These media include compressed air (Mair 1979, Calvello & Taylor 2000) and chemical solids such as Styrofoam (Sharma et al. 2001). In the last case, the removal is effected by chemical solvents.

In this study, the medium used is heavy zinc chloride solution. Zinc chloride solution has been used in experiments related to other phenomena (Lade et al. 1981, Mair et al. 1984, Bolton & Powrie 1987). The other two options, involving compressed air and Styrofoam, were also considered but were not selected owing to the difficulties discussed below. In the case of compressed air, the pressure of the compressed air is uniform throughout the tunnel. Thus, if the air pressure is selected to equilibrate the specific level within the tunnel, then there will be some over-pressure at the shallower regions of the tunnel and under-pressure at the deeper regions. For instance, if the compressed air pressure is selected to equilibrate the overburden pressure at the spring line of the tunnel, then there will be over-pressure at the crown and under-pressure at the invert. Thus, compressed air pressure cannot completely equilibrate geostatic overburden pressure at all points at the same time.