亀裂方位情報を利用したクロスホール試験による岩盤の三次元透水パラメータの測定法

Abstract

We propose a new method to determine the three-dimensional hydraulic parameters of fissured rock mass. More than three boreholes are required for a conventional cross-hole method and many water injection experiments have to be repeated during the test. However, our method make the estimation of hydraulic parameters possible through the use of only two boreholes along with information on fissure orientation. A constant volume of water is injected into one borehole, and the head increase with time in a packed-off interval in the other borehole is monitored. Data of fissure orientation are sampled from boreholes or nearby outcrops. When a rock mass is regarded as an equivalent porous media, the hydraulic conductivity, hydraulically and geostatistically obtained by using in situ information of fissures from the crack tensor theory, will be expressed by a second-rank, symmetric, positivedifinite tensor. Principal directions of the hydraulic conductivity tensor obtained are reliable because orientation and distribution density of the fissures are reliable. Whereas, absolute values of the tensor are not reliable because of low reliability of aperture data. Thus, our proposing method can be applicable for the case of the rock mass that is regarded as an equivalent porous media. The method uses a non-linear least squares iteration to determine the three-dimensional hydraulic conductivity tensor and specific storage, which give the same variation curve of the theoretically calculated water head by using the already known data of orientation and density of fissures as the observed one during crosshole test. The theoretical fitting curve of water head used for this purpose was calculated by Hsieh's three-dimensional theory of transient seepage flow in a homogeneously and anisotropically porous media which has the estimated hydraulic parameters.Finite element analyses of three-dimensio n al seepage for getting the cross-hole test data were applied to two kinds of rock mass models to check the reliability of our method. The observed change of water head with time obtained from a three-dimensional finite element analysis and given the fissure orientation are analysed by our method. The calculated values of hydraulic conductivity tensor and specific storage are sufficiently accurate for practical application.