Groundwater in fractured rock mass (fissure water) which was drained during excavation work recovers its level after pumping is stopped. In the recovery process, it is expected that water goes up much faster in large fissure than in small fissure because the velocity of a laminar flow in an interstice is proportional to the square of the width of the interstice. In result, air is entrapped in smaller fissures while water goes up through larger fissures. As water seeps into smaller fissures in process of time, the entrapped air is compressed and moves slowly through paths of complex structure forming bubbles at the junction of fissures. Thus, it is expected that a mass of the entrapped air is separated into smaller masses or two bubbles are united into a larger mass. Such behavior of air in fissures may affect the storage efficiency, the leakage mechanism and the dispersive convection process of the matter such as oil, gas or nuclear wastes stored in tunnels built in fractured rock mass.
A numerical analysis is carried out to simulate the movement of the air entrapped in fissures in process of groundwater recovery. The model employed for this simulation is a fissure which branches off into two fissures of different width and joins again. This study aims to develop a numerical technique to simulate the process in which air is entrapped in a smaller fissiure and bubbles flow out of the smaller fissure into a larger fissure at the junction of two fissures. The process is treated as the moving-boundary problem. The validy of the simulation technique is confirmed by an experiment.