Post-failure fracture angle of brittle pipes subjected to differential ground movements

Abstract

Centrifuge testing of four glass pipe models has been used to measure the kinematics associated with cast iron pipe fracture and the relative rotation of the pipeline on either side of the fracture when fracture is induced by a normal ground fault. The model pipelines were fabricated at 1/30th scale using solid glass rod of semi-circular cross-section. After placing each model against the transparent sidewall of a test box, the pipe was buried and the tested at 30g. Movements of the soil and the model pipe were monitored using particle image velocimetry, and analysis was used to examine both the curvature distributions before fracture, the fracture pattern (i.e. the fracture locations) expected for cast iron pipes passing across a normal ground fault, and the post-test rotations across the fractures. The depth of soil cover was observed not to make a significant difference to the amount of initial fracture angle for the range of covers investigated in this study (1.125–2.25m in prototype scale). The fracture angle was shown to be well represented by the slope of the pipe displacements at the inflection point. The use of normalized peak curvature to estimate the fracture angle (rotation across the fracture) was demonstrated. The estimated magnitude of rotation after the pipe breaks can then be used to estimate the local stresses and strains that would develop in a polymer liner inserted within the cast iron pipeline before fracture initiates and which is therefore subjected to longitudinal bending stresses after the cast iron pipe fractures, values of ‘demand‘ that can then be evaluated against measurements of liner strength (i.e. its stress and/or strain limits). The initial angle of rotation directly across the fractures after they form is approximately the same regardless of when the pipe initially breaks, and regardless of the soil cover depth and soil density. However, the magnitude of the vertical ground displacement across the normal fault needed to induce fracture varies considerably for the different burial conditions that were considered, and the rotation angle increases steadily after fracture if the amount of vertical ground displacement continues to grow.