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soil densification by external forces

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Mechanical modification means soil densification by external forces. Compaction means densification of an unsaturated soil by reduction in volume of voids filled with air, while the volume of solids and the water content remain essentially the same. It also implies that the soil particles are packed close together by the application of heavy loads. The geotechnical engineer makes a clear distinction between the processes of compaction and those of stabilization and consolidation. Stabilization may refer generally to an increase in strength or a reduction in the deformation of a soil mass, whereas consolidation is a process where the volume of a soil mass is reduced by the expulsion of water. The latter involves stress transfer from the water to the solid phase. This is achieved by long term application of static load.

The major purposes of compacting soil are:

  1. To increase shear strength

  2. To reduce permeability

  3. To reduce compressibility

  4. To reduce liquefaction potential

  5. To control swelling and shrinkage

Improvement of engineering properties by densification is possible for natural soils as well as for soils stabilized with chemicals such as lime and cement. Optimization of the densification process may be carried out by the following steps:

Terminology

Compaction

Compaction means expulsion of air from voids, while the volume of solids and the water content remain the same.

Stabilization

An increase in strength or a reduction in the deformation of the soil mass.

Consolidation

It is the process where the volume of a soil mass is reduced by the expulsion of water.

 

Methods of compaction:

Laboratory compaction tests are conducted to simulate the field compaction procedure. The results are laid in optimization and control of placement condition. Compactions are of three type, viz., static compaction; kneading compaction and dynamic compaction.

Static Compaction:

Soil specimen of required density can be made by compressing a known amount of soil into calibrated cylindrical mould placed over a universal type of testing machine. The compressive force is steadily increased until a desired density is observed. This type of compaction is described as static compaction or odometric compression. It is known to create a soil particle orientation which may differ from that obtained by other methods of compaction. The compactive effort in the field cannot be changed as readily as during laboratory static compaction.

Kneading Compaction:

The soil structure created by this type of compaction closely resembles that obtained with compaction equipment typically used for fine-grained soils in the field such as sheepfoot and tamping rollers. The samples are placed in circular moulds about 127 mm high and 102 mm in diameter and then compacted by kneading 100 times at 2413 kPa. In a Miniature Harvard Compaction Test, a 25.3 mm-diameter specimen is produced by tamping with a calibrated spring-loaded piston, which is small in relation to the mould. Each time the piston is forced down into the soil, it tends to cause shear failure which is characteristic in kneading compaction. It is no longer a recommended standard by American Society for Testing and Materials (ASTM), but it is widely used for research purposes because it allows a large number of specimens to be produced in a short time.

Dynamic Compaction:

The standard Proctor compaction test is described in ASTM. The test is carried out on that portion of the soil which passes the 19 mm sieve. Five or more samples are compacted in three layers, each receiving 25 blows with a 2.7 kg rammer dropping by 300 mm. The diameter of the flat rammer head is approximately half the diameter of the mould. An automatic compaction apparatus may be used provided that the essential dimensions are adhered to. Minor variations in the cylinder size and the number of blows etc. may not cause significant error provided the total energy expended per cubic metre of the compacted soil remains the same. In some cases it may be desirable to use an extra large mould so that the particles larger than 19 mm can be included in the laboratory test. Compaction test results are plotted in terms of dry density versus moisture content.

Shallow Surface Compaction:

Compaction by surface equipment is achieved by static pressure (static rollers) and/or dynamic pressure (vibrating and impact rollers) caused by impact or vibration. There is a long history in the development of surface compaction machinery resulting in a wide variety of equipment, differing in size, shape and the mode of operation.

Static rollers

Rollers of different types are used for compaction of soils. They are discussed below:

Smooth steel rollers and pneumatic tired rollers

Traditional steel rollers are relatively slow compared to newer types of equipment. They exert high static pressure which makes them most suitable for granular soil. In clay, soil is relatively soft; they may have a ploughing effect without causing significant compaction. Rubber-tyred and pneumatic- tyred rollers compact by the static weight of the ballast and the kneading action of the tyres. The compactive effort depends on (i) gross weight, (ii) wheel diameter, (iii) wheel load, (iv) tyre width and size, and (v) inflation pressure.

Sheepfoot rollers

Sheepfoot and tamping, or padfoot, rollers are distinguished by feet protruding from the cylindrical steel shell of the roller. The term tamping or padfoot roller generally refers to the equipment with relatively large foot prints. Generally, the wetter and softer the soil, larger is the contact area required for its optimum compaction. Sheepfoot rollers have proved more suitable for cohesive soils than other rollers. Moisture control is made easier because of the pockmarked surface during compaction. Steel rollers may be used to level off areas worked by sheepfoot or rubber-tyred rollers.

Grid rollers

Grid rollers have drums that are either covered or consist of alieavy steel grid. This creates high contact pressures while preventing excessive shear deformation, which is responsible for plastic wave, ahead of the roll. Grid rollers are suitable for the compaction of weathered rock, such as sand stone, as they break or rearrange gravel and cobble-size particles.

Vibrating rollers

The light weight vibratory rollers are not of much use as their vibrational amplitude is only of the of 1 or 2 mm.

 

Impact rollers

Impact rollers consist of a non-circular mass which is towed along the ground. As its centre rises and falls, its mass exerts a high impact force causing compaction of the soil. Clifford (1980) described a new type of impact roller developed in South Africa. It consists of a 1.5 m thick 'square' roller with rounded edges. It was found suitable for natural ground and the fill. Because the impact roller leaves the surface uneven, it is recommended for sub-grades and earth fills rather than for surface works.


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