collar

In underground mining, the collar is the structural framework that sits at ground level around the perimeter of a mine shaft entrance. It serves as both a safety barrier and a load-bearing ring, preventing the surrounding soil or rock from collapsing into the shaft opening and anchoring the head frame, winding equipment, and hoist mechanisms. The collar is essentially the transition point between surface infrastructure and the underground workings below.

Historically, mine collars were built from timber, typically heavy squared logs or railway sleepers arranged horizontally and secured with cross-bracing. In modern operations, collars are constructed from reinforced concrete, steel plate, or a combination of both. The concrete collar may be poured into a prepared pit around the shaft opening and can reach depths of 3 to 10 metres, depending on soil stability and shaft diameter. The collar must be robust enough to withstand the lateral pressure from surrounding material, the vibration from hoisting operations, and the concentrated loads from head gear and conveyances.

Variants and supporting structures

Shaft collars come in various profiles. A simple collar may be a concrete ring; a more complex one incorporates steel channels or H-beams as reinforcement. Some collars include a sump or collar pond, a small basin at the base of the shaft opening used to collect water seeping into the shaft, which can then be pumped out. The collar also typically defines the position of the shaft guides and may have integral points for anchoring the guide rails.

Problems at the collar are common sources of operational delay and safety hazards. Uneven settlement of the collar relative to surrounding ground can misalign the shaft, causing guide friction and conveyance binding. Water infiltration around a poorly sealed collar creates a source of continuous pumping cost and corrosion of equipment. Ground movement or frost action can crack concrete collars, requiring costly repair or reinforcement.

The term collar derives from its function as a literal neck or rim around the shaft mouth, analogous to the collar of a garment. In some regions, particularly older British mining texts, the collar is called the pit head curb or simply the curb. The collar occupies a critical but often underestimated role in shaft stability and is one of the first components inspected during routine mine safety audits.