Abstract
Specific storage, one of the hydraulic constants, is defined as a rate of volume change caused by the elastic deformation of structural skeleton and pore water in a porous media associating with changing hydraulic pressure. In this report,, the specific storage was theoretically estimated from the combination of compressibility of skeleton of rock mass, compressibility and density of pore water, and porosity.
The basement rocks of the studied area, mainly composed of rhyolitic welded tuff, are considerably fractured from the observation of the survey tunnels having a total length of 300 meters and of the vertically drilled four bore-holes to the tunnels. The basement rocks also have high permeability, determined from the Lugeon water tests carried out at about four bore-holes. Hydraulic conductivities and specific storages have precisely been determined by the cross-hole hydraulic pressure tests performed at selected four bore-holes. An apparent principal hydraulic direction can be distinguishable from the differences of hydraulic pressure propagation among horizontal three directions during cross-hole hydraulic pressure tests. The specific storage value is larger in the apparently minimum principal hydraulic direction than that in the maximum principal hydraulic direction.
The compressibility of skeleton was determined from the data of elastic wave exploration around survey tunnels and bore-hole horizontal loading test at several bore-holes. The specific storage of a fractured rock mass can be calculated from the combination of the compressibility of skeleton, compressibility and density of pore-water and porosity. Consequently, the calculated specific storages is concordant with that of the apparent minimum principal hydraulic direction. Specific storages of the apparent maximum principal hydraulic direction have smaller of 2 orders and more than those calculated above. The smaller values are explained that volume of the pore in fractures varies with ground water seepage along the apparent maximum principal hydraulic direction without the volume change of the rock skeleton. When the rock skeleton along the apparent maximum principal hydraulic direction has no volume change associating with seepage flow, the specific storage is given by the product of compressibility and density of pore water, porosity and coefficient, α, where α is the ratio of pore volume in the main flow paths, which volume changes during seepage flow, to total pore volume. In the studied rock mass here, α ranges from 0.07 to 1.3. The α is smaller than 1, because ground water flows in a part of fractures which regard as channels, and the storage of the ground water is due to the volume change of a certain part of pore space in the main ground water flow path.
