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In this study, the effect of pipe-soil relative stiffness on the behaviour of buried flexible pipes was investigated considering the pipe size, material type, stiffness, pipe-soil and natural soil-backfill interfaces and geometry of the trench using the finite element method. For this purpose, a parametric study was conducted to examine the effect of different variables on the resulting earth loads and deformations imposed on the buried pipes. Various types of trench pipe-soil cases were analysed for a certain natural ground and backfill material by the PLAXIS finite element code which allows simulating non-linear soil behaviour, the stages of construction as well as the pipe-soil interaction aspects of the problem. Loads and deformations obtained by the finite element method were compared with those calculated by the conventional approaches for different pipe-soil stiffness ratios. The finite element results obtained for the deformation of typically flexible Polyethylene pipes were then used to back-calculate the range of modulus of soil reaction, E&#039 / , values for various pipe-soil relative stiffness and they were compared with the suggested value proposed by Howard (1977).

Recent developments in constructions have heightened the need for protecting existing buried infrastructure. New roads and buildings may be constructed over already existing buried infrastructures e.g. buried utility pipes, leading to excessive loads threatening their stability and longevity. Additionally applied loads over water mains led to catastrophic damage, which result in severe damage to the infrastructure surrounding these mains. Therefore, providing protection to these existing buried infrastructure against increased loads due to new constructions is important and necessary. In this research, a solution was proposed and assessed, where the protection concept would be achieved through the inclusion process of geogrid-reinforcing layers in the soil cover above the buried infrastructure. The controlling parameters for the inclusion of geogrid-reinforcing layers was assessed experimentally and numerically. Twenty-three laboratory tests were conducted on buried flexible and rigid pipes under unreinforced and geogrid-reinforced sand beds. All the investigated systems were subjected to incrementally increasing cyclic loading, where the contribution of varying the burial depth of the pipe and the number of the geogrid-reinforcing layers on the overall behaviour of the systems was investigated. To further investigate the contribution of the controlling parameters in the pipe-soil systems performance, thirty-five numerical models were performed using Abaqus software. The contribution of increasing the amplitude of the applied cyclic loading, the number of the geogrid-reinforcing layers, the burial depth of the pipe and the unit-weight of the backfill soil was investigated numerically. The inclusion of the geogrid-reinforcing layers in the investigated pipe-soil systems had a significant influence on decreasing the transferred pressure to the crown of the pipe, generated strains along its crown, invert and spring-line, and its deformation, where reinforcing-layers sustained tensile strains. Concerning rigid pipes, the inclusion of the reinforcing-layers controlled the rebound that occurred in their invert deformation. With respect to the numerical investigation, increasing the number of the reinforcing-layers, the burial depth of the pipe and the unit-weight of the backfill soil had positive effect in decreasing the generated deformations, stresses and strains in the system, until reaching an optimum value for each parameter. Increasing the amplitude of the applied loading profile resulted in remarkable increase in the deformations, stresses and strains generated in the system. Moreover, the location of the maximum tensile strain generated in the soil was varied, as well as the reinforcing-layer, which suffered the maximum tensile strain. / Government of Egypt