Calculation And Analysis Of Geomembrane Air Expansion Pressure Of The Whole Bottom Seepage Prevention Of Datun Reservoir

As a regulation plain reservoir in east route of the Sourth-to-North Water Diversion Project,Datun Reservoir used an anti-seepage scheme that was completely lined with geomembrane.Under the influence of factors such as the rises of groundwater level,the pore air in the unsaturated soil under geomembrane is easily gathered together to form gas pressure under the geomembrane,resulting in geomembrane air expansion phenomenon in local area and even burst the geomembrane.Air expansion increases the risk of reservoir leakage;therefore,it is of great theoretical and practical significance to study geomembrane air expansion and geomembrane strength.Based on the engineering condition of Datun Reservoir,the reasons that may cause geomembrane air expansion was analyzed.According to the Boyle's law and Henry's law,the formulas that were employed to calculate the pore air pressure under geomembrane when the groundwater level rises were derived,it was validated using indoor experiment.Later,the derived formulas was used in calculating and analyzing the pore air pressure under geomembrane when the groundwater level rises in Datun Reservoir.In addition,according to the similarity between the heat conduction model and gas migration model in unsaturated soil,the Abaqus software was employed to simulate the change of gas pressure under geomembrane when the groundwater level rises in Datun Reservoir,the main research results of this paper are as follows:(1)The formulas were derived which can be used to calculate pore air pressure under geomembrane when the groundwater level rises.And then a set of experimental devices was designed,and some experiments were performed to validate the proposed formulas.The validity of the formulas was proved by the comparison between the calculated results and the experimental results.(2)The derived formulas was employed to calculate the pore air pressure under geomembrane in Datun Reservoir when the groundwater level rises by 0.5m,1.5m,3.0m respectively,the calculated results shows that the gas pressure under geomembrane increases as the groundwater level rises.There will not occur the geomembrane air expansion phenomenon when the groundwater level rises by 0.5m in reservoir even without exhaust measures.When the groundwater level rises by 1.5m,there will be may appear the geomembrane air expansion phenomenon in the area away from the dam and the area near the dam,if the reservoir have no good exhaust measures.But in the case of high water level,there will be no geomembrane air expansion phenomenon.When the groundwater level rises by 3.0m,there will be occur geomembrane air expansion phenomenon without good exhaust measures.In order to prevent the occurrence of geomembrane air expansion phenomenon,some exhaust measures are needed taken.(3)Based on heat conduction model,the Abaqus software was employed to simulate the change of gas pressure under geomembrane when the groundwater level rises in Datun Reservoir.The results shows that the gas pressure under geomembrane in area near the dam is less than that in area away from the dam.The results of the simulation are quite consistent with the calculated results,which also prove that this heat transfer model is suitable for the calculation and analysis of pore air pressure under geomembrane in plain reservoir when the groundwater rises.The research results can provide theoretical guidance and technical support for selection of geomembrane type,thickness of soil above the geomembrane and the design of exhaust measures in plain reservoir that was completely lined with geomembrane.