| Final cover is used to reduce rainfall infiltration in municipal solid waste landfill. Compacted clay is easy to crack under dry-wet cycles and cold-warm cycles of compacted clay cover; hence its long-term anti-seepage performance is very poor. Composite cover has excellent service performance, but slides maybe occur at the interface of clay and geomembrane, whose durability is going to be tested through practice and also at a high engineering cost. So evapo-transpiration cover is getting more attractive, which stores water in soils during rainfall and release water through evaporation and transpiration after rainfall. One type of evapo-transpiration is called capillary-barrier cover, which is consisting of fine-soil layer and coarse-soil layer. Research has proved that capillary-barrier cover have an advantage of excellent durability and soils-getting convenient, it works very well in arid and semi-arid regions. We have to increase the feasibility of capillary-barrier cover in humid regions to reducing rainfall infiltration of southern landfills in china.Increase unsaturated drainage layer(UDL) between fine and coarse-soil layer to increase lateral drainage of capillary cover to reduce percolation is one way to increase feasibility of capillary-barrier cover in humid regions. Experiments study have been carried out to study performance of capillary-barrier cover with UDL in humid climatic region of china:first, a set of test equipment has been developed to study the performance of capillary-barrier cover under strong rainfall condition and get methods to control percolation of cover. And then long-time monitoring experiment has been carried out to get response on capillary-barrier cover under actual complex climate in humid region and to verify long-time performance of anti-seepage property of capillary-barrier cover with UDL. Based on these tests results, practical design method is proposed.Developed test equipment includes model box, rainfall simulator and measuring equipments and so on. Material of fine soil, UDL and coarse soil layer of capillary-barrier cover model is silt, sand and gravel respectively, and slope of model isâ…£:3H. Intensity of simulated rainfall is 65-76mm/h. During tests rainfall infiltration process and lateral drainage phenomenon were observed, and surface runoff, lateral drainage and percolation through capillary-barrier cover were measured. In test I, runoff accounts for 69.4% of total rainfall, and most of infiltration is stored in silt soil layer. Lateral drainage of sand layer is very significant, being 3.5% of total rainfall; and percolation through cover is 2.9% of total rainfall. In testâ…¡andâ…¢, infiltration flux was reduced by adding bentonite into silt soil layer. A delayed response on lateral drainage phenomena was observed in sand layer, and percolation is reduced to only 0.8% of total rainfall in testâ…¡and no percolation was observed in testâ…¢. These model tests results indicate percolation through capillary-barrier cover can be controlled under heavy rainfall condition by reducing rainfall infiltration and enhancing capillary barrier effect and lateral drainage capacity.Model used in long-time monitoring experiment is from laboratory testâ…¢by adding vegetative layer and grass. Vegetation growing condition and response of matric suction in soils, fluxes were monitored. In humid region, deep soils were affected during rainfall, moisture transfers to toe of slope after rainfall. Seasonal changes in matric suction were observed:matric suction changes dramatically in summer; effects of evapo-transpiration is weaking with soil depth; the more close to surface, the larger range of matric suction. Silt layer and sand layer is near saturated except the summer. Rainfalls are more frequent in summer with more extreme rainfall. The storage and runoff during rainfall will be significant with significant lateral drainage of UDL, but the percolation is focused in summer. Rainfall reduces in winter with down of storage and runoff during rainfall except lateral drainage, the mount of lateral drainage flux is raising and accompanying well with rainfall.The capillary-barrier cover reduces percolation through water-store and water-releasing. The tested cover can achieve good anti-seepage effect by water store and releasing in summer and by water store, lateral drainage and releasing together in winter. Rainfall is mainly stored or changes to runoff or drainage by UDL in long-weak rainfall; and rainfall transforms to runoff and storage under short-high rainfall condition. Effects of snow in winter can be equivalent to one long-weak rainfall process. And percolation is caused by the non-uniform moving of infiltration or the flux rate into UDL yield the maximum drainage rate of UDL.The capillary-barrier with UDL cover has overcome disadvantage of tradition capillary-barrier cover at long-weak rainfall and high percolation in winter caused by high water content by adding the UDL. The percentage of storage, runoff, lateral drainage and percolation is 70.20%,23.95%,5.81%,0.04% of total rainfall in monitoring period, and total percolation of capillary-barrier cover with UDL is less than 1mm/year. This cover has a excellent feasibility in humid region in china.Lateral drainage length will be increased significant by adding UDL. The higher water entry value or saturated hydraulic conductivity of UDL soil, the longer lateral drainage length. Hence the soils with high permeability and WEV equal to average WEV of fine and coarse soils should be selected as UDL material. The accumulated difference between rainfall and potential evaporation outside of summer can be divided according to water balance in long-time monitoring test as design storage and design lateral drainage. The drainage length can be decide by the flux rate into UDL, which will be 1/2-1 times of the saturated hydraulic conductivity. And then the thickness of fine soil layer and UDL can be decided by storage capacity of fine soil and Darcy's law. The design results show that thickness of fine layer, UDL and coarse layer will be 1m,0.25m,0.2m when the slope of capillary-barrier cover is 1 V:3H in Hangzhou City. |
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