The structural design of non-circular linings

The French National project of research and experimentation named RERAU (Rehabilitation of Urban Network Sewers) has developed a design method for non circular linings subject to groundwater pressure. The method is based on an analytical approach and gives a formula, recently obtains by O. Thepot, for the buckling pressure of a close-fit non-circular lining subject to external water pressure (with initial annular gap). The method puts the liners into two categories according to their shapes: "critical linings" that are liable to buckle but where the blister remains localized and "sub-critical linings" that do not buckle but where the blister may extends to the entire lining. For critical lining (most egg-shapes are critical), this paper presents formulas for buckling pressure, bending moment, axial force and deflection. Thus the engineer is enabled to calculate safety factors for stability and strength.