Analysis and Design of Buried Flexible Pipes.

This thesis describes the development of a new, rational design method for buried pipe based on the soil-structure interaction principle. The method bridges the traditional division between "rigid" and "flexible" pipe behaviour, and takes proper account of the influence of horizontal earth loading which is either misrepresented or neglected altogether in many existing design methods. A unified theoretical model is developed to predict the stresses, deformations and buckling response of the circular buried pipe cross-section. First-order response equations drawn from elastic continuum theory are reinterpreted and extended to take account of the non-linearity of large deformations and the interactions between out-of-roundness, yield and elastic buckling of the pipe wall. Detailed consideration is also given to the effects of initial out-of-roundness, creep and thermal movements of the pipe, and non-homogeneity and settlement of the surrounding soil. The analytical results are presented as a series of design charts which in their general form are valid for pipe diameter/thickness ratios greater than 20 and depth of cover to the crown of at least one diameter. Additional formulae and charts are developed to extend the method to thicker pipes and shallower cover depths, subject to certain restrictions on the form of loading. The comprehensive theory is used as a framework to reinterpret a large quantity of existing experimental data from which provisional design values of backfill stiffness and lateral pressure ratio are derived as a function of soil type and degree of compaction. Simplified procedures are also developed for calculating equivalent loading due to backfill weight, surcharges and both external and internal fluid pressures. Finally a wide-ranging programme of new experimental work serves to validate the main elements of the proposed ring deflection and buckling theory, and to evolve efficient monitoring techniques for the field and laboratory determination of arching factors, equivalent moduli, lateral pressure ratios and locked-in compaction stresses in buried pipe backfill