Thrust Restraint Design of Concrete Pressure Pipe

Abstract

A finite-element model of a buried pipeline consisting of concrete pressure pipe segments and joints is developed for analysis of longitudinal effects resulting from pressure-induced thrust at a bend. The model accounts for longitudinal pipe-to-soil friction, soil stiffness against transverse movement of the pipe, tensile softening of concrete, yielding of steel cylinder, and the nonlinear behavior of the joint. The model is applied to the analysis of simple bends designed following the current thrust restraint design procedure. The results show that for the pipes installed in stiff backfill, the current thrust restraint design procedure is adequate, although not with uniform safety factor over the range of all possible bend angles. However, the current thrust restraint design procedure does not protect the pipes installed in soft highly plastic, clayey soils against failure. Based on the results of the finite-element model, a new design procedure is developed that accounts for thrust resistance by a combination of longitudinal soil friction and soil stiffness against transverse movement of the pipe. The results of finite-element analysis of the designs by this procedure show adequate safety over the range of bend angles and soil stiffness values.