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Soil Compaction

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Caterpillar CP563 Vibratory Padfoot Compactor
Soil compaction
is the process of increasing the density of soil. The compaction of soil is crucial in construction applications that require building or laying foundations for any type of building, structure, bridge, tunnel, dam, or roadway. Soil compaction also plays a role in the agricultural industry. Soil compaction has drawn criticism from many environmental authorities and organizations as being detrimental to wildlife and plants.

Compaction can be achieved several ways: through vibration, impact, kneading, and pressure; there are many types of compaction equipment suitable for this purpose.[1]

Contents

[edit] History

The effects of soil compaction have been recorded since the 1800s when Benjamin Lincoln of New England wrote about the consequences of letting livestock wander and graze throughout wooded areas. He noticed that the weight of livestock contributed greatly to the compaction of soil particles, hardening soil to the point where little or no oxygen could be contained, and resulting in the death of plant life in the surrounding areas. [2]

Livestock was, for better or worse, used for compacting soil in agricultural communities until the arrival of compaction equipment in the later 1800s and early 1900s. Even the use of horses and mules to pull carts and wagons affected the soil to the point of compaction.

The introduction of machinery and the effect of new technology made compaction equipment beneficial to many construction applications. Compacting soil served many different purposes, such as:

  • Preventing soil from damage due to settlement and frost
  • Providing stability (especially beneficial for laying foundations)
  • Reducing contraction, swelling, and seepage.[3]

Compaction equipment, namely rollers and compactors, can be traced as far back as the 1700s when French pioneer Nicholas Cugnot invented a self-propelling steam traction engine. The first steam roller appeared in France in 1860 and made its way to America, thanks to Aveling & Porter. Motor rollers dominated the 1900s and compactors appeared shortly thereafter.

[edit] Type

There are four types of compaction that are commonly used on soil and/or asphalt:

  • Vibration
  • Impact
  • Kneading
  • Pressure

Each of these types is carried out using one of two types of forces: static or vibratory.

Static force relies on the weight of a machine to apply downward pressure on soil, thus compressing the soil particles. Adding weights to, or removing them from, the compaction machine can adjust the amount of pressure. Although effective, static compaction is best suited for the upper soil layers. The types of compaction that fall under static are kneading and pressure.

Padfeet on a Caterpillar CP563
Vibratory force, on the other hand, uses mechanically driven force to apply downward pressure in addition to the weight of a machine. The mechanically driven force is an applied vibratory force that rotates the eccentric weight of a piston and spring combination. Compactors achieve compaction through the use of delivering rapid blows, or impacts, to the surface. This is effective in that it not only compacts the top layers, but the deeper layers as well. With vibration, the particles are set in motion and moved closer together to form a high density.

Different types of compaction are best suited for different soil types and conditions. This is because of the underlying density and moisture that different soil types are able to retain. Soil types are classified in three soil groups, with consideration to grain sizes. These types are:

  • Cohesive
  • Granular 
  • Cohesive and granular

[edit] Cohesive

In cohesive soils, such as clay, the particles contain characteristics that make them easily stick together so compaction can be achieved by high impact, which forces the air out of the particles, pushing them together.

[edit] Granular

Granular soils include sand, gravel, and other particles that typically range in size from 0.003 to 0.08 inches (0.008 to 0.2 cm). Because granular soils have good water-draining properties, they are able to obtain high density when fully dry or saturated. Granular is best compacted by shaking or vibrating the particles. Any type of vibratory equipment is best suited for this type. Depending on the type of granular soil, different degrees of vibration are required.

[edit] Granular and Cohesive

Often, soils are a mixture of both granular and cohesive, requiring more precise compaction equipment. Equipment should be chosen on the basis of the soil in the mix that is present in the highest percentage. Some materials, such as asphalt, require both vibration and static pressure to be compacted effectively.

