Compaction
Compaction
Q: Differentiate between compaction and consolidation
Q: Explain dry density and water content relationship for soil
Q: Explain compaction process and compaction curve.
Mechanical process carried out to reduce the volume of sub-soil through expulsion of air voids below the structure in order to increase the dry density of soil.
The increase in dry density ensures the increase in resistance (bearing capacity) of soil.
Compaction is carried out before construction of structure.
dry density and water content relationship-compaction curve
OMC- optimum moisture content at which dry density is maximum for soil,
MDD – maximum dry density
Explanation of graph:
Optimum Moisture Content (OMC) is the moisture content at which the maximum possible dry density is achieved for a particular compaction energy or compaction method.
The corresponding dry density is called Maximum Dry Density (MDD).
Water is added to lubricate the contact surfaces of soil particles and improve the compressibility of the soil matrix.
It should be noted that increase in water content increases the dry density inmost soils up to one stage (Dry side).
Water acts as lubrication.
Beyond this level, any further increase in water (Wet side)will only add more void space, there by reducing the dry density.
Hence OMC indicates the boundary between the dry side and wet side.
Hence the compaction curve as shown in figure indicates the initial upward trend up to OMC and the down ward trend.
Effect of Compaction process: 1) Increases density 2) Increases strength characteristics 3) Increases load-bearing capacity 4) Decreases undesirable settlement 5) Increases stability of slopes and embankments 6) Decreases permeability 7) Reduces water seepage 8) Reduces Swelling & Shrinkage 9) Reduces frost damage 10) Reduces erosion damage 11) Develops high negative pore pressures (suctions) increasing effective stress
Q: Explain Standard Proctor test/ Modified Proctor test.
Q: differentiate between Standard Proctor test & Modified Proctor test.
Procedure:
About 3 kg of dry soil, passing through 4.75 mm sieve.
Add approx.5-8% of water for the first trial. (Less -Corse grained & more -Fine grained soil)
Find dimensions and weight of mould. Note it down separately.
The inner surfaces of mould, base plate and collar are greased.
Soil is placed in mould and compacted in 3 uniform layers for SPT/ 5 layers for MPT , with 25 blows in each layer of standard rammer (Rammer used- 2.6 kg wt. & drop height =31cm for SPT/4.89 kg wt. & drop height =45cm for SPT for MPT)
After final layer remove excess soil/ trim top surface
Weigh the mould and soil. Take some sample of soil for determination of water content.
The procedure is repeated with increasing water content.
The number of trials shall be at least 6 with a few after the decreasing trend of bulk density.
Plot the graph of dry density vs water content.
Q: Explain compaction methods and its suitability
Rammers:
Rammers are used for compacting small areas by providing impact load to the soil. This equipment is light and can be hand or machine operated.
For machine operated rammers, the usual weight varies from 30kg to 10 tonnes.
These hammers with 2- 3 tonnes weights are allowed to free fall from a height of 1m to 2m on the soil for the compaction of rock fragments.
Rammers are suitable for compacting cohesive soils as well as other soils.
This machine used in areas with difficulty in access.
Vibrating Plate Compactors:
Vibrating plate compactors are used for compaction of coarse soils
These equipments are used for small areas.
Vibro Tampers:
Vibro tampers is used for compaction of small areas in confined space.
This machine is suitable for compaction of all types of soil by
Smooth Wheeled Rollers:
Smooth wheeled rollers are of two types: Static smooth wheeled rollers & Vibrating smooth wheeled rollers
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 performance of smooth wheeled rollers depend on load per cm width it transfers to the soil and diameter of the drum.
The smooth wheeled rollers consists of one large steel drum in front and two steel drums on the rear. The gross weight of these rollers is in the range of 8-10 tonnes.
The other type of smooth wheel roller is called Tandem Roller, which weighs between 6-8 tonne.
The performance of these rollers can be increased by increasing the increasing the weight of the drum by ballasting the inside of drums with wet sand or water.
In case of vibrating smooth wheeled rollers, the drums are made to vibrate by employing rotating or reciprocating mass.
