In the past, several empirical and analytical solution statements have been developed for defining the critical buckling pressure of pipes of circular cross section under external pressure [Guice94d]. Fundamental links to the buckling behaviour of the free pipe were presented by Timoshenko in [Timos61]. Building on this, Gaube [Gaube74] described the particular failure behaviour of bedded plastics pipes by means of semi-empirical statements. Glock [Glock77], Cheney [Chene71] and Chicurel [Chicu68] provided a description of the stability failure of an uncoated gap-free pipe under external pressure, temperature or shrinkage loads. Furthermore, Lo and Zhang [Lo94] also investigated the influence of gap formation between coating and liner on the buckling problem. Falter [Falte94a] presented a method of calculation that, starting with the Glock solution, also accounted for the formation of gaps as well as the pre-deformation on the basis of pre-factors which arose from rigid frame calculations. Wagner [Wagne92] developed a purely empirical sizing statement from results of investigations on liners with special emphasis on the influences of gaps. All the statements provide more or less good approximations of the actual buckling behaviour and assume various simplified assumptions.