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Extensive and thick permeable strata are found beneath many reservoir sites. The leakage problem in these reservoirs can be effectively solved by constructing a full geomembrane liner design. However, applying this particular solution for the reservoir induces novel engineering problems. The air bulging produced beneath the geomembrane results in geomembrane rupture and causes a renewed leakage in reservoirs. The groundwater level may be relatively low during construction because of seasonal variation and dewatering. Geomembranes isolate pore air from the atmosphere, and a significant amount of air is trapped in the pores of the unsaturated soil above the groundwater level. The rise in the groundwater level or the compression of soil decreases pore-air volume and increases pore pressure, thereby causing air bulging to occur and prompting the ultimate destruction of the geomembrane. The presence of air bulging beneath the geomembrane is studied in the design process of the Datun Reservoir in Dezhou City, Shandong Province, China. Investigation results reveal that a rise in the groundwater level significantly influences the air bulging beneath the geomembrane. The French drain is set to prevent air bulging beneath the geomembrane from occurring. The 75-m French drain spacing is not reasonable because the air pressure beneath the geomembrane will result in air bulging. A comparison among French drains with spacings of 25 m and 50 m indicates that the original design comprising French drains spaced 50 m apart is reasonable and economical.