2. Effect of compaction on soil properties

Effect of compaction is to reduce the voids by expelling out air. This results in increasing the dry density of soil mass.

When soil is compacted, air is pushed out from the spaces between soil particles, which is referred to as voids. As a result, the soil particles are brought closer together, which increases the dry density of the soil. Higher dry density can improve the stability and load-bearing capacity of the soil.

Think of a bag of marbles. When you first pour them into a bag, there's a lot of space between the marbles, which represents voids. If you shake the bag, the marbles settle closer together, similar to how soil is compacted, reducing voids and increasing density.

Increase the number of contacts resulting in increased shear strength, especially in granular soils. In clays, shear strength depends on dry density, moulding water content, soil structure, method of compaction, strain drainage condition etc. Shear strength of cohesive soils compacted dry of optimum (flocculated structure) will be higher than those compacted wet of optimum (dispersed structure).

As soil is compacted, the particles create more contacts with each other, leading to increased shear strength. This is important because shear strength defines the soil's ability to resist sliding or failure. In granular soils like sand, the compaction process significantly increases strength. For cohesive soils like clay, shear strength is influenced by several factors like moisture content and compaction technique. Soils compacted drier tend to have a more stable structure and higher shear strength than those compacted when wet.

Consider stacking books on a shelf. If the books are close together (like compacted soil), they support each other better and don't topple easily (higher strength). However, if they are spaced apart, they are more likely to fall (lower strength). The arrangement and spacing of the books represent the structure and moisture content in the soil.

1. Increased dry density reduces the void space, thereby reducing permeability. 2. At same density, soil compacted dry of optimum is more permeable. 3. At same void ratio, soil with bigger particle size is more permeable. 4. Increased compactive effort reduces permeability.

Permeability is the ability of soil to allow water to flow through it. When soil is compacted, its dry density increases, which reduces the void spaces where water can move, leading to lower permeability. Interestingly, if two soils are of the same density, the one compacted on the dry side of optimum will allow more water to flow through compared to the wetter compacted soil. Moreover, soil with larger particles tends to be more permeable even at the same void ratio. The more effort used in compacting the soil can result in even lower permeability.

Imagine trying to push a thick pudding through a strainer versus pushing water through it. The pudding is denser and has smaller gaps for water to flow, which illustrates how compacted soil with less void space limits water flow, whereas a highly permeable soil, like water, allows it to pass through freely.

1. Increase in compaction increases the density and number of contacts between soil particles. 2. This results in increased bearing capacity which is a function of density.

Bearing capacity is the ability of soil to support loads from structures. When soil is compacted, its density increases and the number of particle contacts increases, allowing it to better bear loads. This makes the foundation more stable and reduces the risk of settlement or failure under a building or structure.

Think of a trampoline. When you jump on a trampoline that is fully inflated (well-compacted soil), it can support your weight much better than one that is partially deflated (less compacted). The more air it has, the firmer and more stable it is; similarly, denser soil provides better support.

1. Compaction increases density and decreases void ratio. 2. This results in reduced settlement. 3. Both elastic settlement and consolidation settlement are reduced. 4. Soil compacted dry of optimum experiences greater compression than that compacted wet of optimum.

Settlement refers to the gradual sinking or settling of soil under loads. By compacting the soil, its density increases and voids are reduced, which minimizes the potential for settlement over time. This means structures built on compacted soil are less likely to experience ground settling. Additionally, soils compacted dry of optimum are usually squeezed tighter, leading to less overall compression when loads are applied compared to wetter soils.

Imagine a sponge when you step on it. If it's dry (like dry compacted soil), it flattens out significantly when stepped on, but if it's wet, it compresses less under pressure, simulating the difference between wet and dry compaction in terms of how each type supports loads.

1. On dry side of optimum, the structure is flocculated. The particles repel and density is less. 2. Addition of water increases lubrication and transforms the structure into dispersed structure. 3. In coarse grained soil, single grained structure is maintained. 4. In composite soil, behaviour depends on composition.

Soil structure refers to how soil particles are arranged. On the dry side of optimum moisture, soils tend to clump together (flocculated structure), reducing overall density. However, adding water helps to lubricate the particles, allowing them to spread out more evenly (dispersed structure). This affects how well the soil behaves under load. In coarse-grained soils, the individual grains remain separate, while in composite soils, the behavior can vary based on the mixture of soil types.

Consider a bowl of dry pasta versus one with sauce. The dry pasta sticks together (flocculated structure), making it less dense and harder to move, whereas the saucy pasta is easier to stir due to the lubrication (dispersed structure). This illustrates how moisture affects the arrangement and behavior of soil particles.

1. Clayey soil compacted dry of optimum develops less pore water pressure than that compacted wet of optimum at the same density at low strains. 2. However, at higher strains the effect is the same in both the cases.

Pore pressure refers to the pressure of water within the soil's pores. In clayey soils, when compacted below optimum moisture content, the pore pressure is lower compared to those compacted with excess water. This difference is crucial at lower stress levels (low strains), as it affects how the soil behaves under loads. However, when subjected to high strains, the pore pressures tend to equalize in both scenarios.

Think of a balloon filled with water. When you squeeze it gently, some areas may withstand the pressure better without bursting (representing low strains). However, if you apply a lot of force suddenly, the entire balloon (like the soil at high strain) equalizes the pressure and may react similarly regardless of the initial water content.

The strength and modulus of elasticity of soil on the dry side of optimum will always be better than on the wet side for the same density. Soil compacted dry of optimum shows brittle failure and that compacted on wet side experiences increased strain.

Stress-strain characteristics describe how soil responds to applied forces. Soils compacted on the dry side exhibit better strength and elasticity than those on the wet side when held at the same density. This difference means that dry-compacted soils tend to fail more suddenly (brittle failure), while wet-compacted soils are able to deform more under stress, indicating greater strain.

Think of how a dry cookie breaks apart versus a wet dough. A dry cookie (dry side) crumbles quickly when pressure is applied, while wet dough (wet side) can be stretched and shaped before it breaks. This analogy helps to illustrate the behavior differences between dry and wet-compacted soils.

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