Machinery uses frequency and amplitude to apply a force for compaction. Frequency is the measure of the speed of the eccentric shaft rotation, or of the jumping of the machine, quantifiable by vibrations per minute (vpm). Amplitude measures the maximum movement of a vibrating body from its axis in one direction.

[edit] Process

The process of compaction differs for each type of equipment. Vibratory and rammer equipment achieve compaction by compacting the soil in the same direction, from top to bottom and vice versa. The impact is felt at the top and results in a hardening of the surface. When impact is applied to the surface, it travels deeper down and then compacts the upper part or the surface of the soil. Compaction can also be achieved by running the machine over the surface with several passes. There is such a thing as over-compaction, and when this happens, the surface is likely to be riddled with cracks and seepages.

Some equipment, such as rammers, compacts soil by delivering high-impact blows, or high amplitude, to cohesive soils. The equipment applying the impacts at a rate of 500 to 700 vibrations per minute is powered by gas or diesel engines; these blows are powerful enough to securely compact the material. The rammer is operated at a forward angle and travels in the desired direction while compacting by impact, vibration, and kneading. Rammers are best suited for cohesive clay. While also a potential machine for sand and clay mixture, it is recommended that that testing be carried out to ensure the effectiveness of this machine on those materials.

Vibratory plates are also used for compaction. Using high frequency and low amplitude and driven by gas or diesel engines, the machine moves forward while vibrating the soil. The compaction of soil depends on the heaviness of the plate. The heavier the plate, the more compact the soil will be. The frequency of these machines ranges between 2,500 and 6,000 vibrations per minute. Due to the nature of asphalt, vibratory plates also require a water tank and a sprinkler system. Vibratory plates work best on granular soil and asphalt. While not recommended for use on clay, they may work well on a sand/clay mixture, if tested.

The reversible vibratory plate machine is similar to the standard machine but it also features reversible weights and two eccentric weights. This allows it to travel forward and backward, using the additional rounds to increase the level of compaction. The reversible plate is best applied to sand/clay or clay applications.

Mustang MVP88 Walk-Behind Plate Compactor
Rollers provide compaction by means of vibratory and static force. There are many different types of rollers, including walk-behind and ride-on. Rollers can comprise drums that are smooth or padded, or they can be rubber-tired.

Walk-on rollers can use a set of smooth dual steel drums that are mounted on a rigid frame and powered either by gas or diesel engines. They provide a frequency of about 4,000 vibrations per minute and amplitude of 0.018 to 0.020. Padded walk-on rollers, for example trench rollers, are most effective for compacting trenches and excavation applications. This kind of operation relies on drum pads to knead the soil into compaction through the use of hydraulic or hydrostatic steering.

Ride-on rollers, on the other hand, rely on the static motion of wheels to compact asphalt surfaces. The ride-on can also come with single-drum features, driven by pneumatic-drive wheels and compacting with a vibrating drum. Alternatively, it can include a padfoot trench roller, a highly effective compacting machine that operates pneumatically. The ride-on rubber-tire roller has 7 to 11 pneumatic tires in the front and rear and compacts by adding or removing weight and applying downward pressure on the surface. Weights to be added can range from 10 to 35 tons.

Vibratory rollers are best used on asphalt and sand/clay mixtures and are not recommended for use on granular soils. The Rammax roller works best on sand/clay or clay-only mixtures. [4]

[edit] Compaction Failure

Factors that result in poor compaction can result leaks and cracks in foundations, pipes, slabs, abutments, and trenches.

[edit] Environment

Soil compaction can be bad for the environment in the sense that it prevents plant life from growing. This can have an adverse effect on the ability of wildlife to survive in regions where compaction is rife.[5]

[edit] References

  1. Soil Compaction. Concrete-Catalog. 2008-10-23.
  2. Warren, Louis S. American Environmental History.
  3. Soil Compaction. Concrete-Catalog. 2008-10-14.
  4. Soil Compaction. Concrete-Catalog. 2008-10-14.
  5. Not Conducive to Root Growth. Keengrower. 2008-10-23.