Water Storage And Release Of Bess/Gravel Cover And Seepage Prevention Design Method InNorthwest Of China

Capillary barrier cover (CBC) is mainly composed of natural non expansive soil. It achieves impermeable function perfectly by water storage, water conductivity characteristics of unsaturated soil. It is more economical and durable than the composite covering layer with soil engineering film. In northwest China, the climate is mainly arid, semi-arid and semi-humid, and widely distributed loess is excellent material as landfill final cover materials. In this paper, the meteorological characteristics of nearly 50 years in Northwest China were collected, and the water storage capcity and permeability characteristics of the loess were tested. Relying on the "project landfill final cover ", the author constructed "the first soil cover layer field test base of china" In Xi’an Jiang Chun landfill. Extreme rainfall test was carried out on the loess-gravel capillary barrier cover. The maximum water storage capacity of the Loess/gravel cover layer was determined. The capillary barrier effect between the Loess and the gravel was verified, The influence of vegetation conditions on the hydraulic response of soil cover was clarified, The design method of capillary retardation loess covering layer was put forward according to the meteorological conditions in Northwest China. The long-term service performance of the Loess /gravel capillary barrier layer is predicted. The main research results are as follows:(1) Test results of water holding characteristics of loess in laboratory and field showed that:the water holding characteristics is influenced by the geological age; status and the degree of compaction (dry density). The greater the dry density, the lower the water holding capacity. The water holding capacity of loess is slightly in different scale. With the field capacity θc being of 34.6% in laboratory; the wilting point moisture content is about 12.1%, and plant available water storage θa is 22%-24%.The field capacity, wilting point moisture content AND plant available water storage θa is 31.5%,12.1% and 19.4% in field respectively. The loess as soil cover material for landfills in northwest China has advantages with respect to both technical and economical aspects.(2) The results of permeability of loess tested in laboratory and field showed that:the saturation permeability coefficient of clayey loess is in 10"7m/s. loess saturated permeability coefficient is basically equal to each other in Laboratory and field. With the dry density being of 1.30 g/cm3 and 1.45 g/cm3, the saturated permeability coefficient were 8.3～10-7m/s and 8.1×10-7m/s respectively. The plant has a greater impact on the permeability. Without plant the saturated permeability coefficient of loess is 8.097×10-5cm/s; correspondingly it is bigger than 8.267×10-5cm/s with plant.(3) The loess-gravel capillary barrier cover base which is the first one in China has been built in the Xi’an Jiang-Chun landfill. It is 30 m in longth,20m in with,0.90m in thickness(the vegetation layer, reservoir water functional layer and gravel layer is 0～0.3m,0.3～0.9m and 0.9-1.2m respectively). In Core test area (24m×10m), TDR water content probe, tensionmeters, weather station, air pressure gauge, thermometer, landfill gas collection and injection pipe fitting, surface runoff and leakage collection system are installed.(4) The results of extreme rainfall experiment showed that:The wetting front gradually migrated down in rainfall and formed a "near saturation area" in the rear of wetting front. With the wetting front gradually reacheing the Loess/gravel interface, Water is blocked by the capillary barrier effect. There are higher water content and pore water pressure on the soil surface. When the pore water pressure increases to the water-entry value of the gravel layer, Water breakdowned interface and leakage occurred.(5) The water balance in extreme rainfall experiment showed that:Total rainfall were 199.57mm and 157.81mm respectively with and without plant. Water storaged in soil were 214.8mm and 194.9mm respectively With and without plant. The available water storage capacity Sfac is 259.82 mm and 251.95 mm respectively. The measured value is less than the theoretical value of 6.7%-9.5%; The measured water storage capacity in capillary barrier cover is bigger of 47.17%～51.76% than it in monolithic soil cover.(6) The results of hydraulic response under the condition of natural dry and wetting circle showed that the plant has a greater impact on the anti-seepage performance. Evapotranspiration is the main way of covered soil layer water release. With vegetation, the transpiration rate of the cover is much greater than that of the non-vegetation. During the summer, the former is larger than the latter 70%. With vegetation, the structure of the soil is Uniform and not easy to crack. In addition,Vegetation root system enhanced the impermeable performance of capillary barrier cover layer by absorption WaterThe results of hydraulic response under the condition of natural dry and wetting circle showed that The plant has a greater impact on the anti-seepage performance. Evapotranspiration is the main way of covered soil layer water release. With vegetation, the transpiration rate of the cover is much greater than that of the non-vegetation. During the summer, the former is larger than the latter 70%. With vegetation, the structure of the soil is Uniform and not easy to crack. In addition, vegetation root system enhanced the impermeable performance of capillary barrier cover layer by absorption Water.(7) According to Chen’s water balance model, a preliminary design method for the thickness of the loess-gravel capillary barrier cover in Northwest China is presented and discussed. In the preliminary design process of cover layer thickness, the annual precipitation maximum (Ptw) is key controlling condition to determine thickness, and annual snowfall maximum (Pts), maxima precipitation in non-growth period of vegetation (Pow) and maximum snowfall in non-growing season of vegetation (Pos) were the non controlling conditions. AFTER Preliminary determination of thickness, it is necessary to analyze the condition of the continuous heavy rainfall in the rainy season. In preliminary determination of thickness it is thick, safe, and conservative by using laboratory wetting SWCC, and it is dangerous by using laboratory drying SWCC. The author suggests that it is very appropriate by using field wetting SWCC. In the semi-humid climate area northwest of china (such as Xi’an), the thickness of the clayey loess is suggested 1.30 m and the gravel which Particle size is 0.5～1.0cm should be use as coarse soil.(8) The results of long-term performance of the recommended cover structure showed that: Under the meteorological conditions of northwest, It is high temperature and strong effect of vegetation evapotranspiration in rainy season The relative humidity is low and the evaporation effect is considerable in the dry season. This is advantageous to the soil to cover. The most humid 10 years of the last 50 years(1980-1990 years), the cumulative rainfall is 6081.62mm. Transpiration and soil evaporation were respectively 3002.26mm and 2439.95mm, and which accounting for 89.49% of the total rainfall. Percolation is only 7.16mm, accounting for 0.12% of total rainfall. The recommended cover in this paper is Very suitable in semi humid climate area in northwest of China...