A risk assessment weighs what could happen on exposure to a hazard and how likely it is, and helps decide how severe a risk is, whether existing controls work, what action is needed and how urgently, and which hazards could cause harm ranging from minor discomfort to serious injury or death. Where a hazard and its accepted controls are well known, a formal assessment is unnecessary and the controls can simply be applied. Factors affecting excavation risk include local site conditions (access, slope, nearby buildings and structures, watercourses above or below ground, trees), excavation depth, soil properties (variable types, stability, shear strength, cohesion, groundwater, weathering), rock fractures and faults (joints, bedding planes, dip and strike, clay seams), specialised plant or methods such as ground support, transport, haul routes and disposal, exposures such as noise, UV or chemicals, the number of people, possible unauthorised access, local weather and how long the excavation stays open.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.