How to Compact Granular Soils

Working on and compacting granular soils such as sands and gravels are difficult tasks. Due to the composition of sand and gravel, water can enter or leave the voids in them with relative ease. If voids in the sand are filled with water or are completely dry, no forces are holding the sand particles together.

Applying vibration to those particles tends to create a dense configuration. The amount of water in the voids of the granular soils will affect the cohesion of the particles. Dumping sand or gravel from the bed of a truck or a scraper places the granular material in a relatively loose condition, particularly if the sand contains only a small amount of surface moisture. This material must be compacted to provide the required strength. ​If the material is not compacted, then it can lead to settlements in the future.

Vibratory Rollers

Vibratory rollers are designed to consolidate granular soils to a high density, compacting very efficiently to shallow depths. These rollers come in different sizes and different tons of capacity. This type of equipment works effectively when the soil is placed in a confined space.

Flat Plate Vibrators

Some excellent flat plate vibrators have a gasoline engine mounted to a unit that causes a flat skid plate to vibrate. These do an efficient job on sands and small gravels, compacting to a depth or lift thickness of about 6 inches. Flat plate vibrators are commonly used in small trenches and for small-diameter pipe installation. Because of their limited power and impact force, the use of flat plate vibrators is not recommended when lifts are thicker than 6 inches.

Vibratory Hammer

With a vibratory hammer, the vertical vibration in the pile disturbs or "liquefies" the soil and causes the soil particles to lose their frictional grip on the pile. The pile moves downward under its weight, plus the weight of the vibratory hammer. An amplitude of at least one inch is usually required to cause enough soil disturbance to achieve pile driving. Vibratory compaction works well, as the soil disturbance due to vibration causes the soil particles to move into a denser configuration. Large amplitude results in a high soil strain level, a greater influence radius, and a higher degree of compaction.

Vibroflotation

Vibroflotation is another very effective method for compacting clean, free-draining sands and gravels. It resembles a standard internal concrete vibrator but is much larger and more powerful. The probe provides large-capacity water jets, which act downward and sideways, flooding the soil and breaking the surface tension. This action allows the sand particles more freedom to settle into a compact configuration as the granular particles are vibrated.

Soil Compaction Technology Advances

Technology is also improving in the area of compacting granular soils. Equipment is available for compaction that uses a monitoring system and sensors to measure the soil stiffness and generate a value related to the compaction progress on how good or bad the process is going. This technique provides real-time values by reducing the time wasted compacting an area that is already finished. The tool lets users know when the design value has been obtained, preventing the excessive use of resources in over compacting granular soil.

Other Important Factors

When compacting soils, put safeguards in place for employees' health. An illness called Raynaud's syndrome is very common among workers who operate or manage compacting equipment. The all-day-long vibration transmitted to the hands and arms causes this illness, which can result in your employees feeling a sort of numbness and tingling in the fingers. If that worsens, it can lead to loss of sensation in hand and finger muscles. Some manufacturers have designed equipment with reduced vibration. Certain jurisdictions have laws limiting the daily exposure to vibrational forces because of the potential for such illnesses to develop.