tights

A tight is a column or pillar of intact rock that remains within the intended blast zone after detonation. It happens when explosive charge distribution is uneven, detonation timing misfires, or geological structure (bedding planes, joint sets) creates a pocket of rock that does not fracture as planned. The tight sits surrounded by broken ore or waste that has fragmented properly, making it stand out visually on the blast face.

Tights are common in open pit and underground mining when rock strength varies across the blast block. A strong competent zone, a missed hole, or insufficient powder factor in part of the pattern can leave these pillars. In hard rock mines working granite, basalt, or massive sulfide deposits, tights appear frequently. In softer rock like coal seams or weak sedimentary strata, they are less common because the entire block tends to break uniformly.

Cost and workflow impact

A tight must be removed before mining can continue. The broken rock around it cannot be cleanly separated from the standing pillar, and the tight blocks mucking (loading and hauling). Removal requires a follow-up blast, secondary breaking with hydraulic hammers, or occasionally hand tools depending on size and access. Each method adds time and cost to the mining cycle. On a per-tonne basis, secondary work to clear tights can add 5 to 15 percent to blast cycle time in poorly designed patterns.

Blast engineers reduce tights by increasing hole density, improving powder distribution, optimizing initiation sequences, and adjusting powder factor based on rock type. Presplit or trim blasting techniques, in which a precisely spaced, lightly loaded row of holes contains the blast boundary, also limit tights at pit walls. Geotechnical logging before blasting helps predict where tight-prone zones lie.

The term originates from the tight, cohesive state of the unbroken rock, which grips together and resists separation. It is used primarily in surface mining and underground metal and hard rock extraction, less commonly in coal mining or tunneling where the language tends toward other descriptions.