Background:
When a clay is compacted at a lower moisture content, it possesses a flocculent structure. Approximately at the optimum moisture content of compaction, the clay particles have a lower degree of flocculation. A further increase in the moisture content at compaction provides a greater degree of particle orientation; however, the dry unit weight decreases, because the added water dilutes the concentration of soil solids per unit volume.
Research showed that;
1. For a given compaction effort, the hydraulic conductivity, k, decreases with the increase in molding moisture content, reaching a minimum value at about the optimum moisture content (that is, approximately where the soil has a higher unit weight with the clay particles having a lower degree of flocculation). Beyond the optimum moisture content, the hydraulic conductivity increases slightly.
2. For similar compaction effort and dry unit weight, a soil will have a lower hydraulic conductivity when it is compacted on the wet side of the optimum moisture content.
Since it is a very important criteria that k = 10-7 cm/sec, it is important to establish the moisture content–unit weight criteria in the laboratory for the soil to be used in field construction. This helps in the development of proper specifications.
Compaction of Clay
Clays are commonly used as soil liners in waste containment bins. To prevent groundwater pollution from leachates generated from solid-waste disposal sites, the U.S. Environmental Protection Agency (EPA) requires that clay liners have a hydraulic conductivity of 10-7 cm/sec or less. In the compaction of clayey soils for clay liners and similar works such as core of an earth dam as stated before, the compaction must be done in a manner so that the specified upper level of hydraulic conductivity of the soil is achieved.
To achieve this value, the contractor must ensure that the soil meets the following criteria (Environmental Protection Agency, 1989) are met:
1. The soil should have at least 20% fines (fine silt and clay-sized particles).
2. The plasticity index (PI) should be greater than 10. Soils that have a PI greater than about 30 are difficult to work with in the field.
3. The soil should not include more than 10% gravel-sized particles.
4. The soil should not contain any particles or chunks of rock that are larger than 25 to 50 mm (1 to 2 in.).
5. In many instances, the soil found at the construction site may be somewhat nonplastic. Such soil may be blended with imported clay minerals (like sodium bentonite) to achieve the desired range of hydraulic conductivity. In addition, during field compaction, a heavy sheepsfoot roller can introduce larger shear strains during compaction that create a more dispersed structure in the soil. This type of compacted soil will have an even lower hydraulic conductivity. Small lifts should be used during compaction so that the feet of the compactor can penetrate the full depth of the lift.
6. The size of the clay clods has a strong influence on the hydraulic conductivity of a compacted clay. Hence, during compaction, the clods must be broken down mechanically to as small as possible. A very heavy roller used for compaction helps to break them down. The breakdown of clods is very necessary because according to Benson and Daniel (1990), the magnitude of k decreases with the decrease in clod size.
7. Bonding between successive lifts is also an important factor; otherwise, permeant can move through a vertical crack in the compacted clay and then travel along the interface between two lifts until it finds another crack. Poor bonding can increase substantially the overall hydraulic conductivity of a compacted clay.
8. Scarification and control of the moisture content after compaction of each lift are extremely important in achieving the desired hydraulic conductivity.
