Soil Compaction: An Essential Process for Engineering

Soil Compaction: An Essential Process for Engineering
Soil compaction consists of the procedure of improving terrain properties through manual or mechanical processes.

Generally, a soil when transported and grounded is in a relatively fluffy and heterogeneous state and, therefore, not very resistant and very deformable. The compaction procedures aim to provide the soil with improvements of these aspects.

Compaction is a process that aims to improve soil properties by reducing its voids by applying pressure, impact or vibration. In addition, this process makes the mass more homogeneous. This operation results in an increase in the apparent specific gravity of the soil.

With the reduction of soil voids, a reduction in the variation of moisture content, compressibility and permeability is expected, as well as an increase in shear strength and erosion.

The composting geotechnical studies began with the compaction theory developed by Ralph Proctor. In 1933, he disclosed his method of compaction control and concluded that the density at which a soil is compacted under a particular compaction energy depends on the moisture content of the soil.

Soil compaction is usually plotted on the dry apparent specific gravity (γd) versus the corresponding moisture content (w) during the compaction process.


The upward branch of the compaction curve is called the dry branch and the down branch of humid branch. In the ascending branch, the water lubricates the particles and facilitates the arrangement of these, occurring, for this reason, the increase of the apparent specific dry mass. In the descending branch, the water amortizes the compaction and the sample starts to have more water than solids, leading to a decrease of the apparent specific dry mass.

During the soil compaction process, the water content in the soil remains practically constant, and this characteristic is often referred to as the main difference between compaction and soil consolidation.

Differences between Compaction and Consolidation
Compaction is a process acquired by reducing the volume of voids, or air, between soil particles. In the consolidation, which is also a process that is desired to reduce voids index and soil compressibility, the expulsion of the liquid phase occurs, and there is a change in the water content of the soils.

The consolidation is associated to relatively slow processes, caused by the performance of a static and continuous request, which gives rise to the progressive approximation of the particles, at the same time as the flow (expulsion) of the liquid phase occurs. Compaction is generally understood as a rapid process, with the application of variable loads over time by a mechanical action, through which the soil structure is modified in order to create a new arrangement of the particles. In the compaction the moisture content remains constant.

Compaction Energy
The work done during the compaction process of a final volume soil sample V is called energy or compaction effort (Ec). Consider a mass socket (M) falling (n) times from a height (H) on the So, the layered compaction energy is:

Ec = M.H.N.Nc/ V

At where:

Ec = compaction energy
M = Socket Mass
H = height of socket drop
N = number of strokes per layer
Nc = number of layers
V = sample volume
Some types of laboratory compaction
Normal Proctor Assay
The Proctor Normal test uses the cylinder 10 cm in diameter, height of 12.73 cm and volume of 1,000 cm³ is subjected to 26 strokes of a socket with mass of 2.5 Kg and falling of 30.5 cm. Corresponds to the effect of compaction with conventional field equipment.

Modified Assay
The Modified test uses the cylinder 15.24 cm in diameter, 11.43 cm high, 2.085 cm3 volume, 4,536 kg socket weight and 45.7 cm drop height applying 55 strokes per layer. It is used in the most important layers of the pavement, for which the improvement of the soil properties, justifies the use of greater compaction energy.

Compaction in the field
The principles governing compaction in the field are similar to those in the laboratory. Thus, maximum dry specific weight values are fundamentally obtained depending on the type of soil, the amount of water used and the specific energy applied by the equipment to be used, which depends on the type and weight of the equipment and the number of successive passes applied. The compaction curves for the equipment and the number of passes, play the same role as the number of sockets in the laboratory.

For a given equipment, the compaction energy is directly proportional to the number of passes and inversely proportional to the thickness of the compacted layer.

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