Field Compaction Control

Parameters for Compaction Specification in the Field:
The parameters used to ensure effective compaction in the field include (a) relative compaction and
(b) placement water content.
(a) Relative Compaction:
To Determining the relative compaction using the below equation to specify the limits of dry density
to be achieved for the compacted fill with respect to the MDD obtained from laboratory IS light or
heavy compaction as the case may be –
The Engineer-in-charge will specify the relative compaction to be achieved. For cohesive soils, a
relative compaction of 95% of laboratory standard Proctor test (IS light compaction test) can be
achieved using either sheep’s foot rollers or, in some cases, pneumatic tired rollers. For low plastic
cohesive soils, a relative compaction of 95% of modified Proctor test (IS heavy compaction test) can
be achieved using pneumatic tired rollers.
For cohesionless soils, a relative compaction of the order of 98%-100% or even more of modified
Proctor feat (IS heavy compaction test) can be achieved using vibratory rollers or pneumatic tired
rollers.
(b) Placement Water Content:
The water content to be used for compacting the soil in the field is known as placement water content.
Usually, a placement water content of ±2% of OMC obtained from relevant laboratory compaction
test is adopted.
Water content different from OMC may be occasionally specified by the Engineer-in-charge to achieve
an intended purpose. Placement water content less than OMC (dry of optimum) may be specified for
highway embankments of cohesive soils to achieve higher shear strength and lower compressibility.
Similarly, outer shells of earth dams are compacted at a placement water content dry of optimum (less
then OMC) to achieve higher shear strength, higher permeability, and lower pore pressures.
Compaction of subgrades below pavements and foundation soils below floors may be done at a
placement water content more than the OMC (wet of optimum) to prevent excessive swelling and to
achieve lower swelling pressure. Similarly, the core of an earth dam is compacted at a placement water
content more than the OMC (wet of optimum) to reduce the permeability and pre-empt swelling.
Once a placement water content is specified, which may be equal to the OMC, or dry or wet of
optimum as discussed above, the water content used for compaction should be within ±2% of the
placement water content or it may be within any other limits as specified by the Engineer-in-charge.
Objective of Field Compaction Control:
The following are the important objectives of field compaction control:
1. To determine the in-situ dry density and water content immediately after the compaction of each
lift and to ensure that it satisfies the limits of relative compaction and placement water content as per
compaction specifi-cations of the Engineer-in-charge.
2. To check and ensure that the soils from the prescribed borrow area, having the desired properties,
are used for compaction.
3. 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. It is also necessary to ensure that the required minimum
number of passes of the roller is used for compaction of each lift. The lift thickness, which is the
thickness of each compacted layer, is also to be controlled depending on the type of roller used as
excessive lift thickness that leads to ineffective compaction.
4. Certain minimum number of tests are to be done in the field when the compaction is in progress:
For large fills, one test for every 1000 m2 area/lift

For small fills (< 1000 m2 area), two or three tests/lift
The in-situ density can be computed by any one of the following methods:
➢ Core cutter method (Fine grained Soil)
➢ Sand replacement method (Almost all types of soil)
➢ Rubber balloon method (Well compacted Soil)
The in-situ moisture content can be computed by any one of the following methods:
➢ Proctor Needle Method
➢ Nuclear Method
Improper compaction of soil in the field will lead to the dry density of compacted soil less than the
required maximum achievable dry density, resulting in the following problems:
• Low shear strength of compacted soil leading to shear failure of the soil and foundation
supported on the compacted soil.
• Excess total and differential settlements leading to the failure of foundation and structures
supported on the compacted soil.
• Slope failure of slopes of compacted fills, embankments, and earth dams.
• Excessive volume changes in the compacted fill in terms of excessive swelling and shrinkage.
• Increase of permeability of the compacted fill leading to the loss of water, failure of hydraulic
structures due to piping, etc.
Selection of Fill Material and Borrow Area:

The local soil sources are surveyed and samples are collected and tested. The relative suitability of
different soils is determined and suitable borrow area is selected based on the suitability of soil,
availability in required quantity, the cost of compensation for using the borrow area, and the cost of
transportation. Once the basic properties such as grain size analysis and index properties are
determined, the soil can be classified as per IS codal guidelines that also gives the relative suitability
for compaction. Soils with high plasticity index or large shrinkage ratio should be avoided as they tend
to expand or compress or become too sticky or shrink. Organic matter that can decay should generally
be avoided as fill material unless special precautions are taken.

Field Compaction Control

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Parameters for Compaction Specification in the Field:

For large fills, one test for every 1000 m2 area/lift

Selection of Fill Material and Borrow Area:

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