Compaction of soil (for civil engineering)

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1. Compaction of soil Content – Compaction and related terms Standard proctor test Factors affecting compaction Methods of compaction

2. compaction Definition – it is an instantaneous process in partially saturated/dry soil by which density in increased by compactive efforts. compaction means to press the soil particles to each other by mechanical method. During compaction air is forced out from voids and hence density is increased. Objective of compaction is to increase the density, bearing capacity, shear strength.

3. Optimum water content (optimum moisture content)- It is the water content at which maximum dry unit weight can be achieved after a given compaction effort. A maximum dry unit weight have no voids in it.

4. Zero air void line or 100% saturation line It is graphic representation of degree of saturation. It is plot above the curve, which is formed between dry density and water content. It’s all point represent 100% saturation at any water content.

5. Factors affecting compaction

6. Amount of compaction/compactive effort The effect of increasing the amount of compactive effort is to increase the maximum dry density and decrease the optimum water content. Compactive effort – 1 2 Max Dry density- 1 2 OMC- 1  

7. Type of soil Type of soil has a great influence on its compaction characteristics. Normally, heavy clays, clays & silts offer higher resistance to compaction where as sandy soils and coarse grained or gravelly soils can be easily compacted. The coarse grained soils yield higher densities in comparison to clays. A well graded soil can be compacted to higher density.

8. Water content The dry density of soil increases with an increase in water content till the optimum water content is reached, with further increase in water content, the air voids do not decreases, but total voids increases, and dry density decreases.

9. Method of compaction The dry density achieved not only depend on compactive effort but also method of compaction. For the same amount of compactive effort, dry density will depend upon whether the method of compaction utilizes kneading action, dynamic action or static action.

10. Admixture The compaction characteristic of soil are improved by adding other materials known as admixture. The most commonly used admixtures, are lime, cement, bitumen. Dry density achieved depends upon type and amount of admixture. lime cement bitumen

11. Standard proctor test It is done to find the optimum water content and max dry density of a soil in laboratory. A standard proctor 10 cm internal diameter, height of 12.7cm and 1000ml capacity. The rammer consists of 2.56 kg mass.

12. Procedure – Take soil sample of 3kg, add water to bring the water content to 4%. Mass of empty mould with base plate(without collar) is M₁. Mix the soil thoroughly and compacted by 25 blows of rammer, with free fall of 310mm. The blow are distributed evenly over the surface. The soil is compacted in three layers. The weight after compacting to 3 layer is taken as M ₂.

13. The weight after compacting to 3 layer is taken as M ₂. M ₂- M₁ V = volume of mould =1000ml density = max dry density = density/(1+w) the test is repeated 3 times and the graph is obtained with max dry density and optimum water content.  

14. Method of compaction

15. Tamper It is hand-operated instrument consist of a block of iron/stone , about 3-5kg attached to wooden rod. It is used to compact in confined area like trenches, behind bridge abutment, etc.

16. Roller These are of three type – Smooth wheel roller – The most suitable soils for these roller type are well graded sand, gravel, crushed rock, asphalt etc. where crushing is required. These are used on soils which does not require great pressure for compaction. These rollers are generally used for finishing the upper surface of the soil. These roller are not used for compaction of uniform sands. The smooth wheeled rollers consists of one large steel drum in front and two steel drums on the rear.

17. Pneumatic-tyred roller- Pneumatic-tyred roller have kneading action and can convert flocculated structure into dispersed structure. These rollers are used for compaction of coarse grained soils with some fines. These rollers are least suitable for uniform coarse soils and rocks.

18. Sheep foot roller It consists of a hollow drum with a large number of small projection (known as feet) on its surface. These projection penetrates in soil layer during rolling operation and causes compaction. Suitable for cohesive soil only.

19. Vibratory compactor In the vibratory compactor, vibrations are induced in the soil during compaction. These are suitable for cohesion less soil, especially confined condition. When it mounted on drum, it is called a vibratory roller. These roller are available both pneumatic tyred and smooth wheel type.

20. Difference between compaction and consolidation Compaction Consolidation 1) It is a instant process 1) It is a slow process 2)It is an artificial process 2)It is a natural process 3)Dynamic loading are applied 3)Static load are applied 4)In compaction, soil changes from partially to fully saturation condition 4)In consolidation, soil remains in fully saturation condition 5) Compaction is due to expulsion and compression of air in soil mass 5) Consolidation is due to expulsion of pore water from voids