Abstract
Hill surfaces in the Kanto district are generally covered by fine-grained Kanto barns and underlying coarse-grained sand and/or gravel. Water tables are usually found in the lower coarse materials at some ten meters below ground surface. In spite of considerably low permeabilities of the overlying Kanto loam, unexpected quick responses of groundwater to heavy rainfall, for example water table rise and groundwater discharge, have been recognized by many workers.
In order to clarify the characteristics of soil water movement during infiltration and the processes of groundwater recharge (or discharge) in these stratified two-layered soils, laboratory experiments were carried out using a soil column (15 cm×15 cm×150 cm) with a water table fixed at 125 cm below the soil surface. A stratified two-layered formation which consists of a fine-grained upper layer and a coarse-grained lower layer was produced in the column by sands with different particle sizes. An interface between the layers was set at 55 cm below the soil surface. A ratio of saturated hydraulic conductivities was measured to be about 1: 6. After the cessation of the gravity drainage, that is, just before the beginnings of experiments, a tension-saturated layer of capillary supported-suspended water (about 7 cm thick) was found in the lower part of the overlying fine materials.
Discharge from the column, hereafter referred to the groundwater discharge, and matrix suctions at the depths of 7 cm,30 cm,55cm,80 cm,105 cm,117.5 cm and 130 cm were measured continuously. Artificial steady rainfalls were supplied to the top of the soils until they became equal to groundwater discharges. An air valve which controls pore-air pressure in the underlying coarse materials by opening and shutting it was equipped just below the interface of the layers in order to check a role of the entrapped air in the groundwater discharge.
The results and conclusions of the study are summarized as follows;
The groundwater discharge increases with showing distinct three stages and finally balances the rainfall.
The groundwater discharge takes place as soon as a wetting front reaches the tensionsaturated layer of the capillary supported-suspended water in the overlying fine materials and thereafter it continues to increase dramatically (the 1st. stage).
These phenomena are considered to be caused by an effect of the pore-air pressure in the underlying coarse materials. A break of an equilibrium of forces acting on the capillary supported-suspended water is commenced by the downward-advancing wetting front, followed by an intrusion of pre-event waters into the underlying layer. The air in the coarse materials between the tension-saturated layer of the capillary supported-suspended water and the water table cannot escape vertically and is compressed. Resulting pore-air pressure increases subsequently and breaks an equilibrium of the water in the capillary fringe above the water table, causing an unexpected quick response of the groundwater discharge to the rainfall without an arrival of the wetting front near to the water table.
After a temporal decrease of the groundwater discharge in the 2nd stage, the discharge begins to increase rapidly again when the wetting front reaches the capillary fringe above the water table (the 3rd stage). This is due to a direct destruction of the equilibrium of the water in the capillary fringe by an arrival of the wetting front.
