Theoretical analysis and field test are carried out on the mechanism of bulge phenomenon under geomembrane in plain reservoir of which the bottom paved with geomembrane. It is pointed out that the uplifting or bursting problem will occur during surrounding embankment construction

reservoir water quick reduction or groundwater level rising. A field test is carried out at Dezhou Datun reservoir

during which different loadings over geomembrane

different amplitudes and rates of groundwater level rising are simulated and controlled. The pore pressure at different depths in soil is measured and the bulge phenomena under geomembrane are observed. The influences of different influence factors on gas field are analyzed. The results show that the groundwater level rapidly rising results in gas pressure increasing under geomembrane and the gas pressure value depends on the rising amplitude of groundwater level; different uplift rates have little influence on gas field under geomembrane. A certain loading over these geomembrane can effectively ensure the safety of geomembrane.