Soil-structure Interaction Associated with Buried PVC Sewers with Vertical Risers

Abstract

The design of service connections to deeply buried sewers involves a number of challenges. In practice, the loads that develop from vertical risers can damage the Tee or Wye fitting to which it is connected. This thesis studies the expected loads and resistance of these connections, and provides some recommendations for the solution of this engineering problem. Laboratory tests have been performed to explore the capacities of the existing fittings both in air and when buried in uniform sand. A test procedure different from the standard quality control test methods described in ASTM F1336-02 is used to study the performance of the fittings in air. A special test configuration was also developed for an existing pipe test cell to explore the capacities of the existing PVC Tee and Wye fittings when buried in uniform sand. An analytical formula analogous to pile downdrag and numerical analyses have been used to evaluate the test results, to calculate the capacities of the buried heavy-wall fittings, and to explore the downdrag forces that develop along vertical risers. Through comparisons with experimental measurements, it was demonstrated that these methods of analysis can be used to estimate the downdrag forces and determine the adequacy of specific fittings to resist those forces. The major conclusions drawn for the specific fittings tested in this project are summarized as follows. When axially loaded in air, both Tee and Wye fittings experience plastic yield failure. When buried, the Tee fitting fractures or yields only along the base of the riser part; while the Wye fitting itself does not fail, the riser cracks near its base where it connects to the curved pipe (450 elbow) above the Wye. Both the capacities and stiffness of the buried fitting system (either Tee or Wye) are approximately linear functions of the confining stress supplied by the surrounding soil. The accumulated downdrag along the riser in the coarse-grained soil is much smaller than that in fine-grained soil. Various practical solutions for the vertical riser problem are then discussed and recommended.