Back to Basics: Soil Compaction

Compaction is the process by which air pockets are removed from soil, reducing the surface area around each particle, and increasing the soil density. This alteration of the soil structure produces a higher unit weight than non-compacted soil and is a necessary process to create a strong foundation for construction. Construction of buildings, roads and pavements are all common examples of when compaction is required to ensure their stability and longevity.

Soil is prone to natural compaction, however, when building upon a surface, the level of compaction needs to be much greater to bear the weight of the structure on top. There are multiple gauges to test the level of compaction of soil, with the Dynamic Cone Penetrometer or Proctor Test, common in Australia. In each case, soil samples are taken from the site and tested in a lab, to determine the moisture level and density required. Too little moisture can inhibit proper compaction and too much can cause pockets to form between the soil particles, weakening its capacity to bear the weight of a structure above. A result of 95% is most commonly considered the appropriate level of compaction, being within 5% of the soil’s maximum compaction capacity. 

Settlement of soil, where soil naturally shifts with the weight bearing of the structure above, is much more likely to occur if compaction isn’t completed properly and if left, the structure can become unbalanced with shifting soil particles un-levelling its base.
 
Soil shifting occurs over time, as it shrinks or expands, with the structure above doing so in response. Without compaction to reduce the pore size between soil particles, water has a greater ability to pass through it, which is immensely troublesome to the structure above if the soil shifts as a result. Soil can “shrink” under the shear weight of the building or road above if it isn’t first compacted, ultimately causing the cracking you see in structures where soil has been poorly compacted.


 

Methods of Compaction

Methods of compaction differ depending on the soil content and requirements of the job. There are two main forces driving compaction: static force and vibratory force. The use of either is dependent on the type of soil and site size. Static force compression is achieved when a machine bearing significant weight is applied to the soil, compressing its top layer. Compression via vibratory force is the combination of both the machines weight and mechanical forces, driving soil down using strong vibrating blows to the soil surface. This method of compaction not only compresses the topsoil, but also layers below.
 
From here, there are four types of compaction, which apply one of these two forces:

Pressure

Pressure as a method of compaction, is the practice of exerting static force to compress the soil with the machines bear weight. Machines that utilise pressure are inclusive of 3 Pin Static Rollers and Multi Tyre Pneumatic Rollers, which roll over the surface of the soil, compressing its top layer.

Kneading

Kneading is the use of pressure, coupled with the reduction of surface area to produce deeper compaction of soil. This is achieved with the machine’s design utilising block-like protrusions on its rollers, which press into the soil at a greater depth than a regular flat roller such as a 3 Pin Static Roller. Padfoot Rollers and Trench Rollers apply this kind of pressure to complete compaction, applying static force to do so.

Impact

Unlike pressure and kneading, using impact as a form of compression includes a mechanical force to strike the surface of the soil continuously, at rapid speeds. This use of vibratory force allows for greater penetration, reaching deeper layers of the soil. Machines that achieve this form of compression include tampers, and plate compactors.

Vibration

Like impact, vibration as a form of compaction, is achieved through mechanical forces against the surface of the soil, which are expelled into deeper layers using vibratory force. Where impact forces the soil downwards in one place, vibration creates friction of soil particles, dispersing it in all directions. Machines that can accomplish this type of compaction, include double drum vibratory rollers, smooth drum vibratory rollers, and pad foot vibratory rollers.

When choosing the right compaction machine for the job, distinguishing the soil type is necessary to ensure the soil particles bond properly when the compaction force is applied to them. Soil types suitable for compaction are split into the categories of granular soils and cohesive soils. Each class reacts differently to certain compaction methods. Cohesive soils are fine, with high plasticity, high porosity, and low permeability. They include silt and clay. Granular soils are course, have low porosity, high permeability and do not exhibit plasticity. Sand and gravel are examples of granular soils. Granular soils respond better to vibratory forces, which disperse soil particles in all directions, whereas cohesive soils are better compacted using the shear force of a machine.  

To learn more about the types of compaction machines available for hire or purchase at Brooks, visit www.brookshire.com.au.