Volume 26, Issue 3 (September 2003)
Variable-Head Field Permeability Tests in Driven Flush-Joint Casings: Physical and Numerical Modeling
Two types of variable-head permeability tests in driven flush-joint casings, the end-of-casing test and the lateral injection test (Lefranc test), were performed in two sand tanks. Hydraulic conditions in the sand tanks included a constant vertical hydraulic gradient that was null, positive, or negative. The gradients were monitored by lateral piezometers, which confirmed that the large-scale hydraulic conductivity (k) value of the sand layer remained constant throughout the test program. Any interpretation method for these variable-head tests implicitly requires an assumed piezometric level (APL) for analysis. According to a common practice, the elevation of the ponded water surface above the sand deposit was taken as the APL to plot the test results as the logarithm of the difference in total head versus time. After each casing installation, permeability tests were performed for the three gradient conditions. When the vertical gradient was null in the tank, the test data provided straight lines as assumed in theory. When the gradient was positive or negative, the test data provided curved graphs that may be interpreted as giving k values that vary with time. The velocity graph method, however, provided the same constant k value for the three gradient conditions. It also detected the error in the APL and provided in all cases a local piezometric level (PL) equal to that given by lateral piezometer monitoring. Thus, the velocity graph method, a graphical representation of the conservation equation, eliminated the potential for misanalysis of variable-head tests. The experimental k values, as determined by the two types of tests, usually ranged between 0.5 and 2 times the larger-scale values determined independently by the monitored flow rates and gradients in the sand tanks. The variability in experimental k values was partly due to the local variations of void ratio and effective diameter of the sand, and partly due to the influence of casing installation. Numerical simulations of the tests confirmed the validity of the velocity graph method for providing k and the local piezometric level for a test.