ABSTRACT
Unexpected can mean that an event was never thought of previously. The aircraft and process industries it is now common to classify risks to projects as either unknown unknowns' or known unknowns'. Dalton suggested two categories of choices in preparing to avoid or minimize the risk of hitherto inconceivable events. In the first category may be events for which no reasonable person would prepare. In the second category are those which, if they do occur, should have been anticipated notwithstanding how improbable the eventuality. The weakening of a road traffic viaduct by an industrial waste fire was unexpected in its nature. The amount of propping was greatly in excess of the temporary support occasionally required for maintenance and repairs, so this project can be classified as unexpected in its scale. In some cases the events led to reconsideration of what resources and other preparations should in future be available at short notice for that sort of event.
