Method of repairing buried sewage pipes

A method of patching up from within at least one selected portion of an at least partially buried conduit wherein the at least one selected portion exhibits at least one crack or analogous defective zone, comprising the steps of impregnating a mat containing at least one layer of fibrous material with a hardenable plastic compound;

• convoluting the impregnated mat around a radially expandable and contractible carrier having an external surface, to form a tubular mat shell having overlapping portions, an internal surface and first and second axial ends and to permit frictional engagement between the external surface of the carrier and the internal surface of the shell;
• establishing friction between the external surface of the carrier and the internal surface of the shell by introducing a polyethylene film therebetween before expansion of the carrier so that radial expansion of the carrier entails simultaneous radial expansion of the shell and slippage of the overlapping portions relative to each other;
• introducing the carrier, the polyethylene film and the shell into the at least one selected portion of the conduit;
• expanding the carrier, the polyethylene film and the shell in and against the at least one selected portion of the conduit to cause (a) flow of the hardenable plastic compound out of the mat into the at least one crack or analogous defective zone and (b) rotation of the carrier relative to the conduit due to pronounced friction between the external surface of the carrier and an adjacent internal surface of the polyethylene film, which polyethylene film constitutes a coupling which prevents turning of the carrier relative to an adjacent turn of the shell;
• contracting the expanded carrier to cause separation of the contracted carrier from the expanded shell;
• withdrawing the contracted carrier and the polyethylene film from the shell;
• and causing or permitting the hardenable compound to set thus ensuring reliable sealing of the at least one crack or analogous defective zone and ensuring reliable adherence of the expanded mat against the at least one selected portion of the conduit, wherein the convoluting step includes converting the impregnated mat into a shell having an axial length such that the carrier extends beyond the axial ends of the shell, and wherein the convoluting step further includes converting the film into a tubular body which surrounds and extends axially beyond the carrier.