4.1. Simulation Under No Rain Condition
 The temporal changes of the simulated and observed air pressures at point O and the observed tidal rate are shown in Figure 2b, which indicates that the model can reproduce reasonably well the dynamic behavior of the air pressure. An important observation from the simulation results is that the air pressure is approximately proportional to the rate of tidal rise and fall. The times when the air pressure reaches its maximums and minimums are in line with the times when the rate of tidal rise and fall reaches its maximums and minimums (Figure 2b). This suggests that the maximum air pressure is determined by the maximum rising rate, rather than the tidal amplitude. A high tidal level will not necessarily lead to high air pressure if the sea level rises slowly. On the other hand, a sudden rise of the sea level will induce high air pressure, although the amplitude of the sea level fluctuation may not be great. This finding explains why the semi-diurnal tide with daily small and large peaks can create similar air pressures as shown in Figure 2a.
 Figure 3 presents the simulated air pressure (color) and velocity (arrows) distributions when t = 26.2 (a) and 44.3 h (b). When t = 26.2 h, the tide approximately reaches its maximum falling rate and the air pressure reaches its minimum. The fall of the water table leaves extra pore space above the water table and the ground surface then inhales, as clearly indicated by the airflow velocity field. The relative air pressure in the system ranges from −2.4 kPa near the water table to barometric pressure of 0 near the ground surface. Inside the marine sand, columns of low pressure are formed below the pavement-covered surfaces.
Figure 3. Simulated air pressure (color) and flow velocity (arrows) distributions when water table falls (a) and rises (b). Negative pressure is created and the ground inhales when the water table falls. High pressure is created and the ground exhales when the water table rises.
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 When t = 44.3 h, the rate of tidal rise approximately reaches its maximum and so does the air pressure. The rise of the water table compresses the overlying air, forcing the air to exhale through the ground surface, as is clearly indicated by the airflow velocity in Figure 3b. The pressure changes from 2.0 kPa near the water table to barometric pressure near the surface. Inside the marine sand, columns of high pressure are formed below the pavement- covered surfaces.
4.2. Simulation Under Rain Conditions
 The asphalt pavement in the study area, which has a thickness of 0.10–0.15 m was domed upward following heavy rainfall, suggesting that air pressure during storms is very significant. Thus, we also used our model to simulate the development of abnormal pressures during typical storms. Rain gauge data near the site from 1999 to 2002 show that the maximum daily rainfall was 95 mm, and that there were 7 days in which the daily rainfall exceeded 80 mm. On some days the 80 mm of rain fell within a few hours.
 Three hypothetical rain patterns with maximum daily precipitation of 80 mm were used in the simulation. Rain pattern 1 has one 37-hour rain period [8h, 45h] with a total rainfall of 160 mm (intensity = 4.3 mm/h). Rain patterns 2 and 3 both have two 3-hour-long rain periods with rainfall of 80 mm in each period (intensity = 26.7 mm/h). The difference between the two is the timing of the rainfall; intervals of [3h, 6h] and [24h, 27h] were used for pattern 2, and [16h, 19h] and [28h, 31h] for pattern 3. Ponding occurs over the pavement because its hydraulic conductivity is less than the rain intensity in all the rain events, so a ponded boundary condition is used on the pavement. The depth of ponded water is set to be 0.01 m.
 Figure 4 presents the simulated water saturation (color) and air velocity (arrows) distributions corresponding to rain pattern 1 when t = 26.2 and 44.3 h at which time the air pressure reaches its minimum and maximum, respectively. Because the low permeability of the pavement, infiltrated water under the pavement is limited and the soil below the pavement is relatively dry. Perched water can form above the geotextile, but where the geotextile is overlain by pavement, perched water is either absent or forms much later than in areas not covered by pavement. Even when perched water does form above the geotextile it can be ‘blown off’ by air flow when the water table rises (Figure 4b). When a perched water table is formed, the soil between the ground surface and the geotextile has very low air permeability. The relatively dry area below the pavement becomes the main storage space and pathway of the air.
Figure 4. Simulated degree of water saturation (color) and flow velocity (arrows) distributions when the water table falls (a) and rises (b). Perched water is formed above the geotextile. Soil below the surfaces covered by pavement is relatively dry and the thickness of the perched water on the geotextile below the pavement is relatively small. The areas below the pavement become the main pathway and storage of air when the water table rises.
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 When the water table falls, negative pressures are created (Figure 4a). This is because extra pore space is formed when water table falls, but meanwhile the ponded surface of the pavement almost completely blocks off air inflow directly through the pavement to fill the extra pore space. When the water table rises, the perched water upon and within the geotextile below the areas covered by marine sand virtually seals off upward airflow and the air is forced to flow to, and accumulate in, the relatively dry areas below the pavement-covered surfaces (Figure 4b). As a result, the air pressure under the pavement is significantly increased.
 Figure 2c shows the temporal changes of the simulated air pressure at point O corresponding to the rain and no rain conditions in the 72-hour period. The maximum air pressures when there is rain are significantly greater than when there is no rain. The maximum air pressure under no-rain conditions is 1.64 kPa, whereas the maximum air pressures corresponding to rain patterns 1, 2 and 3 are 5.06, 3.58 and 3.24 kPa, respectively. The significant increase in maximum pressures produced by heavy rainfall is due to a combination of two factors; a decrease in relative permeability with respect to the air phase in the unsaturated zone, and the formation of perched water when the infiltrated rain water through the marine sand accumulates on the geotextile. Of the three rain patterns, pattern 1 leads to the greatest maximum air pressure because it has the longest rainfall period which causes the most significant reduction in relative air permeability in the soils.
 Similarly, the minimum air pressures under rain conditions are significantly lower than under no-rain conditions. The simulated minimum air pressure under no-rain conditions is −0.56 kPa, whereas the minimum air pressures corresponding to rain patterns 1, 2 and 3 are −3.17, −5.20 and −2.82 kPa, respectively. Among the three patterns, pattern 2 leads to the lowest pressure of −5.20 kPa. In this pattern the rainfall and the formation of perched water on the geotextile both occur at the same time the water table falls. The rainfall periods of pattern 3 do not coincide with the fall of the water table, so its minimum pressure is least significant among the three. The simulation results show that abnormal air pressures depend on a combination of the geological structure, length of rain period, rain intensity, and the timing of rainfall with respect to the maximum or minimum tidal rate.