The restraints from the side fill on a buried pipe influence its deformed shape. It is often very difficult to compact well the side fill in narrow trenches. The trench width to pipe diameter ratio must necessarily be an important parameter in the design of buried flexible pipes. A 2m long, 1.5m high soil box with variable width was used to observe the pipe deformations, pipe strains, normal and shear stresses at the soil-pipe interface of a 2mm thick, 150mm diameter glass reinforced plastic pipe. Instrumentation was also provided for direct measurement of the horizontal and vertical soil stresses at the boundaries of the trench. The soil box test data are complemented with numerical analysis results. The observations showed that a distinctive soil-pipe system is developed when the initial layers of fill are placed on a very flexible pipe (Ring stiffness - EpI/D3 < 4 KPa). The rate of increase of soil load/ radial deformation of the pipe with increasing heights of fill decreases with increasing trench width parameter. In a narrow trench, the proportion of the sharing of the soil load between the pipe and the side fill is increased due to the weakening of the soil arch resulting from the limited soil fill at the springing.


Recent development in flexible pipe manufacture and competition drives pipe manufacturers to develop thinner walled pipes with low cost pipe materials. Hence the more flexible thin walled glass reinforced plastic (GRP) pipes are becoming increasingly popular in the pipeline construction industry. Molin(1981) and Gumble (1983) used numerical modelling techniques to show that in flexible pipe design the effect of horizontal earth loading is a parameter not to be ignored. The performance of the pipe depends not only on the pipe material but also on the interactive soil-pipe system.

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