Today in this article we have discussed the primary compaction effect on soil properties which are generally occurred due to overall compaction process.
The primary goal of compacting a soil is to enhance certain desirable properties such as compressibility, water absorption, and permeability, as well as increase soil strength, bearing capacity, and change in swelling and shrinkage characteristics.
The influence of compaction effect on soil properties, on the other hand, is highly dependent on the structure achieved by the soil during compaction.
Effects of Compaction on Soil Properties
Following are the compaction effect on soil properties which takes place due to compaction process :
- change in soil structure
- permeability
- shrinkage
- swelling
- pore pressure
- compressibility
- stress-strain characteristics
- shear strength
1. Changes in Soil Structure :
The structure of a soil during compaction is determined by :
- The type of soil.
- The amount of moulding water.
- The type and amount of compaction.
Let us divide the soils into three forms for the sake of discussion.
- Soils with coarse grains and little to no fines.
- Composite soils.
- Purely Cohesive.
At any voids ratio or water content, the soils of the first type (i.e. coarse grained soils) retain a single grained structure. However, the structure of composite soils after compacting is determined by the relative proportions of coarse and fine particles, and their structure may be coarse grained skeleton or cohesive matrix.
Compacted clay, on the other hand, has a complex structure.
With growing water content, the soil structure becomes increasingly focused, i.e. dispersed, at the same compactive effort. The structure of clay is often flocculated when compacted to the best of its ability.
The graph above illustrates two compaction curves for clay, one at a lower compactive effort and the other at a higher compactive effort.
The soil at points B and D of the two curves is more oriented than at point A when the water content is higher than optimum. The structure, on the other hand, is more directed at C than at A.
2. Permeability :
Take note of the following points:
- As the dry density rises due to compaction, the voids shrink, lowering the permeability.
- Fine-grained samples compacted dry of optimum are more permeable than those compacted wet of optimum for the same density. This is because when compacted dry of optimum, these soils have flocculated structure, and when compacted wet of optimum, they have dispersed structure (i.e. parallel orientation).
- The larger the individual pores are for a given voids ratio, the greater the permeability.
- Because of the increased dry density and improved particle orientation as the compactive effort is increased, the permeability of the soil decreases.
3. Shrinkage :
A soil sample compacted dry of optimum shrinks considerably less than a sample compacted wet of optimum for the same density.
Since soil particles with a scattered composition have a nearly parallel orientation and can pack more efficiently, this is the case.
4. Swelling :
A clayey soil sample compacted dry of optimum water content has a high water shortage and a more random orientation, so it imparts more swelling pressure and swells to a high moisture content than a sample of the same density obtained though the wet side compaction.
5. Pore Pressure :
In an undrained shear test, a saturated sample of clay compacted dry of optimum appears to develop significantly lower pore pressure at low strains than a sample of the same soil compacted wet of optimum.
At higher strains, however, both samples have the same pore pressure.
6. Compressibility :
When the applied pressure is in the low pressure range, a saturated sample of clay compacted on the wet side of the optimum is more compressible than a sample of the same soil compacted on the dry side of the optimum with the same voids ratio.
This is because the optimal dry compacted sample has a flocculated structure that necessitates additional pressure to induce parallel particle orientation.
However, in the high pressure range, the optimum sample compacted dry is more compressible than the optimum sample compacted wet.
7. Stress-strain Characteristics :
A sample compacted dry side of optimum has a steeper stress-strain curve and therefore a higher modulus of elasticity than one compacted wet side of optimum at the same density for a given soil.
Soil compacted wet of optimum fails brittlely, while soil compacted wet of optimum, with a scattered structure, continues to strengthen even at higher strains.
8. Shear Strength :
Shear strength of compacted clays is determined by :
- Dry Density
- Moulding Water Content
- Soil Structure
- Compaction Process
- Strain used to determine Strength
- Drainage Condition
- Soil Type
In general, the strength of cohesive soils compacted dry of optimum is higher than that of cohesive soils compacted wet of optimum at low strains.
The failure envelope of 2 specimens of the same soil, one compacted dry of optimum whilst the other compacted wet of optimum, both compacted at the same density, as shown in above figure.
The flocculated structure of the same compacted on the dry side is disrupted at higher strains, resulting in ultimate strength for both samples.
The strength of the soil sample compacted wet of optimum is also affected by the process of compaction. It’s worth noting that clay cores in earth dams are normally compacted wet to allow for large settlements without cracking.