In Japan, the construction or planning of large-scale underground facilities; such as underground electric power station, unlined underground oil storage cavern and nuclear power plant underground have been coming into consideration in recent years. To construct these underground structures safely and also to keep stability for a long time, it is one of the great important problems for the design and construction to make clear the existence and the movement of groundwater in the bedrock. Especially in case of unlined underground oil storage cavern, excavated in the bedrock below the natural groundwater level and used the sorrounding groundwater pressure to prevent the leakage of the fuel such as petroleum or LPG, the movement of the groundwater in the bedrock is of great importance.

As for the movement of the groundwater in the bedrock, it is general to consider the groundwater pressure having the hydrostatic pressure which correspond to the water level in the bore-hole. The groundwater in the bedrock exists in cracks and depends on geologic factors such as joint or fractured zone. In spite of many studies on the relation betweenthe theoretical groundwater flowand the geologic factor in the bedrock, the actual behavior of groundwater flow is very complex and there have been few studies especially on the actual groundwater flow in the bedrock (Witherspoon,1977; Rocha,1977; Motojima,1979; Watanabe & Kojima, 1980). The actual measurement of the groundwater flow was observed only in an aluvium deposit by Meyboom (1966), and the groundwater flow in the bedrock has not been measured systematically.

In the present study, the authors have in- vestigated the movement of the groundwater in the bedrock using the groundwater pressure measured in the bore-hole.

Measuring apparatus

The apparatus for the measurement of groundwater pressure has been developed by Central Research Institute of Electric Power Industry (Motojima,1979). The measuring probe of this apparatus is consisted of the upper and lower packers and the pressure transducer set in the interval zone between packers. The packer is a kind of gum tube air packer having 1 m length, 55 mm outer diameter.

(Table in full paper)

Figure 1 shows the system of the groundwater pressure measurement by this apparatus and Table 1 shows main properties of this apparatus.

The groundwater pressure in the space between packers is converted into electric signal by the pressure transducer and are sent to the measuring devices on the ground which are composed of control amplifier, recorder and also digital indicator for direct reading of the actual groundwater pressure. By this apparatus, the permeability of the bedrock is also able to be measured accurately.

Measuring method

The measured space between the upper and lower packers for the groundwater pressure measurement is 5 m in length. The measurement for one stage in the bore-hole consists of the following five steps controlled by the expansion state of each packers.

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