Steel-lined pressure shafts - appropriate design for external pressure

During inspection work, when the pipe is empty, a thin-walled steel pipe is susceptible to shell buckling due to external pressure. Comprehensive research activities, including tests, began in the 1960s, and analytical and empirical formulae were developed for design.
Some of these different analytical design concepts are presented in the first part of the paper, and their results are compared for the whole range of pipe slenderness in practice.
Afterwards, the results of extensive numerical calculations are shown. These deal with different imperfections in practice and are not considered in the current design solutions. In addition, the effect of the radial flexibility of the surrounding rock mass has been studied - an issue ignored up to now. All these results are compared with the design formulae in practice.
It is therefore possible to conclude the paper with an appraisal of the individual solutions in practice and sum up appropriate design rules for external pressure.