| A large amount of leachate is produced by degradation and rainfall infiltration of urban solid waste heaps in China,and the leachate with poor drainage is deposited in the heaps for a long time,resulting in generally high leachate water level in the heaps.High leachate water level can easily induce the instability of solid waste reactor,soil and water pollution and landfill gas can not be collected effectively,which pose a serious threat to public health and the surrounding environment.According to the high leachate water level,the leachate is usually drained by the vertical shaft combined with the vacuum method,so as to improve the stability and security of the reactor and the landfill gas collection rate.Vertical shaft combined vacuum method has a broad application prospect in leachate level control and landfill gas collection optimization in China's high water level landfill,but the lack of understanding of the mechanism of vertical shaft combined vacuum method restricts the development of this method.Therefore,the study on the precipitation mechanism of vertical shaft combined with vacuum method in high water level landfill is helpful for the more reasonable and efficient control of leachate water level and landfill gas collection.In this paper,sand soil is used to simulate the deep garbage in the landfill.The vertical shaft combined with the vacuum precipitation model system designed and made independently is used to inject gas into the saturated soil at different rates(gas injection stage),simulate the gas generation of the garbage in the landfill,and then let it stand for a period of time(static stage).At the end of the static stage,the vertical shaft combined with vacuum pumping method was used to simulate the precipitation of vertical shaft combined with vacuum method in the urban solid waste landfill with high water level.The main research contents and results are as follows:(1)According to the mass conservation and volume conservation relationship of solid liquid gas three-phase material in the process of model test,the variation law of gas volume and average pressure in the soil below water level in the process of gas injection and standing was analyzed.The formation and development of the internal sealing gas encapsulation zone in soil below water level were studied by means of the hole pressure monitoring data in the gas injection and static stage.The results show that the average air pressure in the soil below the water level is about 1.35?4.26 times of the ambient air pressure until the end of the static stage,which indicates that the accumulation of the enclosed air zone in the soil is very obvious.The higher the air injection rate is,the larger the volume of the internal sealing air envelope of the soil below the water level is,and the higher the water level is,the lower the average air pressure of the internal sealing air envelope of the soil below the water level is.At the beginning of the static stage,the gas in the soil body below the water level escapes to the water surface freely,so the volume of the closed air pocket decreases,and the pore pressure decreases in a short time.At the end of the static stage,the gas escape from the soil below the water level is completed,and the remaining gas forms a stable closed gas envelope in the pores of the soil.(2)Through there will be a shaft joint vacuum gas injection method(the second group)precipitation model test results with no joint shaft vacuum gas injection method of precipitation,the results of model test(the first group)analyzed the shaft precipitation under different vacuum degree in the process of water level and change law of pumping rate,obtain the shaft combined vacuum method precipitation on soil water levels fall and closed covering the influence law of gas,and the soil photosensitive breath covering on the influence law of water transport to move.The results showed that: the higher vacuum degree(6k Pa,8k Pa,9.5k Pa)significantly accelerated the diffusion of the closed gas belt below the water level,and had a stronger blocking effect on the water pumping and drainage near the vertical shaft area,resulting in the irregular precipitation funnel in the early pumping of the second group under the higher vacuum degree(6k Pa,8k Pa,9.5k Pa).This was not the case in the first trial.In the initial stage of pumping,the pumping rate decreases with the increase of vacuum.In the late pumping stage,the pumping rate increases with the increase of vacuum degree.At the initial pumping stage,with the increase of vacuum,the cumulative pumping water gradually decreases.In the later pumping stage,the larger the vacuum degree is,the faster the breakthrough and dissipation speed of the closed air bag belt will be,and the cumulative pumping volume will gradually increase.(3)Based on the analysis of the test results,the relevant mechanism was discussed: the water in the pores of the soil was affected by gas extrusion,soil particle extrusion and confining pressure before and during the gas injection stage,and it flowed out of the pores.At the late stage of gas injection,a large amount of gas is injected through the pores of the soil,forming a gas channel.The movement of the channel gas is mainly affected by capillary force.At the late stage of gas injection and before and during the middle stage of standing stage,gas escapes from the soil body below the water level,which is accompanied by water flowing out from the pores of the soil body.At the end of the static stage,part of the gas is enclosed in the soil body,and the movement of the gas is mainly affected by buoyancy.At the end of the static stage,there are closed air pockets in the soil below the water level which are difficult to escape and block some pore channels of the soil.The application of vacuum degree and the lowering of soil water level by shaft pumping will reduce the breakthrough pressure of the closed air pocket,making the closed air pocket easy to diffuse. |
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