Numerical Simulation of Tide-Induced Periodic Air Pressure Fluctuations in Coastal Vadose Zone

While it is well known that the groundwater level in coastal areas fluctuates frequently due to tidal seawater, it is not so well-recognized that air pressure in the subsurface soil near the coast may also fluctuate in response to sea tides. If the soil is well capped by less permeable materials such as concrete or asphalt, which is common in the extensively urbanized coastal areas in Hong Kong, significant high pressure may be produced by the watertable fluctuation and causes geotechnical problems such as surface heave. Therefore, understanding air flow induced by tidal fluctuation is of not only theoretical significance but also engineering implications. In this paper, a case study is conducted to explore the air pressure fluctuations observed below the asphalt pavement in a coastal reclamation area of Hong Kong. An isothermal, two-dimensional, cross-sectional model was built to describe the water-air two-phase flow in the unsaturated zone of the area. TOUGH2 simulator was used to obtain the numerical solutions. Sensitivity analysis was carried out to investigate the effects of model parameters on the air pressure fluctuations. After the boundary conditions and the model parameters were derived from the hydrogeological background in the area, the simulation reproduced the observed air pressure below the asphalt pavement with a satisfactorily small error.