Numerical Model for Analyzing Slug Tests in Vertical Cutoff Walls

Analysis of a slug test estimating the hydraulic conductivity of a vertical cutoff wall is complicated by the high compressibility of backfill materials and by the proximity of a well to the edge of the cutoff wall. An implicit finite-difference program, named Slug_3, was developed to analyze results of slug tests in the vertical cutoff wall. The program uses block-centered mesh formulation, considers variable hydraulic conductivity and specific storage, and has automatic time-step control and mesh generation. The geometry and flux -boundary condition in the well-intake section is fully considered, and the interface between a cutoff wall and natural soil formation is modeled as a constant head-boundary condition. Also, a filter cake can be simulated in Slug_3. Slug_3 is verified by comparing results with an analytical solution for a partially penetrating well in aquifers and another numerical code, MODFLOW-96, for a vertical cutoff wall. The program provides a new analytic tool for analyzing slug-test results from vertical cutoff walls and is unique in the ability to simulate variable hydraulic properties, which can be particularly important for highly compressible materials such as soil–bentonite backfill in a cutoff wall.