inset

An inset is a horizontal or near-horizontal passageway driven from the wall of a vertical or near-vertical shaft at an intermediate depth, typically to provide access to ore bodies or work areas without requiring miners to descend the full length of the shaft. It functions as a branch off the main shaft, allowing personnel and equipment to enter and exit at specific levels rather than at surface or at the bottom of the shaft alone.

Insets are driven perpendicular or at an angle to the shaft wall and may connect to stoping areas, development headings, or lateral working zones. The opening itself is usually framed with timber or steel to maintain stability and prevent dislodgement of rock from the shaft wall. In deep mines, multiple insets at different depths allow parallel work on several ore horizons simultaneously, reducing congestion on the shaft and improving ventilation and haulage efficiency.

The term reflects its position: the inset is literally set into the wall, or more accurately, set back into the rock at a point partway down the shaft's depth. Mines with thick ore bodies or multiple mineralized zones will have numerous insets, often designated by depth or level number. The cost of driving an inset is significant, so their location is determined during mine design based on geological mapping and planned extraction schedules.

An inset differs from a shaft sump or bottom, which is the terminal chamber at the base of the shaft. It also differs from a raise, which connects two levels from below upward. The inset is planned and driven as part of shaft development, whereas raises are often secondary connections driven later during stoping.

In hard rock mining, insets into a shaft may experience ground settling or rock pressure after periods of extraction nearby, sometimes causing the opening to tighten or become unstable. Regular inspection and support maintenance are essential, particularly in active mining zones where vibration from blasting or heavy equipment can destabilize poorly installed timber sets or loose rock around the opening.