Moisture Content
- At low moisture contents, the strength of clay lumps and friction between granular particles is high; so a given compactive effort will not be able to remove all air voids leaving the soil in an overall compressible state when it is subjected to stresses from further layers of fill or a structure, and in a potentially collapsible state.
- At higher moisture contents, clay lumps become weaker and friction between granular particles reduces so the air voids are easily removed during compaction. The moisture content at which the maximum dry unit weight is attained is generally referred to as the optimum moisture content.
- At moisture contents above OMC, the soil particles cannot move any closer together because even though most of the air has been expelled there is more water present in voids.
Soil Type
- Soil type, grain size, shape of the soil grains, amount and type of clay minerals present and the specific gravity of soil solids, have a great influence on the unit dry density and OMC.
- In poorly graded soils, dry unit density initially decreases as the moisture content increases, and then increases to a maximum value with further increase in moisture.
- At lower moisture contents, the capillary tension inhibits the tendency of the soil particles to move around and can be compacted.
Compactive Effort
Amount of Compactive Effort
If a soil is already moist, weaker and above OMC, then applying more energy is wasteful since air can quickly be removed.
Applying large amounts of energy to a very moist soil may be damaging since no more soil can be expelled but high pore water pressures can build up which should cause:
Nature of Effort - Load Duration and Contact Area
Amount of Compactive Effort
- Maximum dry unit weight increases with increase in compactive effort.
- Increase in compactive effort decreases optimum moisture content to some extent.
If a soil is already moist, weaker and above OMC, then applying more energy is wasteful since air can quickly be removed.
Applying large amounts of energy to a very moist soil may be damaging since no more soil can be expelled but high pore water pressures can build up which should cause:
- Slope instability during construction.
- Consolidation settlements as they dissipate after construction.
Nature of Effort - Load Duration and Contact Area
- Longer time duration leads to reduced shear stiffness response and greater compaction.
- Greater contact area leads to greater depth of influence.