Hydrogeological investigations for tunnels and large excavations routinely use packer tests to estimate the large-scale hydraulic conductivity of the rock mass. The standard five-step method of testing is quite time consuming, which leads investigators to test selectively in order to complete the work within time and budget. In our experience, this is the wrong approach; we see much greater value in testing all the rock drilled, with only one step per test. We are proposing a change in how geotechnical investigations are conducted for rock-tunnel projects in order to provide results with more scientific and engineering value for less time and money.
These proposed changes are based on several thousand packer tests from over a dozen, hard-rock tunnel projects, both in Australia and in the United States. Many of these tests were run with five pressure steps, as described in Houlsby (1976) and many were run with only one step as advocated in this paper. The results are compared statistically and no significant difference was found in how the large-scale hydraulic conductivity was interpreted; the main difference was that the five-step method required about four times the time and money as the one-step method.
In the proposed method, the entire relevant portion of each borehole is divided into stages of equal length (3 or 6 m are typical). No stages are skipped, no matter how solid the rock might appear. Each stage is tested using a single pressure step lasting about 10 minutes. Then the packers are moved to the next stage. The procedure and equipment are kept simple because, in over 90 percent of the tests, sophisticated methods and equipment are not needed. If an excessively high-taking stage is encountered, then there may be value in coming back to that stage later and using more sophisticated methods and equipment.