Benching or battering excavation walls is a fairly simple way to control collapse. A slope counts as safe where the ground is stable: it does not flatten over a long period, no material moves down it, and its toe stays put. Without shoring, ongoing safety depends on conditions during construction; if they differ from those expected or change, for example different soils or heavy rain or flooding, action should be taken at once to protect workers, others and property, such as stopping work for a time until the ground stabilises, or installing positive ground support.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.