Effect of Backfill Erosion on Moments in Buried Rigid Pipes

The service life of rigid sewer pipes is often controlled by joint integrity. Leakage at the joint can result in ingress of water and the development of a void where soil has been eroded from beside the pipe. The influence of soil voids on the stability of buried rigid pipes is investigated, considering the effects of void size, void location and void shape. A series of simplified void geometries are defined, and elastic and elastic-plastic finite element analyses are performed to study how those voids influence bending moments in the rigid sewer. Erosion voids at the springlines lead to increased bending moments at crown, invert and springlines. Elastic analysis indicates that the bending moments approximately double once the void contacts the external surface of the pipe over a 90 degree arc. If shear failure is included and the loosened backfill is modeled, the moments approximately triple for that void geometry. In contrast, a void under the invert leads to decreases in the magnitude of bending moments, and for large void size, the moments can reverse sign. The moment increases slowly when a void starts to grow at the springline, but these moment changes accelerate once the void contacts the pipe over a 45 degree arc. This preliminary study suggests that efforts to arrest the growth of erosion voids at the springlines should be undertaken before the voids reach this size. All results presented are theoretical in nature, and physical testing is needed to evaluate the performance of these calculations.