Soil compaction is one of the most critical components in the construction of highway embankments, earth dams, foundations and many other engineering structure. Soil must be compacted to their unit weights, increase the strength characteristic which increase the bearing capacity of foundations constructed over them. Soil is compacted by removing air and water from its pore space.
Compaction is a change in soil structure, not just an increase in soil density. Healthy soils have a diversity of pore sizes, while compacted soils have mostly small pores. In general, Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. Almost all types of building sites and construction projects utilize mechanical compaction techniques.
During the compaction process soil partials are pushed closer together. This reduces the size of pores, the continuity of pores, and the size and stability of aggregates. Only under severe compaction will aggregates break down.This soil density chart shows properly compacted soil.
Purpose of soil Compaction:
The principal reason for compacting soil is to reduce subsequent settlement under working loads. Also in a construction material, the significant engineering properties of soil are its shear strength, its compressibility, and its permeability. Compaction of the soil generally increases its shear strength, decreases its compressibility, and decreases its permeability. in addition, compaction reduces the voids ratio making it more difficult for water to flow through soil. This is important if the soil is being used to retain water such as would be required for an earth dams. So we conclude that there are four reasons to compact the soil:
Increase load-bearing capacity.
Prevent soil settlement.
Provides stability .
Reduce water seepage, swilling and construction.
Poor compaction can lead to unwanted results.
Figure: result of poor compaction
Types of compaction:
There are four types of compaction that are commonly used on soil and/or asphalt:
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
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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
Compaction soil types:
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 and granular
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.
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.
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
It is important to know and control the soil density during compaction. Following are common field tests to determine on the spot if compaction densities are being reached.
Sand Cone Test (ASTM D1556-90):
A small hole (6″ x 6″ deep) is dug in the compacted material to be tested. The soil is removed and weighed, then dried and weighed again to determine its moisture content. A soil’s moisture is figured as a percentage. The specific volume of the hole is determined by filling it with calibrated dry sand from a jar and cone device. The dry weight of the soil removed is divided by the volume of sand needed to fill the hole. This gives us the density of the compacted soil in lbs per cubic foot. This density is compared to the maximum Proctor density obtained earlier, which gives us the relative density of the soil that was just compacted.
Nuclear Density (ASTM D2292-91):
Nuclear Density meters are a quick and fairly accurate way of determining density and moisture content. The meter uses a radioactive isotope source (Cesium 137) at the soil surface (backscatter) or from a probe placed into the soil (direct transmission). The isotope source gives off photons (usually Gamma rays) which radiate back to the mater’s detectors on the bottom of the unit. Dense soil absorbs more radiation than loose soil and the readings reflect overall density. Water content (ASTM D3017) can also be read, all within a few minutes. A relative Proctor density with the compaction results from the test.
How the Nuclear Density test works
How the Sand Cone test works
FACTORS AFFECTING COMPACTION IN THE FIELD:
Compaction of a particular soil is affected by following given factors:
Proper control of moisture content in soil is necessary for achieving desired density. Maximum density with minimum compacting effort can be achieved by compaction of soil near its OMC (Optimum Moisture Content). If natural moisture content of the soil is less than OMC, calculated amount of water should be added to soil with sprinkler attached to water tanker and mixed with soil by motor grader for uniform moisture content. When soil is too wet, it is required to be dried by aeration to reach up to OMC. Moisture content of the soil is vital to proper compaction
Type of soil has a great influence on its compaction characteristics. Normally, heavy clays, clays and silt offer higher resistance to compaction where as sandy soils and coarse grained or gravelly soils are amenable for easy compaction. The coarse-grained soils yield higher densities in comparison to clays. A well-graded soil can be compacted to higher density.
The more the thickness of layer of earth subjected to field compaction, the less the energy input per unit weight of soil and hence, less is the compaction under each pass of the roller. Suitable thickness of soil of each layer is necessary to achieve uniform thickness. Layer thickness depends upon type of soil involved and type of roller, its weight and contact pressure of its drums. Normally, 200-300 mm layer thickness is optimum in the field for achieving homogeneous compaction.
Contact pressure depends on the weight of the roller wheel and the contact area. In case of pneumatic roller, the tyre inflation pressure also determines the contact pressure in addition to wheel load. A higher contact pressure increases the dry density and lowers the optimum moisture content
SPEED OF ROLLING
Speed of rolling has a very important bearing on the roller output. The greater the speed of rolling, the more the length of embankment that can be compacted in one day. Speed was found to be a significant factor for vibratory rollers because its number of vibrations per minute is not related to its forward speed. Therefore, the slower the speed of travel, the more vibrations at a given point and lesser number of pass required to attain a given density.
TYPE OF COMPACTING EQUIPMENTS
A large variety of mechanical equipments is available for compaction of soil but soil type and moisture condition will often dictate the type of equipments and method of use. Some important compacting equipment are given below: –
1. Light compacting equipments (Rammers/Plate compactors)
2. Smooth wheel rollers
3. Sheepsfoot rollers
4. Pneumatic tyred rollers
5. Vibratory rollers
6. Grid rollers
2003 Dynapac CC122 Tandem Vibratory Smooth Drum Roller .
Figure: Smooth wheel-Roller
figure: Light compacting equipments (Rammers)
Soil compaction is an important part of the construction process. It is used for support of structural entities such as building foundations, roadways, walkways, and earth retaining structures to name a few. In general, the preselected soil should have adequate strength, be relatively incompressible so that future settlement is not significant, be stable against volume change as water content or other factors vary, be durable and safe against deterioration, and possess prop
Das, Braja M. (2002). Principles of Geotechnical Engineering.fourth edition. P100
IS: 2720-1983 (Part-14)- Determination of density index (Relative Density) of cohesion
New & Used Heavy Equipment http://www.ritchiewiki.com/wiki/index.php/soil_compaction#ixzz2CglEjAcM
Engineering Properties of Soils Based on Laboratory Testing
Prof. Krishna Reddy, UIC
Das, Braja M. (2002). Principles of Geotechnical Engineering.fourth edition. P100
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