1.2. Total Settlement Components

The total settlement consists of three components:

1. Immediate settlement.
2. Primary consolidation settlement
3. Secondary consolidation settlement (Creep settlement) St = Si + Sc + Ssc

Total settlement in soil engineering refers to the overall change in vertical position of the ground surface due to applied loads. It has three components:

1. Immediate Settlement (Si) - This happens right after the load is applied. It reflects the elasticity of the soil and requires little to no water movement in this stage.
2. Primary Consolidation Settlement (Sc) - This takes time and involves water being expelled from the soil's pores as pressure is applied. The soil rearranges itself over time as pore water pressure reduces.
3. Secondary Consolidation Settlement (Ssc) - Also known as creep, it continues after primary consolidation and involves gradual volume change due to rearrangement of soil particles under constant effective stress. These three components when summed give the total settlement (St).

Imagine putting a heavy book on a sponge. Initially, the sponge squishes down (immediate settlement). Over time, if you keep the book on the sponge, some water might seep out of it, causing further squishing (primary consolidation). Even after you remove the book, the sponge continues to shrink a bit due to its internal structure rearranging (secondary consolidation).

Elastic Settlement or Immediate Settlement occurs immediately after the load is applied. This is due to distortion (change in shape) at constant volume. There is negligible flow of water in less pervious soils. In case of pervious soils, the flow of water is quick at constant volume. This is determined by elastic theory.

Immediate settlement is the part of total settlement that occurs right after a load is applied to the soil. It involves the soil temporarily deforming under load without any significant water movement. In less permeable soils, like clay, the change happens fast in terms of shape but doesn’t involve much water displacement. On the other hand, in more permeable soils, like sand, immediate water movement might occur but still within a short time frame. This phenomenon is described by elastic theory, which considers how materials deform under load.

Think of it like using a rubber band. When you stretch it quickly, it deforms (immediate settlement) but returns to its original shape after the load is removed, showing that the change was temporary.

Primary consolidation settlement occurs due to expulsion of pore water from the voids of a saturated soil. In case of saturated fine grained soils, the deformation is due to squeezing of water from the pores leading to rearrangement of soil particles. The movement of pore water depends on the permeability and dissipation of pore water pressure, ultimately leading to effective stress increase.

Primary consolidation occurs after the immediate response to a load, as excess pore water pressure is reduced over time. In saturated fine-grained soils, when a load is applied, the water trapped in soil pores is gradually pushed out. As this excess pore water is expelled, the soil particles start to reorganize and settle into a denser formation due to the increase in effective stress (the stress carried by soil particles after accounting for pore water pressure). This process can take a significant amount of time depending on the soil's permeability.

Consider a sponge soaked in water. When you apply pressure, the water starts to squeeze out. Initially, the sponge doesn't change much in shape, but over time, as you keep pressing, the water escapes, and the sponge slowly becomes denser and settles down into a smaller form. This illustrates primary consolidation.

Secondary Consolidation Settlement (Creep) is defined as the change in volume of a fine-grained soil due to rearrangement of soil particles (fabric) at constant effective stress. The rate of secondary consolidation is very slow when compared with primary consolidation.

Secondary consolidation is the gradual, long-term settlement that occurs in saturated fine-grained soils after primary consolidation has concluded. Unlike primary consolidation, where pore water is expelled, secondary consolidation involves the slow rearrangement of soil particles that occurs even when the effective stress remains constant. This process is typically much slower and can continue for years after the initial loading.

Think of secondary consolidation like a bowl of jello settling at room temperature. Even after you remove any force that was applied to it (like shaking it), the jello continues to settle and reshape itself very slowly over time, reflecting the slow and gradual changes that occur in the soil.

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