The basic mechanisms of restrained hydrostatic buckling of nominally close-fitting, circular sewer liners are described, and the origins and limitations of current design formulae based on simple linear ring bending theory are explained. A fully consistent, non-linear theory, already the subject of several years of academic research in Europe and North America, is introduced as the basis of a new, practical approach for dimensioning liner pipe. Simple design charts and algebraic formulae are developed to reflect the important influence on buckling pressure of liner ring compressive strains and separately identified "characteristic" (i.e. renovation technique related) and "system" (host pipe related) imperfections. It is shown how the incorporation of these features enables the new theory to be applied with much greater confidence to a wider range of renovation techniques and liner pipe dimension ratios, and to provide the basis for better targeted product characterisation and proving tests. The predictions of old and new design theories are compared, circumstances in which there is significant divergence identified, and the extent of experimental justification of the new approach in such cases briefly reviewed.
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