Stress re-distribution may occur by excavation around an excavated cavern, and it is important to correctly identify subsequent mechanical and hydraulic change in the Excavation Disturbed Zone (EDZ) so as to produce a reliable safety assessment. In this paper, we introduce the results of application of a specially designed device for detecting a hydraulic EDZ to one of the fractured rock cavern of a talc mine in Korea. It was observed that the hydraulic EDZ ranges to the limited distance of 1~2m from the cavern wall, and hydraulic conductivities of the EDZ were orders of magnitude higher than those of undisturbed region. Flow through the connected fractures within EDZ was evaluated by comparing in-situ measurement and numerical borehole hydraulic test using the constructed DFN model.
Changes of a rock mass around an excavated cavern are usually expressed as the term of Excavation Disturbed or Damage Zone (EDZ). Within the EDZ area, both mechanical and hydrogeological properties are changed, but both behavior are not always consistent (Alheid et al, 1999).
Well known conventional methods in investigating a hydrogeological EDZ around an excavated cavern can be grouped into the use of geophysical methods, overcoring methods after injection of a resin-like fluid, and laboratory tests of drilled core specimen from cavern wall, but they are all indirect measurements of hydrogeological characteristics.
Among these indirect methods, the use of geophysical methods contains sufficient geophysical information of a regional range, however the geophysical information is not always accordance with hydrogeological property. The use of injection method has a difficulty in selecting an injection material as well as an injection conditions, and it is hard to acquire specimen in-situ as well. The conversion of laboratory test results such as uni-axial compressive strength, porosity, seismic velocity etc. into hydrogeological properties has a limit in the number of cores to be tested, and it is not easy to find out general correlation between mechanical properties and hydrogeological ones. And the use of uncertain correlation in conversion may results. Thus, it is desirable to measures hydrogeological property of EDZ around an excavated cavern directly, if possible.
In this study, in-situ borehole hydraulic test using specially designed system was carried out, and hydraulic conductivity transition along the distance from a cavern wall was investigated in order to identify hydrogeological EDZ. Conventional borehole hydraulic test device has a limit in detecting hydrogeological EDZ, which is formed at a limited region around an excavated cavern, so that the system with short test interval of 10cm was applied.
And flow model represented by a flow dimension should be well understood in analyzing borehole hydraulic test and deriving hydraulic conductivity from the test (Kim et al, 2007). In a fracture rock mass, Discrete Fracture Network (DFN) model analysis can be applicable and is tested to identify a flow model around a borehole hydraulic test.
In-situ borehole hydraulic test was carried out at the Underground Research Laboratory (URL) cavern of Korea Institute.
