Soil Compaction Basics

• Compaction is a method of mechanically increasing the density of soil.
• Compaction is a significant part of the building process in construction.
• Settlement of the soil could occur if compaction is not performed properly.
• Unnecessary maintenance costs or structure failure can result from improper compaction.
• Compaction can be described as the densification of soil using external compression forces.
• External compression forces can be static or dynamic.
• Compaction is performed to expel air from the soil voids.
• The volume of solids and the water content remain essentially the same.
• Every soil type behaves differently with respect to maximum density and optimum moisture.
• Each soil type has its own unique requirements and controls both in the field and for testing purposes.
• Soil types are commonly classified by grain size, determined by passing the soil through a series of sieves to screen or separate the different grain sizes.
• There are three basic soil groups: cohesive, granular, and organic.
• Organic soil is not suitable for compaction.
• Coarse grains can be seen with granular soils, fine sands, and silts.
• Grains cannot be seen by the naked eye with with cohesive soils, mixes, and clays
• Coarse grains have a gritty feeling when rubbed between fingers.
• Grains are smooth and greasy when rubbed between fingers.
• When shaken in a palm of hand, water and soil will mix with granular soils, fine sands, and silts, when shaking stops, they separate.
• Water and soil will not mix after shaken in a palm of hand with cohesive soils, mixes, and clays.
• Granular soils, fine sands, and silts have very little or no plasticity when moist.
• Cohesive soils, mixes, and clays are plastic and sticky when moist, and can be rolled.
• Granular soils, fine sands, and silts have little or no cohesive strength when dry.
• A soil sample of granular soils, fine sands, and silts will crumble easily when dry.
• Cohesive soils, mixes, and clays have high strength when dry, crumble with difficulty, and have low saturation in water.
• Fill materials such as gravel has very high permeability, excellent foundation support, excellent pavement subgrade, is not expensive and is very easy to compact.
• Fill materials such as sand has medium permeability, good foundation support, a good pavement subgrade, is not expensive, and is easy to compact.
• Fill materials such as silt has medium low permeability, poor foundation support and pavement subgrade, some expansion, and some difficulty to compact.
• Fill materials such as clay has none+ permeability, moderate foundation support, poor pavement subgrade, difficult expansion, and is very difficult to compact.
• Fill materials such as organic materials has low permeability, very poor foundation and pavement subgrade, some expansion, and is very difficult to compact.

General Principles of Compaction

• Compaction is done to increase in the soil shear strength.
• Compaction is done to increase in unit weight or dry density of the soil.
• Compaction is done to reduce soil permeability.
• Compaction is done to reduce soil settlement, especially if the soil is subjected to dynamic load such as traffic load.
• Compaction is done to control further change in soil moisture content.

Optimization of the Soil Densification

• Optimize soil placement conditions such as water content, density, and depth of layer.
• Appropriate equipment use, is selected and followed by the method of operations such as the number of passes of rollers, patterns of tamping (in dynamic compaction).
• Implement an adequate quality control procedure, type and number of tests, statistical evaluation, etc.

Laboratory Compaction Test

• Laboratory tests are carried out to simulate the field compaction procedure.
• Laboratory compaction tests are done to determine the maximum dry density and optimum moisture content for various types of soils.

Proctor Test

• Proctor or Modified Proctor Test, determines the maximum density of a soil needed for a specific job site.
• First, the test determines the maximum density achievable for the materials and uses this figure as a reference.
• Secondly, it tests the effects of moisture on soil density.
• The soil reference value is expressed as a percentage of density.
• Standard Proctor Test, a small soil sample is taken from the jobsite.
  • A standard weight is dropped several times on the soil.
  • The material is weighed and then oven-dried for 12 hours in order to evaluate the water content.
• Modified Proctor Test is similar to the Proctor test except a hammer is used to compact material for greater impact.
  • The Modified Proctor Test is normally preferred in testing materials for higher shearing strength.

Field Compaction

• Field compactions are usually carried out using equipment or machinery utilizing static pressure.
• The machinery is either self-propelled or needs to be driven.
• Compaction equipment includes smooth wheel rollers, pneumatic tire rollers, sheep foot rollers, grid rollers, and vibrating rollers.
• Smooth wheel rollers comprise a steel drum whose mass could be increased by adding water or sand ballast.
  • The roller is suitable for most types of soils, especially where crushing action is needed, except for silty and uniform sand.
• Pneumatic Tired Rollers are is suitable for most types of fine and coarse-grained soils but not for uniformly graded soils.
  • Increasing the tire inflation pressure could increase the compactive effort.
  • The maximum depth of soil that could be compacted is about 250-300 mm.
• Sheep Foot Rollers comprises a hollow steel drum with numerous feet in the form of clubs protruding from its surface.
  • The feet will exert relatively high pressure over a small surface.
  • Generally, the wetter and softer the soil, the larger is the contact area required. for its optimum compaction.
  • The sheep foot roller is more suitable for fine-grained soils, especially those with water content on the dry side of the optimum.
  • It is also suitable for coarse-grained soils with over 20% fines.
  • Moisture control is made easier because of the pockmarked surface during compaction.
  • Steel rollers may be used to level off areas worked by sheep foot or rubber-tired rollers.
• Grid Rollers comprises a network of steel bars in the form of heavy square grids.
  • These creates high contact pressures while preventing excessive shear deformation, which is responsible for plastic wave ahead of the roll.
  • Kentledge can be added to the roller to increase its weight.
  • These rollers are only suitable for most fine-grained soils, as they break or rearrange gravel or cobble-sized particles.
• Vibrating Rollers is actually a smooth wheel roller fitted with a vibrator.
  • The roller is suitable for most types of soils, particularly effective for those soils whose moisture contents are slightly on the wet side of optimum.
  • The roller is most suitable for coarse-grained soils with little or no fine.

Field Specification and Control

• The method used for specification and control of field compaction will depend on the condition of the site.
• In cases where the work is confined to a small area, such as a building or a commercial complex, most likely the soil is quite uniform.
  • The compaction operation can be easily controlled.
  • Specifications are then made in terms of operational methods, such as the maximum thickness of each soil layer to be compacted, the weight and type of compaction equipment to be used and the number of passes for each layer.
• In larger work, for example dam construction, the operational method is usually determined after a field trial has been made on the actual soil and using the same equipment.
• Field Tests include the Sand Cone, Balloon Densometer, Shelby Tube, and Nuclear Guage.
• Sand Cone advantages: Large sample and accurate.
  • Disadvantages: Many steps, large area required, slow, requires halt equipment and is tempting to accept flukes.
• Balloon Densometer advantages: Large sample and direct reading obtained and open graded material.
  • Disadvantages: Slow, balloon breakage and awkward to use. Cost is moderate.
• Shelby Tube advantages: Fast, deep sample, under takes sample of pipe haunches.
  • Disadvantages: Small sample, no gravel,sample is not always retained.
• Nuclear Gauge advantages: Fast, and easy to redo. More tests for statistical reliability
  • Disadvantages: No sample, radiation, moisture suspect and encourages amateurs. high cost.