Compaction of soil, test for compaction & Factors affecting compaction

Hello my friends, welcome to iconicccivil. I being a civil engineer pass my knowledge to you and the society. Have a look on the process of Compaction in depth through this article. 

   :COMPACTION:

Compaction of soil is the process of increasing the unit weight of soil due to decrease in volume of air, volume of void hence volume of soil by forcing soil solids into denser state.

• It is done by mechanical means in order to decrease void ratio, permeability etc. and to increase denseness, unit weight, bearing capacity of soil etc.
• Compaction can be achieved by use of ROLLERS, RAMMERS and VIBERATORS etc. 
• It is instantaneous process in which soil is always unsaturated( partially saturated) & dry.
• Compaction of soil is represented in terms of dry Unit Weight of soil(Yd).
• More the dry Unit Weight, denser will be the soil.

:COMPACTION CHARACTERISTICS OF SOIL CAN BE ANALYSED IN LABORATORY BY PERFORMING THE FOLLOWING TEST:

1. STANDARD PROCTOR TEST:

This test consists of standard cylindrical mould of volume(1/30)cubic feet (~945cc) in which soil to be tested is filled in three layers and each layer is compacted by subjecting it to 25 numbers of blow with the help of hammer(12 inches) (~304.8mm).

• Compaction energy provided in this proctor test is approximately ~593 KJ/m3.
•  Aim of compaction is to find try Unit Weight of soil for given compactive effort. 
• After compaction soil is tested for its water content by oven drying method to find dry unit weight of soil. This indicates compactness achieved at this particular water content.
• This test is repeated at different water content and different dry Unit Weight of soil are found for the same soil and Same compactive effort.
• Result of this test is represented in the form of curve known as compaction curve in which water content is expressed on x-axis and dry Unit Weight of soil on y- axis.
• This compaction is used to find water content at which maximum dry unit weight is obtained and is termed as OPTIMUM moisture content.

NOTE: For given water content of soil maximum dry Unit Weight is obtained when compaction is such that all air is removed from the soil(zero air).

• Maximum dry unit weight for a soil at a given water content is when Degree of saturation =100% or air content =0% and air void =0.
• This is called THEORETICAL MAXIMUM DRY unit weight of soil. 
• Theoretical maximum dry unit wt. Decreases as water content increases. 

AIR VOID LINE: A line (curve) showing relationship between water content and dry Unit Weight of compacted soil at constant percentage of air void is termed as air void line.

SATURATION LINE: 

A line showing relationship between water content and dry Unit Weight of compacted soil at constant percentage of degree of saturation is termed as saturation line.

NOTE: 

1. 0% air void line is same as 100%saturation line but 90% saturation line does not mean 10% air void line. 

• 90% saturation line means 10% air content line(not 10% air void line). 

2. In Gravels, Silt and Clay maximum dry unit weight is practically achieved at 85% to 95% degree of saturation as it is not possible to remove the entire from soil sample.

3. In case of sand, behaviour of which is different from gravels, silt and clay in terms of compaction. Maximum dry unit weight can be achieved at 100% saturation.

LINE OF OPTIMUS: 

A line joining or showing relationship between optimum moisture content and practically obtained maximum dry unit for a particular soil is termed as LINE OF OPTIMUS. 

• With increase in compactive effort (compaction energy) on given soil, maximum dry unit weight increases and corresponding OMC decreases.
• It corresponds to 85% to 95% saturation line and roughly parallel to ZERO AIR void line. 

NOTE: PROCTOR needle test is also used to find water content in field. 

:FACTORS AFFECTING COMPACTION :

A. Water Content : 

• At lower water content, soil is relatively stiff and Hence Offers higher resistance against compaction and does not pack into denser state of contact resulting in a low Dry Unit Weight. 
• With increase in water content more and more layers of water is formed surrounding the soil solids which increases the lubrication effect under which soil solids can be worked therefore soil solids become workable and came into denser state of packing resulting in higher dry Unit Weight.
• This increase in density of soil takes place upto particular water content termed as OMC at which lubrication effect is maximum, beyond which if water content increases, there is no change in lubrication effect and water starts replacing the soil solids resulting in decreased denseness, Hence dry Unit Weight decreases.

B. Degree of Compaction :

• Degree of compaction means energy provided in compaction (light or heavy heavy compaction).
• With increase in degree of compaction Maximum dry Unit Weight increases and corresponding a OMC decreases but there is no direct relation in between this increase and decrease.

C. Method of Compaction :

• Method of compaction also affects compaction of soil in terms of weight of compacting element, time and area of contact between soil and compactive element and nature of effect induced by compactive element.
• Nature induced:(Impact, pressure, Vibration and kneading). 
• Compaction of soil can be done by Rollers, Rammers and Viberators. 

i). ROLLERS:

Types of Rollers:

a). Smooth Wheeled Rollers:

• Compaction is achieved by application of pressure over soil. 
• Suitable for coarse grained soil, like (crushed stones and gravels).
• Used in compaction of roads. 
• Tendom roller is a type of smooth wheeled roller. 

b). Sheep Foot Roller:

• Compaction by kneading action. 
• Used for cohesive soil like clay.
• Used in construction of Earthen Dams. 

c). Pneumatic Type Roller:

• Compaction by combined action of pressure and kneading. 
• Suitable for all types of soil but generally suitable for cohesive soils. 
• Used in construction of roadways, airfields & Homogeneous Dam.

ii). RAMMERS :

• Compaction by impact.
• Used in confined areas. 
• Suitable for all types of soils but generally used for cohesionless soils. 
• Used in basements, Retaining walls, Trench fills etc.

iii). VIBERATORS :

• Compaction by vibration, used in confined areas. 
• Used for cohesionless soil(used for compaction of sand). 

D. TYPES OF SOIL:

• Well graded coarse grained soils with little fineness ( up to 12% fineness) is found to have maximum dry unit weight. 
• With increase in fineness in this soils their maximum dry Unit Weight decreases and corresponding OMC increases. 
• Poorly graded coarse grained soil is found to have minimum dry Unit Weight.
• Fine grained soil(silt/clay) generally have lower maximum dry Unit Weight and higher OMC( because in fine grained soil there is huge number of a small voids).
• With increase in plasticity of soil Dry Unit Weight decreases and OMC increases.
• With increase in organic content dry Unit Weight decreases & OMC increases.

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