These rollers are helpful from several considerations like:- (i) Higher compaction level can be achieved with maximum work (ii) Compaction can be done up to greater depths (iii) Output is many times more than conventional rollers
Although these rollers are expensive but in the long term the cost becomes economical due to their higher outputs and improved performance.
The latest work specifications for excavation recommends the use of vibratory rollers due to their advantage over static smooth wheeled rollers.
Sheepsfoot roller Roller:
Sheepsfoot rollers are used for compacting fine grained soils such as heavy clays and silty clays.
Sheepsfoot rollers are used for compaction of soils in dams, embankments, subgrade layers in pavements and rail road construction projects.
Sheepsfoot rollers are of static and vibratory types.
Vibratory types rollers are used for compaction of all fine grained soils and also soil with sand-gravel mixes.
Generally this roller is used for compaction of subgrade layers in road and rail projects.
The weight of drums can be increased as in the case of smooth wheeled rollers by ballasting with water, wet sand or by mounting steel sections.
The efficiency of sheepsfoot rollers compaction can be achieved when lugs are gradual walkout of the roller lugs with successive coverage. The efficiency is affected by the pressure on the foot and coverage of ground obtained per pass. For required pressure and coverage of ground, the parameters such as gross weight of the roller, the area of each foot, the number of lugs in contact with the ground at any time and total number of feet per drum are considered.
The compaction of soil is mainly due to foots penetrating and exerting pressure on the soil. The pressure is maximum when a foot is vertical.
Pneumatic Tyred Rollers:
Pneumatic tyred rollers are also called as rubber tyred rollers.
These rollers are used for compaction of coarse grained soils with some fines.
These rollers are least suitable for uniform coarse soils and rocks.
Generally pneumatic tyred rollers are used in pavement subgrade works both earthwork and bituminous works.
Pneumatic rollers have wheels on both axles. These wheels are staggered for compaction of soil layers with uniform pressure throughout the width of the roller.
The factor which affects the degree of compaction is tyre inflation pressure and the area of the contact.
Grid Rollers:
Grid rollers are used for compaction of weathered rocks, well graded coarse soils.
These rollers are not suitable for clayey soils, silty clays and uniform soils.
The main use of these rollers are in subgrade and sub-base in road constructions.
As the name suggests, these rollers have a cylindrical heavy steel surface consisting of a network of steel bars forming a grid with squire holes.
The weight of this roller can be increased by ballasting with concrete blocks.
Typical weights vary between 5.5 tonnes net and 15 tonnes ballasted.
Grid rollers provide high contact pressure but little kneading action and are suitable for compacting most coarse grained soils.
Pad Foot / Tamping Rollers:
These rollers are similar to sheep foot rollers with lugs of larger area than sheep foot rollers.
The static pad foot rollers also called tamping rollers have static weights in the range of 15 to 40 tonnes and their static linear drum loads are between 30 and 80 kg/cm.
These rollers are more preferable than sheep foot roller due to their high production capacity, and they are replacing sheep foot rollers.
The degree of compaction achieved is more than sheep foot rollers.
The density of soil achieved after compaction with this roller is more uniform.
These rollers operate at high speeds, and are capable to breaking large lumps.
These rollers also consists of leveling blades to spread the material.
Pad foot or tamping rollers are best suitable for compacting cohesive soils.
Q: Explain compaction Field control of compaction methods and its suitability
Q: Explain field compaction control
Objective of Field Compaction Control:
The following are the important objectives of field compaction control:
To determine the in-situ dry density and water content immediately after the compaction
To check and ensure that the soils from the prescribed borrow area, having the desired properties, are used for compaction.
To check and ensure that the required compaction energy is used in compacting the soil. This consists of ensuring that the required type of roller suitable to the soils being compacted is used as well as that the roller is of required capacity.
Certain minimum number of tests are to be done in the field when the compaction is in progress
Determination of In-Situ Density:
The determination of relative compaction requires the following finding:
In-situ bulk density.
Field moisture content.
The in-situ density can be computed by following methods:
Core cutter method.
Sand replacement method.
Rubber balloon method.
Determination of water content of compacted soil can be done by following methods:
Calcium carbide method
Proctor’s Needle Method:
Nuclear Method
Comments
Post a Comment