New Approach to Design of Circular Liner Pipe to Resist External Hydrostatic Pressure

Abstract

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. Introduction Current methods for structural design of flexible gravity sewer liners, as typified in North America by the Appendix X1 to ASTM F1216 (1998), generally consider two external load cases for dimensioning of the liner pipe. The first is sustained hydrostatic pressure due to groundwater acting in the annular space between the liner and the host pipe. Since gravity sewers in need of renovation invariably leak, this load case applies regardless of host pipe condition or whether the aim of the rehabilitation is primarily structural or simply to provide a barrier against internal corrosion or exfiltration. Even where the permanent groundwater table is below pipe invert level, the liner must generally be designed to resist a short-term hydrostatic head which could arise under storm conditions. The second load case supposes that earth and traffic loads will in due course be transferred from the existing pipe-soil structure to the liner pipe. The likelihood of this occurring is assumed to be a function of the condition of the sewer at the time of lining. In the great majority of practical situations, however, little or no such load transfer ever takes place because the existing Technical Director, Insituform Technologies, Dolphin House, St. Peter Street, Winchester SO23 8BW, U.K.