ABSTRACT
Hydraulic fracturing and hydraulic jacking tests were performed in a series 18 to 24 meter deep holes drilled in a pressure tunnel. Hydraulic jacking is a variant of the standard constant pressure test (also known as packer test or Lugeon test) where the injection pressure range is sufficiently high to open the fractures in the rock. Hydraulic jacking has been used on (1) 1.5 to 12 meter straddled sections containing natural fractures, and (2) as a final pressurization cycle for hydraulic fracturing tests. The jacking pressure, defined as the pressure at which the pressure-flow relationship becomes non-linear, provides a lower minimum stress estimate (by about 0.7 MPa) than pressure transient methods. Hydraulic jacking is an attractive method for estimating minimum stress values particularly in fractured rock masses where hydraulic fractures are likely to intersect open, natural fractures close to the borehole.
1 INTRODUCTION
This paper discusses hydraulic fracturing and hydraulic jacking results from a program of measurements for a hydroelectric project using pressure tunnels in the mountains of Northern California. "Hydraulic jacking" is defined here as an injection test performed in multiple fixed pressure steps at pressures both above and below those necessary to open the fractures in the rock. The greatest potential for water loss from the tunnels may be in fractures opened by water pressure, hence the focus of the testing program was to determine the fracture opening pressures through stress measurements and water injection tests. This paper points out some of the similarities of the hydraulic fracturing and hydraulic jacking approaches, and suggests the hydraulic jacking approach may be incorporated in the hydraulic fracturing as an attractive alternative for determining minimum stress values. Discussion of results focuses primarily on the interpretation of minimum stress values, as these are of greatest concern for the design of the pressure tunnels. The hydraulic fracturing and hydraulic jacking tests are particularly appealing for supporting the design of pressure tunnels. The hydraulic fracturing and hydraulic jacking tests not only provide estimates of the stress, but also provide a direct simulation of fracture opening that could occur in an unlined pressure tunnel. The tunneling project involves diversion of surface water into a series of tunnels leading into a shaft. From the shaft the flow proceeds along a pressure tunnel leading to the power house. The stress measurements reported in this paper were performed in the lower pressure tunnel. The measurements support the design of the liner for the pressure tunnel. The liner limits or eliminates water loss from the tunnel. Aside from major fracture zones, the primary source of water loss comes from the effect of water pressure opening existing fractures or creating new hydraulic fractures. In either case, the water loss is controlled by the in situ stress. Where the minimum stress is sufficiently high, tunnels may be left unlined or lined using concrete. A steel lining may be used where the in situ stresses are insufficient to contain the water pressure in the tunnel. The minimum stress may be estimated from the thickness and density of the rock overlying the tunnel.
