1. Compression of Fine Grained Soil

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

The compressibility of fine-grained soils is influenced by the relationship between the voids ratio (a measure of soil volume that is not occupied by solid particles) and effective stress (the stress carried by the soil skeleton). In a laboratory setting, a small cylindrical soil sample is taken from a larger, undisturbed field sample to study its properties under controlled conditions. This sample is cylindrical, measuring 60 mm in diameter and 20 mm in height, and is subjected to One-Dimensional (1D) consolidation. This process involves applying different levels of pressure incrementally, allowing us to understand how the soil reacts under varying loads.

Imagine a sponge that you press down on while it's submerged in water. Initially, the sponge compresses slightly, but as you apply more pressure, it gets significantly squished. This is similar to how fine-grained soils behave when pressure is applied – they may compress a little at first but compress significantly when subjected to higher stresses.

Each pressure increment is maintained for 24 hrs and equilibrium void ratio is recorded before the application of the next pressure increment.

In the consolidation test, each time a new pressure is applied, it is held constant for 24 hours, allowing the soil sample to settle and reach equilibrium. During this time, water can either escape from or enter the soil pores. Once the soil reaches a steady state, the void ratio is measured, which represents the amount of space available in the soil structure. This helps in understanding how the soil volume changes with increased stress.

Think of this process like letting a cake cool down after baking. The cake initially appears fluffy, but as it cools (similar to the pressure being applied), the moisture evaporates, and it settles into a denser form. Similarly, the soil settles and records a new void ratio under constant pressure.

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.

Upon applying pressure to the soil sample, it follows specific paths on a graph that plots void ratio against effective stress. Initially, when the sample is recompressed from a point marked as D, it follows a designated path DE. Once it reaches a limiting point C, any further compression goes along a line designated as BCF, indicating a marked change in behavior beyond this point. The path illustrates how much the soil compresses and its capacity to handle stress.

This is like using a rubber band. If you stretch it a little, it returns to its original shape. However, if you stretch it too much, it may not fully return to its original size. The paths on the graph indicate similar behavior in the soil as pressures change.

During the initial stages (at low effective stress) sample follows recompression path (portion AB) and undergoes less compression. Beyond this is the virgin compression line (portion BC) also called the normal compression line and the sample undergoes large compression.

Initially, when the stress is low, the soil sample follows a path AB on the plot, indicating that it undergoes less compression. At this stage, the soil can recover relatively well. However, as pressure increases and the stress crosses a certain threshold, the behavior changes, and the sample begins to follow the virgin compression line (BC), which indicates that it is undergoing larger amounts of compression. This transition is significant in geotechnical engineering, as it can inform how the soil will behave under different loading conditions.

Think of a pillow. When you initially sit on it lightly, it compresses only a little, but if you add more weight (like lying down), it gets compressed significantly. The different stages of pressure applied to the soil are reflected in how compressible the soil becomes.

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

When the stress is released (unloading) at point C, the soil sample begins to expand following a path CD. This behavior indicates that when the pressure is removed, the soil attempts to return to its original state. However, this recovery is not always complete due to factors like permanent strain caused by the soil structure's irreversible changes during compression.

This expansion can be likened to a balloon. If you let the air out of a balloon, it expands again, but might not fully return to its original shape if it’s been stretched too far. Similarly, once the soil is compressed and then unloaded, it expands but might not reach its original void ratio.

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

Even after unloading, the sample experiences permanent strain. This means that some deformation remains due to the changes in the soil structure which cannot fully recover to its initial state. However, there is still a degree of elastic recovery, which refers to the temporary deformation that can revert once the load is removed.

This concept can be illustrated using a clay figurine. If you press and reshape it, when you release the pressure, some parts might return to their original form, while others stay changed. Soil behaves similarly; not all changes revert after unloading.

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

Upon reloading the sample, the new loading curve will be positioned above the path of recovery from the previous unloading, creating an area known as the hysteresis loop. This phenomenon demonstrates that the soil behaves differently when it is loaded and unloaded in successive cycles. The presence of the hysteresis loop indicates that the reloaded soil compresses less than it originally did, suggesting a history of loading has affected its compressibility.

Consider a spring that has been stretched multiple times. Each time it’s reloaded, it doesn’t stretch as much as the initial stretch because the material has become 'used' to being stretched. The hysteresis loop shows this decrease in ability to compress further under identical conditions.

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 soil behavior displays a continuous reaction without returning to its previous state, merging into the section EF on the plot. This indicates that once the soil is subjected to higher pressures, it behaves as if it has never been unloaded, and the soil retains memory of the new stress state. The implication is that previously unloading does not affect how the soil behaves under subsequent loading.

Think of it like a piece of clay that, once molded under heavy pressure, doesn’t return to its previous shape when new pressure is applied. Even if you release the pressure momentarily, if you push harder afterward, it continues to respond as if it had never been lightened.

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