Compaction of Soil - Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

Introduction

Compaction is the application of mechanical energy to soil so as to rearrange its particles and reduce the void ratio. It is employed to improve the engineering properties of existing soils or during placement of fills for embankments, road bases, runways, earth dams and reinforced earth walls. Compaction is also used to prepare a firm and level foundation for buildings. Compaction normally rearranges particles and pore spaces; it does not change the water content significantly nor the size of individual soil grains.

The primary objectives of compaction are:

• To increase soil shear strength and therefore its bearing capacity.
• To reduce subsequent settlement under working loads.
• To reduce soil permeability, making it more difficult for water to flow through the soil mass.

Laboratory Compaction

Laboratory compaction tests determine the variation of dry density with water content for a given compactive effort. These results are used to select field compaction procedures and to specify compaction control values for construction.

Standard laboratory procedures commonly used are:

• Indian Standard Light Compaction Test - equivalent in principle to the Standard Proctor test. Soil is compacted in a 1000 cm³ mould in three equal layers. Each layer receives 25 blows of a 2.6 kg rammer falling from a height of 310 mm. The test is repeated at several moisture contents to obtain a dry density versus moisture content curve.
• Indian Standard Heavy Compaction Test - equivalent in principle to the Modified Proctor test. To represent heavier field compactive effort, soil is compacted in five equal layers in the same 1000 cm³ mould. Each layer receives 25 blows of a 4.9 kg rammer falling from a height of 450 mm. This produces a larger compactive energy per unit volume than the light test.

The compactive energy applied per unit volume in a rammer-type laboratory test may be written as

E = (W × H × N) / V

where W = weight of rammer, H = fall height, N = total number of blows applied and V = mould volume. This expression shows why different combinations of rammer weight, drop height and number of layers produce different compactive efforts.

=4J

Dry Density - Water Content Relationship

To measure the degree of compaction, the dry unit weight (dry density) is used because it indicates how closely the solid particles are packed in a given volume. Laboratory testing establishes the maximum dry density attainable for a specified compactive effort and the corresponding water content at which this maximum occurs.

In practice, the bulk (wet) density (γ) and the moisture content (w) are measured. The dry density (γ_d) is then calculated as

where γ is the bulk (wet) unit weight and w is the water content (expressed as a decimal). The test is carried out at a range of moisture contents and a plot of dry density versus water content is drawn.

• The typical plot for cohesive or fine-grained soils has an inverted-V shape with a clear peak. This peak identifies the maximum dry density (MDD) and the corresponding optimum moisture content (OMC).
• Dry density can be related to water content and degree of saturation (S) by standard soil mass-volume relations.

An increase in dry density represents a decrease in voids ratio and a more compact soil. Dry density may also be related to the percentage of air voids (n_a) as shown below.

The line obtained by assuming full saturation (S = 100 %) is called the zero air-voids line. It represents the theoretical maximum dry density for a given water content if all air were expelled. In practice, complete elimination of air is not possible, so the zero air-voids line represents an upper bound and is not achievable by compaction.

Effect of Increasing Water Content

• At low moisture contents, added water acts as a lubricant between soil particles. The lubrication reduces internal friction and allows particles to rearrange more easily under compactive effort. This leads to a reduction in voids and an increase in dry density.
• As moisture content increases further, water occupies space that could otherwise be occupied by the soil solids. Beyond a certain point, additional water hinders closer packing and the dry density begins to decrease.
• The dry density reaches a maximum at the optimum moisture content (OMC). Values of MDD and OMC are obtained from the dry density versus moisture content curve.

Effect of Increasing Compactive Effort

• Different compactive efforts produce different compaction curves. Increasing the compactive effort generally increases the MDD and tends to reduce the OMC.
• An increase in compactive effort produces a large increase in dry density when compacting at moisture contents drier than OMC. For moisture contents wetter than OMC, increasing compactive effort has only a small effect on dry density.
• Because the compaction curve depends on the compactive effort, any specification of MDD and OMC must state the compaction procedure used (for example, light or heavy/modified).

Effect of Method of Compaction

The final dry density achieved in the field depends on the compaction technique. Important factors include compaction energy, moisture content at compaction, compaction method (static, impact, kneading, vibration), number of passes, type and size of compactor, soil gradation and particle shape, and environmental conditions. Choice of method should match soil type and the required field density.

Effect of Type of Soil

Coarse-grained soils (sands and gravels) more readily attain higher dry densities at lower water contents. Fine-grained soils (silts and clays) typically achieve lower maximum dry densities and require higher moisture contents to reach MDD because of the presence of fines and adsorbed water films.

