Planning Economical Tunnel Surveys

Economical tunnel surveys require careful advance planning. Methods and criteria for planning the surveys are examined, as are the factors affecting the choice of method to be used. A well-planned basic control system is shown to be essential for keeping survey costs at a minimum. Traverse, triangulation, and trilateration methods are compared and recommendations given for their use. The amount of positional uncertainty which can be tolerated in the tunnel alignment may be determinative in the choice of method, but often other considerations may take overriding precedence. Problems in reproducing the design alignment in the tunnel are greatly simplified by the advance in instrumentation and horizontal accuracies of one part in 30,000 can be obtained with comparative ease and economy when supported by a properly designed basic control system.