1.1. Voids Ratio versus Effective Stress

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

The relationship between voids ratio and effective stress is crucial in understanding how fine-grained soils behave under load. The voids ratio measures the volume of voids or empty spaces in soil compared to the volume of solids. Effective stress, on the other hand, refers to the stress that contributes to soil strength and stability, factoring out pore water pressure. Together, these concepts help us analyze soil compression and consolidation behavior in engineering applications.

You can think of voids ratio like a sponge. If it's full of water (high void ratio), it's softer and can be compressed easily (low effective stress). When you squeeze the sponge (increasing effective stress), the water is expelled, making it firmer (lower voids ratio), hence it can support more weight.

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.

In the laboratory, a cylindrical soil sample was collected and subjected to a process called one-dimensional (1D) consolidation. This means the soil was pressed down under a series of increasing pressures over several days. After each pressure was applied for a set amount of time (24 hours), the voids ratio of the soil was measured again to see how the soil reacted to the stress. This experiment helps in gathering data about how soil compresses over time under load.

Imagine pressing a pack of sugar cubes. When you apply weight on top, the cubes start to settle and compress. After you keep the weight for a while and then check, you notice the cubes are denser. The same principle applies to the soil - the longer it is under pressure, the more it compresses.

Then a plot of void ratio versus effective stress is made as shown in above figure.

Once the data from the experiment is collected, the information is represented graphically by plotting the voids ratio against effective stress. This visual representation helps in understanding the relationship between these two parameters. The behavior of the soil can be observed more clearly, showing how it compresses or expands under varying levels of stress.

Think of a graph like a seesaw. Depending on how much weight you put on one end (effective stress), the other end (voids ratio) will either go up or down. The graph illustrates how they balance each other out.

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.

In the experiment, the soil sample displays different compression behaviors depending on the stress levels it has experienced. When reloading the soil, it follows a specific path (DE) and then merges into another path (BCF) which shows its compressive behavior at higher stress levels. This merging indicates that the soil's behavior changes at different stresses, emphasizing the concept of soil memory in how it responds to loading.

Consider a rubber band. If you stretch it gently (low stress), it returns to its original shape (recompression path). If you stretch it too much (high stress), it might not return completely - just like the soil’s behavior on different paths after stress is applied.

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

In the initial phases of loading the soil, the effective stress is low, and the soil does not compress significantly. During this phase (path AB), it tends to follow a path of recompression where the change in voids ratio is relatively small. This behavior indicates that the soil is still capable of adjusting without significant deformation.

It’s like filling a balloon with a little air. Initially, it stretches only slightly without much effort. However, once you keep adding more air (increasing effective stress), it starts to bulge more significantly, similar to how soil behaves under higher stress.

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

Once the soil reaches a threshold stress level, it enters the virgin compression phase (path BC). In this phase, the soil experiences significant compression. Here, the relationship between voids ratio and effective stress becomes more pronounced, indicating that the soil structure has changed and is now denser under the applied load.

Imagine a wet sponge. When you first press it gently, it only compresses slightly (initial stages). But if you press it hard (moving to the virgin compression line), the sponge compresses more significantly, squeezing out the water and reducing its volume drastically.

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

When the stress is removed from point C, the soil sample begins to expand again, tracing the path labeled as CD. This expansion shows that even after the load is removed, there is still some recovery of the voids ratio, although it may not return completely to the original state. This is critical to understanding how soil behaves when loads are taken off.

Think of a sponge again. If you press it down and then let go, it doesn't revert back to its original shape instantly. Instead, it slowly expands back – similar to what happens with soil during unloading.

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

As the soil is loaded and then unloaded, it experiences what is known as permanent strain, which means some deformation remains even after the load is removed. This permanent change is attributed to the way soil particles rearrange during loading. However, there is also an elastic recovery component, which is the ability of the soil to bounce back slightly when unloaded.

This is like clay. If you shape it into a ball and then squish it, it won't return perfectly to a ball shape (permanent strain). But if you let it sit and it slowly dries out, it might regain some of its original shape (elastic recovery).

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 is reloaded after some unloading, it does not follow the same path back to its original condition but instead takes a higher path, indicating that it behaves differently upon reloading. This results in a hysteresis loop, showcasing the difference between expansion and compression paths, which is important in understanding the resilience of soil.

Consider a rubber band again. If you stretch it and then relax it, and finally stretch it again, it won’t go back to its original stretch midway, indicating how the material has changed after each cycle (hysteresis).

The reloaded soils show less compression.

As previously outlined, soils display less compression when reloaded as compared to initial loading due to the changes in soil structure from the previous loads. This phenomenon demonstrates the cumulative effects of stress on soil mechanics and shows that soils can become more resilient with repeated loading.

Imagine you have repeatedly squished a sponge. After a point, every time you squish it, the sponge doesn’t respond as much compared to the first few times – it has adapted to the force applied.

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

When loading progresses beyond the critical point indicated by 'C', the stress and void ratio relationship shifts again, and the curve merges into a new phase (EF). This suggests that once the soil has been loaded beyond the unloading point, it behaves as if the previous unload did not significantly affect its resilience, indicating the capability of soils to be effective in bearing loads, even after prior cycles of stress.

Similar to pushing a swing. After a swing goes back and forth, if you push it again from a further point, it might seem as if it’s consistently responding to your pushes without registering earlier movements – that’s the merging of load behaviors.

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