Factors Affecting Compaction

In the laboratory, the main factors influencing the degree of compaction are:

• Plasticity of the soil - clays with higher plasticity tend to have lower achievable dry densities for a given compactive effort and require more water.
• Water content - moisture acts as a lubricant up to OMC and reduces efficiency beyond OMC.
• Compactive effort - greater energy generally increases achievable dry density and changes OMC.

Compaction of Cohesionless Soils

For cohesionless soils (clean sands and gravels) the standard Proctor-type compaction test is less useful because these soils are compacted effectively by vibration rather than by tamping. Vibration allows particles to rearrange into a dense packing. Watering during compaction may also help achieve higher densities for some granular materials.

For cohesionless soils, field specifications commonly use relative density (I_D) as a requirement. Relative density is defined by reference to the maximum and minimum void ratios or corresponding dry densities determined in laboratory tests.

Values of maximum and minimum dry densities (or void ratios) depend on grain gradation and angularity, so relative density does not directly give an absolute dry density without knowing these bounds.

On the basis of relative density, sands and gravels are commonly classified into categories of loose, medium and dense condition as shown in laboratory tables and charts.

Engineering Behaviour of Compacted Soils

Soils compacted at the same dry density but on different sides of OMC (dry side or wet side) show different structures and engineering behaviour. The water content at compaction is therefore important, not only the dry density achieved.

1. Soil Structure

   For a given compactive effort, soils compacted on the dry side of OMC tend to have a flocculated or open structure where particles interlock in a more random arrangement. Soils compacted on the wet side tend to have a dispersed, more parallel particle orientation, caused by thicker adsorbed water films around particles.

2. Swelling

   Soils compacted dry of OMC have drier particles and partially developed water films; when they gain access to water they tend to absorb more and swell more than soils compacted wet of OMC.

3. Shrinkage

   Soils compacted wet of OMC usually exhibit greater shrinkage on drying because their particle arrangement allows greater change in volume as water is removed.

4. Construction Pore Water Pressure

   Compaction proceeds layer by layer in fills, generating pore water pressures in underlying layers. Soils compacted wet of OMC develop higher positive pore pressures, whereas soils compacted dry of OMC may develop negative pore pressures initially, affecting stability and consolidation behaviour.

5. Permeability

   Dry-side compacted soils with a random particle orientation tend to have more isotropic permeability. Wet-side compacted soils with aligned particle fabrics may be more permeable along particle orientation and less across it.

6. Compressibility

   At low applied stresses, soils compacted dry of OMC are generally less compressible because of the truss-like flocculated arrangement. Wet-side compacted soils are more compressible at low stresses. Under high stresses where particle reorientation occurs, both initially dry-side and wet-side soils tend to converge to similar structures, compressibility and strength.

Field Compaction Methods and Quality Control

In the field, compaction is achieved using a variety of equipment chosen according to soil type and the required compaction. Common types of rollers and compactors include:

• Sheepsfoot (padfoot) rollers - effective for cohesive soils because impact and kneading action compresses and helps remove voids.
• Smooth-wheeled rollers - used for granular soils and for finishing layers; provide static and some dynamic compression.
• Pneumatic-tyred rollers - provide kneading action, effective on mixtures of fines and sands.
• Vibratory rollers - effective on cohesionless soils; vibration aids particle rearrangement into dense packing.
• Tamping rammers and plate compactors - used for confined areas and for thin layers.

Field quality control ensures that the specified compaction (often expressed as a percentage of laboratory MDD) is achieved. Common field tests include:

• Sand replacement (sand patch) test - a reliable method to determine in-situ dry density where a hole is excavated, the mass of the removed soil is measured and replaced with calibrated sand.
• Core cutter method - useful for cohesive soils in compacted layers to obtain bulk density directly.
• Nuclear density gauge - provides rapid estimates of in-situ wet and dry density and moisture content; requires calibration and adherence to safety rules.
• Plate load and CBR tests - used for assessing stiffness and bearing performance of compacted layers in pavement and foundation design.

Typical field specification requires compaction to a percentage of the laboratory MDD (for example, 95% of MDD by Standard Proctor) and control testing is performed at regular intervals and at locations specified by the engineer.

Some extra details about compaction

1. Coarse-grained well-graded soils normally attain higher maximum dry densities (γ_d).
2. Clays with higher plasticity typically show a decrease in maximum dry density for a given compactive effort.
3. Pure clean sand may show a V-shaped compaction curve because of bulking and particle rearrangement effects.

Practical implications and summary

Compaction is a crucial construction operation: selecting the correct moisture content, compactive effort and compaction method for the soil type ensures a compacted layer with suitable strength, stiffness and permeability. Laboratory compaction tests provide MDD and OMC for specified compactive efforts; field compaction must be controlled against these laboratory values using appropriate field testing. Understanding differences between dry-side and wet-side compaction helps predict swelling, shrinkage, permeability and compressibility behaviour of compacted fills.