| Waste containment is one of the most important issues among waste management practices. Bottom liner system is the most essential element of waste containment so that critical emphasis of low permeability, strong adsorption capacity, enough pressive strength and sun-crack resistance should be given to the materials uesd. Silty clay has good compactibility and relatively high shear strength, while dredged soil has the performance of high adsorption capacity and low water permeability. The mixture of these two materials possesses the combination of their advantages. Moreover, the source of high quality clay is limited and large amount of dredged soil is under disposal. Hence, the application of silty clay mixed with dredged soil to the landfill is beneficial. It is required that waste containment system has effective long-term performance of obstructing leachate and pollutants within its design period. The basic problems of clay liners, hydraulic conductivity, strength and adsorption capacity, are considered, and the long-term performance of the mixed soil (silty clay and dredged soil) with the condition of wetting-drying cycle, freezing-thawing cycle and leaching cycle is investigated in the following aspects:1. Wetting-drying cycle often results in the dehiscence of the clay liner. The basic physical and mechanical properties of the silty clay and the mixed soil were examined after 0, 5, 10 and 20 times of wetting-drying cycles, respectively. The result showed that coefficient of permeability of both the two soils increased with the increase of cycle times. After 20 cycles, the coefficient of permeability of the silty clay was 3.21 × 10-7cm/s, and that of 25% mixed soil (silty clay mixed with 25% dredged soil) 8.61 × 10-8 cm/s, meeting the demand of criterion, k≤1 × 10-7 cm/s. Under the condition of wetting-drying cycle, the shear strength of the silty clay and the mixed soil decreased with the increase of cycle times, but the strengh of 25% mixed soil was higher than that of the crude soil generally.2. The repeated change of temperature has a great influence on soil property, hence the permeability and strength of mixed soil were examined after 0, 5, 10 and 20 times of freezing-thawing cycle. The result showed that with the increase of cycle time the permeability gained a rise and strength decreased. After repeated cycle, thefriction angle changed a little but cohesion had a tendency of continued decrease. The friction angle and cohesion of 25% mixed soil were higher than that of the crude soil.3. It is theoretically hold that wetting-drying cycle and freezing-thawing cycle will result in cumulative damage to the clay so that the strength will reduce and coefficient of permeability increase. The investigation showed that mixed soil has a better resistance to sun crack and wetting-drying cycle. This should be greatly due to the bonding capacity of organic material in the dredged soil, which cements fine granules of dredged soil and corase particles of silty clay together.4. The change of physical and mechanical property and microstructure of the clay after adsorbing cations was investigated, and the result showed that both the strength and permeability coefficient increased. Scanning electron microscopy (SEM) was done to study the microstructure of the crude clay and the mixed soil. The photos indicated agglomeration took place in the tested soil and large and deep holes formed.5. The basic performance of the mixed soil (silty clay and dredged soil) with the condition of leaching cycle was investigated. The result showed that both the penneability of tested soils obtianed an increase, but the coefficient of permeability of the crude soil did not satisfy the request while the mixed soil did after four times cycle. Meanwhile, the cohesion decreased to some extent and friction angle keep consistent. It is obvious that the mixed soil had good resistent with the crude soil.It is shown in this discussion the geotechnical and long-term performance of the 25% mixed soil are better than that of the crude soil.
|
PDF Full Text Request