Determination of in situ stresses normal to a fracture around an internal pressure tunnel by hydrojacking testing

• A correlation between fracture opening and fluid pressure is important for stability of spillway tunnel.
• Coupled problem of fluid flow and fracture deformation can be analyzed by P–T data from hydrojacking test.
• Minimum principal stress is mandatory in calculating factor of safety of hydraulic confinement.
• Hydrojacking test is useful for analyzing the fracture behavior in an internal pressure tunnel.

When designing an internally pressurized tunnel, the normal stress required to reopen the natural fractures should be a major parameter for its stability. Also the relationship between the fracture opening and the fluid pressure should be pre-interpreted for the stability of structures, such as the spillway tunnel of a dam. Apart from the general hydraulic fracturing in situ stress measurements, the stress normal to the natural fracture plane should be identified with respect to the dip direction and the dip angle of the pre-existing fracture. Hydrojacking test can be adopted for this purpose, and is used to determine the mechanical and hydraulic behaviors of pre-existing fractures around an internal pressure tunnel site. This paper summarizes the results and evaluation of the hydrojacking test conducted in the internally pressured spillway tunnel site of Juam dam, located 300 km south of Seoul, near Suncheon, South Korea. The test intervals were selected after inspecting borehole televiewer images and rock cores, and the jacking pressure and the normal stress to the fracture plane have been determined.Considering that the new spillway tunnel will be adjacent to the existing drainage tunnel next to the main dam, the mechanical and hydraulic behaviors of the natural fractures in the rock mass should be thoroughly interpreted because the spillway tunnel will be pressurized internally. A proper determination of the minimum principal stress should be related to the applied safety factors for hydraulic confinement. The tests indicated that the hydrojacking test is a useful method for determining the hydro-mechanical behaviors of fractures in an unlined pressure tunnel.

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