In situ test and analysis method of air bulging under geomembranes in a shallow-lined reservoir

Air bulging under a geomembrane is a problem in reservoirs that are lined with geomembranes. Three essential conditions influence the air bulging under geomembranes: earthworks construction, rapid decrease in reservoir water level, and rising of the underground water table for a geomembrane liner without defects. The theory of unsaturated soil mechanics is proposed to analyze the mechanism of air bulging under geomembranes by using a one-dimensional method. In the absence of a liner, no air bulging is produced because any air could be squeezed out easily. However, air pressure could increase in the presence of a liner because the pore air is isolated from the atmosphere. Whether water infiltration is caused by the rising of groundwater table or soil is compressed, pore air volume is reduced in the unsaturated soil, and the pore air pressure increases. Air bulging could be produced if the pore air pressure is greater than the load over the geomembrane. Then, the pore air pressure is computed with the two-dimensional consolidation method of unsaturated soil. A small-scale in situ test of the Datun reservoir was performed to analyze the air bulging phenomenon under the geomembrane in Shangdong Province, China. The earthworks construction and the rapid decrease in reservoir water level have insignificant effects on air bulging under the geomembrane because the pore air pressure under the geomembrane depends on the compression strain of the pore. The increment of the pore air pressure is always low because of less compression strain. The rise in groundwater tables has a significant effect on air bulging under the geomembrane because a linear relationship exists between the pore air pressure under the geomembrane and the increased amplitude of the underground water level.