blind level

A blind level is a horizontal or near-horizontal mine passage driven between two vertical shafts, designed to collect and discharge water from deeper working areas. Unlike an ordinary level that opens to the surface or daylight, a blind level remains completely enclosed within the rock mass, with its only connections to the surface or other workings being the two shafts at its ends. Water enters at the lower shaft, flows through the passage, and rises through the upper shaft, functioning as an inverted siphon to lift water above the level of the lower working zone.

The blind level serves a critical role in mine dewatering, particularly in deep hard rock mines where natural drainage cannot reach isolated wet zones. The passage must be driven with a slight upward gradient toward the discharge shaft to prevent water stagnation, though the gradient is gentler than a standard drainage level because the inverted siphon action itself provides lifting force. The two shafts must be sunk to intersect the blind level at precise depths; miscalculation during the drive means the level will not connect properly or will function inefficiently.

Water enters the blind level through small tributary drives or natural seepage from the surrounding rock. As the water column in the discharge shaft fills and rises, it creates sufficient pressure to push more water up and out, establishing continuous flow. The system works best when the upper shaft reaches into a zone where water can be discharged into a higher level, a natural watercourse, or a pump station. If the upper shaft does not extend high enough, the siphon cannot establish, and the passage becomes merely a shallow sump.

The term blind refers to the level's complete isolation: it has no open daylight portal at either end, only vertical shafts. This distinguishes it from an adit level, which typically emerges as a horizontal entrance at the surface. The inverted siphon principle is the defining mechanical feature; without the siphoning action created by the water column in the discharge shaft, the blind level would merely be a trapped, stagnant gallery.

Blind levels are vulnerable to operational problems if the gradient is miscalculated, if both shafts are not properly surveyed into the drive, or if the discharge shaft blockage or siltation occurs. Maintenance requires access to both shafts for clearing sediment and checking water flow. Modern mines often supplement blind levels with mechanical pumping rather than rely entirely on siphoning, since siphon action is sensitive to water chemistry, temperature, and air locks within the passage.