Modern High-Precision, High-Speed Measurement of Segments and Molds

Abstract

The precision required for the manufacture of molds used in the casting of concrete segments for tunnel linings is often underappreciated. The tender documents for a typical segmentally lined tunnelling project frequently only give the dimensional tolerances for the finished segments. In some cases, however, only the tolerances for a sample build of two test rings is given. Increasingly tight tolerances now included in specifications for segment dimensional accuracy demand a fast, accurate and reliable method for determining these values. Delays in the overall progress of these projects are increasingly considered to be detrimental; consequently geometrical control of the molds now frequently takes place at an early stage in the project, directly at the mold supplier’s fabrication plant. This calls for easily transportable measurement equipment. In addition to checking linear dimensions, the angular properties also need confirmation to fine tune the molds, if and when neces- sary, before any segment fabrication. Should any modification be necessary, the mechanics need to know exactly where to modify, by what amount and in which direction, with verification occurring thereafter. The use of a laser interferometer (or laser tracker, as they are often called) now enables the accurate digitization of surfaces by direct polar coordinate measurement. Measurement to a single spherical retroreflective prism enables the skilled operator to comprehensively measure the full profile of the