Thrust Restraint Design of Buried Flexible Pipes

Unbalanced thrust at bends in buried pipelines has historically been resisted by cast in-situ concrete thrust blocks. In recent years methods have been developed that allow the design of buried piping systems without the need for thrust blocks, i.e. pipes and fittings that carry axial thrust and bending moments. Stiff pipes made of isotropic materials behave in a manner, and have enough reserve axial strength, that warrants simplified analytical methods. Because of the low strain levels of such pipes, the effects of displacements and bending stresses can be ignored in many cases. Pipes made of flexible and/or orthotropic materials such as glass fibre reinforced plastics (GRP), however, require more rigorous analysis, to ensure that displacements and stresses are kept within limits. This paper presents a structural analysis method that incorporates the necessary parameters for such a design. The non-linear soil-pipe interaction, both in the lateral and axial directions, is accounted for and the appropriate stiffness matrix for each pipe element is compiled. A solution based on a simple spreadsheet algorithm, is presented.