Soil compaction is the removal of voids in soil by applying static or vibratory vertical forces, resulting in higher soil density that can withstand greater loads. Various types of compaction machinery are available that can be selected for any given project.
The choice of too light or too heavy a compaction machine can influence the compaction quality and cost. The former would require a higher number of passes to achieve the required compaction degree, whereas the latter may crush the material and require replacement. Therefore, improper selection of the compaction machine may lead to an increased cost and delay project completion.
Types of Compaction Machines
There are three primary types of compaction machines based on the forces they apply to soils: static, vibratory, and impact equipment.
1. Static Compaction Machinery
The static compaction equipment exerts vertical weight on the soil layer. The static compaction rollers are suitable for compaction of thin soil layers. Sheepsfoot rollers, grid rollers, rubber-tired rollers, drum rollers, loaders, and scrapers are examples of equipment that apply static vertical force.
2. Vibratory Compaction Machinery
The vibratory compaction machinery applies rapid succession of impacts on the soil surface. It is suitable to consolidate soil with friction and cohesive soil.
3. Impact Compaction Machinery
The impact compaction machinery works at high speed and exerts a high force on the soil surface. It is suitable for the compaction of large areas due to its high-speed operation. Vibrating sheepsfoot and impact rammers are examples of impact compactors.
The soil type is one of the critical factors that control the selection of a suitable compaction machine. If the soil is brought from another place to the job site, then the soil type and equipment can be selected based on the requirement to achieve a designated compaction degree.
However, if in-situ soil is sufficient for the job, then the soil type and its conditions need to be specified first to allocate suitable equipment to get desirable compaction.
1. Granular Soil
Granular soil includes gravel and sand whose particles are visible with the naked eye. Compact granular soil lifts range from 25-30cm. The lifts are soil layers that are being compacted.
Vibration energy is suitable to consolidate granular soil. The vibration force decreases surface friction between particles. As a result, particles fall under gravity and move around till they find a position that restricts their movement.
The granular soil can be compacted easily; gravel is easily compacted compared to sand. Table-1 represents guidance on the selection of suitable compaction machine for gravel and sand.
Table-1: Performance of Various Compaction Machines for Granular Soil Compaction
|-||-||Static compaction machine types||Dynamic (vibration) compaction machine types||Dynamic (impact) compaction machine types|
|Soil type||Lift thickness, cm||Scraper, rubber-tired roller, loader, grid roller||Vibrating plate compactor, vibrating roller, vibrating sheepsfoot roller||Vibrating sheepsfoot rammer|
|Note: Both static sheepsfoot grid roller and scraper are not applicable for gravel and sand compaction.|
2. Cohesive Soil
Cohesive soils are mostly composed of sand and clay with fine particles. The lifts for cohesive soils are commonly around 15cm. The most suitable compaction machine to compact cohesive soils is vibratory rammer or sheepsfoot roller.
The impact force creates a shearing effect and expels air and excess water between particles. Table-2 represents guidelines for choosing appropriate equipment to compact clay and silt.
Table-2: Performance of Various Compaction Machines for Cohesive Soil Compaction
|-||Static compaction machine types (pressure with kneading)||Static compaction machine types (kneading with pressure)||Dynamic (vibration) compaction machine types||Dynamic (impact) compaction machine types|
|Soil type||Static sheepsfoot grid, scraper||Scraper, rubber-tired roller, loader, grid roller||Vibrating plate compactor, vibrating roller, vibrating sheepsfoot roller||Vibrating sheepsfoot rammer|
|Note: Lifts of both silt and clay are 15cm|
FAQs on selection of compaction equipment
Vibrating plate compactor, vibrating roller, vibrating sheepsfoot roller, rubber-tired roller, loader, and grid roller can compact granular soil (gravel and sand) very well.
It means that in-situ soil would be compacted to 95% of the maximum dry density. Commonly, the proctor compaction test is used to determine the maximum dry density of soil and its optimum moisture content.
Soil compaction is the removal of voids in soil by applying static or vibratory vertical forces, resulting in higher soil density that can withstand greater loads.
The primary types of soil are granular soil and cohesive soil.
The compaction effort types are static, dynamic, and impact.
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