Numerical Analysis and Soil Box Study of the Influence of Trench

Gurnble (1983) and Molin (1981) used numerical analysis to demonstrate the significance of lateral earth loading on flexible pipe design. The restraint provided by the stiffness of the side fills on the pipe influences the shape of the deformed pipe. Low trench width to pipe diameter ratios can hinder the process of compaction and consequently the compactness of the surrounding soil. The influence from such effects can not be ignored and accordingly the trench width to pipe diameter ratio must therefore be a necessary parameter in the design of buried flexible pipes. This paper reports observations of pipe deformations, pipe strains, normal and shear stresses on the soil-pipe interface of a 2mm thick, 150mm-diameter GRP pipe buried, in dry sharp river sand contained in a soil box. The width of the 2m long 1.5m high soil box can be varied and accordingly results from tests for trench widths of 350,450, 600 and 800mm are given in the paper. Instrumentation provides direct measurements of the horizontal and vertical stresses at the trench boundary. Observation of pipe ovality and the strains on the pipe wall for different burial depths and trench widths are presented. These soil box test observations are complimented with numerical analysis results from a time marching 3D finite difference package. The effects of creep in changing the ovality of the 150mm pipe buried in a 800mm wide trench observed over a period of 4 months is also presented. This study concludes on the varying ways in which trench width effects the structural performance of buried flexible pipes and illustrates that single wave buckling at the top of the pipe as a most probable mode of failure.