Reminder No.2 - Length of Test Interval and Measured Hydraulic Conductivity

Packer testing (water pressure test) is a commonly used technique to measure the hydraulic conductivity of rock mass. In this test, a section of borehole is isolated (single packer/double packers) and water is injected to reach the steady state flow condition for each injection pressure (3 or 5 pressure steps). The results from this test is used to calculate hydraulic conductivity of rock mass between the tested intervals. In all the existing equations from simple Lugeon formulation (Houlsby, 1976) to Heok and Bray (1974) or CanMet (1977), the test interval length has a direct impact on the calculated hydraulic conductivity. For example, Lugeon value is calculated as Lu=10Q/PL (Q in L/minute is flow rate, P test pressure for each test step in bars, and L is the length of test interval, 10 is the correction for standard pressure of 10 bar. 

In the most rock masses, the pre-existing rock fractures are the main controlling parameters on the flow rate in the test section (hydraulic conductivity of intact rocks are relatively low compared to the fractures). Therefore, number of open fractures in the test section would have a significant impact on the flow rate. Now, let's assume the length of test interval is 5 m and we have only one open fracture in the test section taking all the flow rate of Q under total pressure of P, the Lugeon would be Lu=10Q/5P. Then if that single fracture is isolated by a straddle packer with 1 m interval length, the flow rate is still equal with Q under same pressure, but the Lugeon would be Lu=10Q/P which is 5 times higher than the Lu measured from 5 m test interval (impact of nonlinear flow condition on flow rate in the shorter test interval is not considered in this comparison). This fact must be considered for the packer test interpretation, particularly for estimating the groundwater inflow (tunnels, underground mines, rock cuts and open pit) and quality control of dam foundation grouting.