subjected to incremental loads, with each load applied for 24 hours to allow the soil to stabilize. The void ratio is measured after each load to track how the soil compresses.

Think of this process like a slow game of 'musical chairs.' Each round (pressure increment) lasts for a specific time, and players (soil particles) slowly settle down. By observing how the players (particles) adjust after each round, we can learn about their behaviors under stress.

When the sample is recompressed from point D it follows DE and beyond C it merges along BCF and it compresses as it moves along BCF.

The testing results in a graph showing different paths of compression. When the soil is recompressed from a certain point, it follows a specific path (DE). Once it reaches point C, the graph shows that beyond this point, the behavior of the soil changes to a new compression path (BCF), indicating a different state of compressibility.

Imagine a bouncy ball. When you press it slightly (recompression), it returns to its original shape somewhat. If you press it harder (moving beyond point C), it gets squished into a new shape that it won't return from completely, similar to how soil behaves under increasing loads.

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

The Virgin Compression Curve indicates the behavior of a soil sample when it is compressed for the first time without any prior loading. This line shows significant compression as the effective stress increases, reflecting the soil's tendency to compact under load.

Think of a vacuum-sealed bag of chips. When you first seal it (virgin compression), the chips compress significantly as the air is sucked out. This is akin to how soil behaves initially when subjected to loads.

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

When the applied load on the soil sample is removed after reaching point C, the sample begins to expand again. This expansion follows a specific path (CD) that traces the soil's recovery behavior after unloading. However, not all deformation is recovered due to the nature of the soil structure.

Consider a spring. When you stretch it, it compresses. When you let go, it partially returns to its original shape, but not perfectly. This is similar to the soil's behavior after the load is removed, recovering to some extent but remaining altered.

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

Permanent strain refers to the deformation that remains in the soil after unloading. While there is some elastic recovery (the ability to return to its original shape), a portion of the deformation becomes permanent due to the soil's structure being altered under load.

Imagine a playdough sculpture. When you shape it (load it), it forms into a new shape. When you stop pressing (unload), it returns to a different shape that is not exactly like the original. This exemplifies how soil can retain some of their deformation permanently.

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

When the soil sample is subjected to reloading after unloading, the path it takes on the graph is different from the initial compression path. The loading path lies above the previous rebound curve, creating a hysteresis loop, which indicates that the soil behaves differently upon reloading compared to when it was first loaded.

Think of riding a bike. The first time you ride it down a hill (initial load), you gain speed (compression). If you stop and try to ride back up (unloading), you may use more energy than the first run. This difference illustrates the varied behavior under different loading scenarios.

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

When the loading continues beyond point C on the compression curve, the graph shows a smooth transition into another portion (EF), indicating that the soil behaves as if it had never been unloaded. This reflects the soil's ability to bear additional loads without a significant drop in compressibility.

Imagine stacking books on a thin table. If you remove a few and then add more (like unloading and reloading), the table might still hold them without collapsing completely. This illustrates how soil continues to support weight even after some unloading.

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