A trench blast is the most confined work in commercial blasting. A production bench has a free face standing open in front of it. A trench has only the ground surface above, so the rock has almost nowhere to go, and every kilogramme of explosive is asked to do considerably more work. That confinement dictates the entire design. - Powder factor climbs to roughly 1.5 to 3.0 kg per cubic metre, against 0.3 to 0.6 on an open bench - Hole diameters stay small, generally 32 to 76 mm, with burden and spacing frequently under a metre - Subdrilling of about 0.3 times the burden carries the break below grade so the floor lifts flat rather than humped - A stemming column of angular crushed aggregate, roughly one burden in length, confines the gas long enough for it to work Sequencing matters more than charge mass. Detonation drives a compressive wave through the rock at 3,000 to 6,000 m/s. That wave reflects off the free surface and returns in tension, and rock fails in tension at roughly a te...