Behavior of Large Diameter Buried Flexible pipes under Surface Live Loads

Abstract

Surfacc load due to vehicular traffic influences the performance of shallow buried pipcs. This thcsis prcscnts an invcstigation of the behavior of buried flexible pipes due to surlacc load. Two-dimensional finite element analysis was performed through idealization of the concentrated surface load as an equivalent line load. A general purpose f1nite clement program ABAQUS was used for modeling of the pipe-soil intcraction under surface loads. Finite element model was developed and evaluated using full scale test data of buried flexible pipe from the literature, where responses of buricd flexible pipe were measured under surface live loads. Through a comprehensive study on different parameters of buried structure under different burial depth, it was observed for large diameter flexible pipe that influences of surface loads are localized within a zone around the pipe crown for shallow buried pipes «0.5D) and for pipes with low material modulus. However, the influence extends downward covering the full pipe circumference for deeper pipes and pipes with high material modulus. For a particular pipe, the effects of the surface load reduced rapidly with the depths of soil cover up to a depth of half of thc pipe diametcr, beyond which the effect reduced steadily. The conccntrated sur/ace load induces compressive wall thrust and negative bending moment (outward concave bending) at the shoulder, and positive bending moment (outward concave bending) at the crown and invert. The study revealed that the effects of the concentrated surface load depend on the burial depth, pipe material, and geometry of the pipe wall. Although sectional area of pipe wall do not affect largely, moment of inertia of pipe wall affect the thrust and moments that develop around the pipe. The effects are very significant on the' dcvelopment of bending moment. Material modulus of pipe also affects the thrust and moment developing around the pipe circumference. However, the influence is small on the thrusts, while the bending moment is significantly affected. The maximum momcnt induccd duc to surface load can be expressed as a function of the relative bending stiftlless of the pipe-soil system. The mechanism of the stress development around the pipe was found different for shallow and deep burial Vll •.••••••..••.. -.41 conditions lor High-Density polyethylene (HOPE) pipe. Boussinesq solution always over-predicted the crown level stress Jor HOPE pipe. However lor the stiffer (steel) pipe, Boussinesq equation under predicted the stress. These are due to development of arching from soil-pipe interaction that is not captured in lloussincSl(s cquation. AASHTO. AseE codc yielded conservative valucs of the average soil strcsscs lor pipcs with greater burial depths.