Effects of capillary fringe and truncation factor on pore-water pressure and water table variation in dewatering and watering sand column experiments

Spatiotemporal variations of the pore-water pressure and water table response to small amounts of water exchange were measured in a sand column. In the dewatering experiment, the pressure head variation can be divided into three stages: the initial pressure head declining phase when water table is above the sand surface, a sudden drop in accordance with the capillary fringe when water level passes through the sand surface, and a slow declining phase with respect to the specific yield of the sand. As for the watering experiment of the shallow groundwater with different initial truncation factors (TF), the addition of a minute amount of water results in a rapid rise of the pressure head initially, then a gradually increase to its equilibrium. When the upper extent of the fully saturated capillary fringe coincides with the sand surface (TF ≤ 1), a thin film of water addition produces nearly instantaneous increase in pore-water pressure, corresponding to a rapid rise of groundwater table to the sand surface. Pressure head increment increases linearly with the TF value and reaches its maximum when TF value approaches unity. Considering the vertical distribution of the pressure head, its gradient becomes larger right after the water addition, then decreases to its initial value of hydrostatic condition in the end. In the case of TF > 1, there is an unsaturated layer above the groundwater table, the pressure head increases after the water addition, whose magnitude however decreases as the increase of TF value. Nevertheless, the final position of the groundwater table is still below the sand surface, and the gradient of the pressure head vertical distribution dose not vary significantly throughout the entire process. In consideration of hydrostatic pressure assumption, temporal variation of the water table was also quantitatively obtained based on the measured instantaneous pore-water pressure data, which is consistent with time-varying process of the pressure head during both the dewatering and watering experiments.