Defects in design, in materials, in specification of materials and workmanship, in standards of workmanship or in any combination of these features-and it is most frequently that mutual incompatibility or misunderstanding lies at the root of a problem-demand rigorous control. The costs of rebuilding or in finding alternative solutions in tunnelling are likely to be high, particularly on account of the linear process of construction and constraints on access to working faces. There is no better safeguard than a continuity in direction of the project by an engineer perceptive to the nature of potential hazards and how they may be anticipated. This, however frequently disregarded, is no new concept. Muir Wood (1975b) states:
The processes of planning, design, construction and maintenance of tunnels are (or should be) closely inter-related. In certain countries, current practices tend towards artificial barriers and discontinuities between certain of these aspects, particularly that of time separation between design and construction; it is not surprising there to find a high frequency of avoidable hazards and frustrated contracts.