Published: May 31, 2018 | Last updated: July 5, 2023

What Does Compaction Mean?

Compaction is the process of increasing the density of soil by mechanical means. It results in the rearrangement of soil particles and the reduction of voids. Soils that are highly compacted contain very few voids resulting in soil having a higher unit weight. The soil in its natural state is loosely bound and therefore compaction is necessary to increase its bearing capacity.

All types of construction processes require soil compaction to prevent the settlement of soil over time. Settlement can cause damage to the structure by inducing bending stresses and even tilt the structure where compaction is not adequate. Compaction ensures stable and settlement-free construction or installation.

Compaction increases the bearing capacity of the soil and provides stability by increasing its shear strength. Compaction also helps prevent damage by frost, reduces settlement under dead and live loads, and reduces the permeability of the soil. Less permeable soil prevents water from easily flowing through the soil and causing contraction and expansion.

Compaction can be carried out by vibration, impact, kneading, or pressure, depending on the soil type whether cohesive or granular. Cohesive soils like clay, which are dense and closely bound, become plastic when wet and hard when dry. These soils require proper distribution of moisture to ensure good compaction; impact and pressure methods work best.

Granular soils like sand and medium-sized gravel have water draining properties and are in the state of maximum density when fully dry or fully saturated; vibration works well for this soil type.


Trenchlesspedia Explains Compaction

Soil compaction takes place when pore spaces between soil particles are drastically reduced by compacting using mechanical means. Well-compacted soils are highly dense with less total pore volume and very few large pores. Compacted soils also have a lower rate of water infiltration and drainage. Soils with a higher percentage of clay and silt have more pore space and hence a lower bulk density than sandy soils.

Importance of Soil Classification

Soil classification is done based on particle size or by Atterberg limits. The particles in soils or sediments are of different sizes and are sized based on their diameter. The distribution of particle size determines the texture of the soil if it is predominantly sandy, loamy, or clayey. Atterberg limits evaluate the shrink/swell potential of near-surface soil and can be used to distinguish between silt and clay and its different types. It gives the shrinkage limit, plastic limit, and liquid limit of the soil sample.

Factors Affecting Soil Compaction

Many factors affect soil compaction such as soil type, moisture content, compactor types, the thickness of lift, contact pressure, speed of rolling, and the number of rollers passes. The first three factors are explained in brief:

· Soil Type

The degree of soil compaction depends on the type of soil being compacted. Well-graded granular soils are preferred for construction applications as they can be easily compacted, removing voids by interlocking the particles. This resists water absorption and is, therefore, able to support heavier loads.

· Moisture Content

When the water content in soil is at the optimum level, its maximum dry density is achieved known as the optimum moisture content. These factors are determined in a laboratory using soil samples taken from the site. Water content in dry soil can be increased by spreading water. For wet soils, chemical drying is preferred by using Portland cement or lime-based reagents that absorb water.

· Compactor Types

Different types of soil compaction rollers can be used such as single or double drums, vibratory mechanisms, or dozer blades. Smooth rollers use static pressure along with vibration and impact. Padfoot and tamping foot rollers use manipulative force to break the bonds between soil particles. Pneumatic rollers use staggered rubber tires with differing pressure.


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