1.2. Laboratory Soil Specimen and Testing Procedure

A laboratory soil specimen of dia 60mm and height 20mm is extracted from the undisturbed soil sample obtained from the field.

In this step, we extract a small cylinder of soil from the original, undisturbed sample taken from the field. The dimensions of this specimen are 60mm in diameter and 20mm in height. This is crucial because maintaining the soil's natural structure as much as possible ensures accurate test results when we assess its behavior under load.

Think of this like taking a slice from a cake. If you want to evaluate the texture and flavor of the cake, you need to cut a slice carefully without disturbing it too much. If you were to crumble the cake before taking your slice, it would no longer reflect the original cake's qualities.

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.

The extracted soil sample undergoes a test called one-dimensional consolidation. In this test, the specimen is placed under increasing amounts of pressure - also known as load - incrementally. After applying each load, the sample is left for 24 hours, allowing it to settle and stabilize. This period lets us measure how much the soil compresses, specifically recording the 'voids ratio,' which reflects the amount of empty space in the soil compared to the volume of solids.

Imagine stacking books on a soft mattress. As you put more books on, the mattress compresses. After some time, the mattress stops changing shape significantly even when more books are added. This is similar to how soil behaves under load in the consolidation test.

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

After completing the 1D consolidation test, we create a graph that plots the void ratio against the effective stress applied. The effective stress is the stress that contributes to the soil's ability to support loads, minus the pore water pressure in the soil. This plot helps us visualize the relationship between how compacted the soil becomes (void ratio) as the pressure increases.

Think of this graph as a visual representation of how much a sponge compresses when you squeeze it. The more you squeeze (increase effective stress), the lesser the air and water pockets are inside (the void ratio decreases).

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.

As we analyze the outcomes from the plot, we notice different paths the soil specimen takes during compression. When recompressed, the soil initially follows a certain path labeled as DE. Then, as we increase the load beyond a certain point (C), the behavior changes and the path merges into a new line that reflects further compression patterns labeled as BCF. This shows how the soil's response to pressure changes based on its previous loading history.

Imagine inflating a balloon. When you first start blowing air into it, the balloon expands easily. But as the balloon gets bigger, it becomes harder to inflate—it follows a different path of expansion due to the way the rubber stretches. Similarly, the soil's behavior changes as it gets loaded.

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.

The graph established in our earlier step indicates that at low effective stress, the soil operates along a path called the recompression path (AB). Here, it exhibits minimal compression. Once we move beyond this stage, we enter the 'virgin compression' region (BC), which shows a substantial increase in compression with added stress. This differentiation is important in understanding how previously loaded soil behaves compared to untouched soil.

Think about a spring. When you gently compress it (low stress), it doesn’t change much. But if you apply more force (beyond the spring's natural position), it compresses significantly and is more noticeable. The spring's response mirrors the soil behavior in low versus high stress.

From ‘C’ when the sample is unloaded, sample expands and traces path CD (expansion curve unloading). Sample undergoes Permanent strain due to irreversible soil structure and there is a small elastic recovery.

When the load on the soil sample is removed from point C, the sample will start to expand again, retracing a path labeled as CD. However, it's noteworthy that some deformation is permanent even after unloading, indicating that soil structure has changed irreversibly. There's also a minor elastic recovery, where the soil partially returns to its original shape. This behavior showcases the differences between how materials react elastically (they return to original shape) versus plastically (they retain some deformation).

Picture a piece of clay: If you press it down, it flattens out (compresses), but when you lift the pressure, it doesn't spring back entirely to its original shape. Some of the flattening remains, demonstrating similar behavior to how soil behaves under and after pressure.

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

When the specimen is subjected to reloading, the response follows a different curve that lies above the previous rebound curve, creating what is identified as a hysteresis loop. This loop illustrates the differences between the soil's response during unloading and reloading, showcasing how previous loading alters the daily performance of the soil. The area between the loading and unloading paths signifies the energy lost (hysteresis).

This can be likened to a rubber band. If you stretch it multiple times, each successive stretch may not return to exactly the same shape as before, and there might be some slack or energy loss. Similarly, each reloading alters how the soil behaves compared to its initial state.

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