Compaction is the process of applying a load or force to a material to increase its density. As a load is applied, it pushes the particles in the soil closer together, reducing air voids.

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Removing air voids in soil increases its stability and strength as the particles interlock with each other. It also helps to stop settlement and unwanted movement over time, very important if a road or building is being placed on top.

 

Compaction is also important as it helps to prevent water soaking into the soil. If there are no air voids, the water can’t penetrate the soil layer as easily.

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There are 3 main factors that affect how well you compact your soil:

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Moisture Content

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Water in the soil acts as a lubricant, and enables the soil particles to slide over and around each other as the compactive force is applied.

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If a soil is too dry, the particles bind up and don't get the chance to fill the air voids.

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If the soil is too wet, all the air voids are expelled, but instead of filling gaps with soil particles, the water pushes the soil out of the way.

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Water can't be compressed, so any force placed on it just moves the water elsewhere through the soil. This will result in the soil 'heaving'. Keep an eye out for this when you compact in your mould, or watch a roller compacting on site. Because water weighs less than soil, you'll also notice a drop in density as the heavier soil particles are replaced by water.

 

Optimum Moisture Content (OMC) is the term used to describe the perfect amount of water in your soil to allow the most compaction

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Compactive Effort

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Compactive effort is the amount of force that is applied to the soil during compaction. Basically, the heavier the object that is compacting, the better the compaction will be. Compactive effort in the field is the roller, in the lab it's your compaction hammer. Compactive effort will also change the OMC of a material. If you are using a modified hammer instead of a standard hammer, you don't need as much lubrication in your sample to get the particles sliding over each other and filling voids. This applies in the field too, with smaller and larger roller size

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Layer Thickness

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Layer thickness or layer depth will affect compaction, especially in the lower part of the layer. As compactive effort is applied, the force dissipates through the layer, resulting in less compaction at the bottom of the layer. For a standard 1 litre lab compaction, this is why we compact in 3 layers of 25 blows, instead of 1 layer of 75 blows, even though that is the same amount of compactive effort. Earthworks layers on site are also generally limited to a maximum 300mm thickness to ensure proper compaction with the roller.

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