Hydraulic characterization of fault F31 at the Xinchang underground research laboratory site for geological disposal of high-level radioactive waste

Abstract

As part of the suitability evaluation of the underground research laboratory (URL) candidate sites, the hydrogeological investigation activity was conducted at the Xinchang site in Beishan area from 2016 to 2019. A previous study discovered that the rock masses at the Xinchang URL site, which is surrounded by north–east trend and east–west faults, have low permeability and shortage of water-bearing structures. This article aimed to describe the hydraulic properties of fault F31, which is located on the west boundary of the Xinchang URL site, and evaluate the fault hydraulic characterization methodology for site characterization of the URL and repository sites. The surface hydrology monitoring activity at the Xinchang site showed that the water flow occurring through the gully where fault F31 is located after precipitation is the main recharge resource of groundwater. Accompanied with hydraulic monitoring during the drilling period of inclined boreholes (BSQ04–BSQ06, BS35, and BS36) crossing fault F31, the survey revealed that groundwater was stored in the weathered zones of the middle and north parts along fault 31. Single-hole and interference tests were performed later with the Heavy-Duty Double Packer system. The hydraulic conductivity (K) and storativity (S) were interpreted from the test data using analytical solutions. Diagnostic plots were also used in determining the reliability of results and identifying the flow regime. By considering the interpretation of borehole acoustic television logging as reference, the spatial morphological statistics of the structures are mainly parallel with the fault plane, wherein K is lower than 1 × 10−9 m/s depth to −250 m below the surface. Thus, the strike-dip parameter of the water resistance structures can be roughly confirmed. Six sections were selected for the evaluation of the water-conductive structures; these sections exhibited depths of more than −250 m below the surface and K values of equal to or more than 1 × 10−9 m/s. After the structures with a strike-dip similar to that in the water-resistant sections were removed, the remaining structural planes were used to build a flow model path during the hydraulic test. Compared with flow regime derived from the diagnostic plots, the structural planes fitting the real flow regime were identified as water-conductive structures. The results showed the following: (1) K and S increased from the south to the north in the shallow zone along fault F31, where water-conductive structures were surrounded by water-resistant structures; (2) the section located −250 m below the surface of F31 was water resistant, and no-flow channel existed in the deformation zone along fault F31; and (3) the fault hydraulic characterization methodology combining hydraulic test, hydrological survey, and geophysical logging is suitable for the site characterization of URL and repository.