Analysis of a 60-In. PCCP that Failed without Warning from Acoustic Fiber Optic System

Abstract

Listen

Translate article

1.0 ABSTRACT: The consequences of failure for Prestressed Concrete Cylinder Pipe (PCCP) are usually very high due to its large diameter and high internal pressure. To monitor these critical pipes, acoustic fiber optic (AFO) monitoring systems are now being used to observe the real-time breakage of prestressing wires in the pipe. However, if PCCP is not adequately prestressed, the wire break does not generate obvious acoustic noise that the AFO system is able to capture. The “un-recorded” wire breaks or inadequate prestress, together with other defects (such as corroded cylinder), can cause the failure of the pipe. This paper presents the results of an analysis conducted for a PCCP with inadequate prestress that failed without warning from the AFO system. The findings of this research will improve the current understandings of PCCP with inadequate prestress and help utilities better manage PCCPs with similar conditions. A 60-inch PCCP within the Washington Suburban Sanitary Commission (WSSC) jurisdiction failed without warning from the installed AFO system. This failure was unanticipated to because the number of broken wires recorded by AFO system was not severe enough to cause pipe failure under normal conditions. This failure also revealed the limitation of this technology in PCCP management, as the number of broken wires recorded is not enough to evaluate the pipe condition. Some other critical factors can also cause the failure of the pipe. Forensic investigations showed that the pipe had inadequate prestress in the failure area and that the water corroded the wires and cylinder which AFO system cannot detect. In this paper, a finite element (FE) model is developed to analyze the performance of the pipe with inadequate prestress in the prestressing wire. Scenarios such as prestressing wire with full prestress, no prestress, no prestressing wires, and with 6 broken wires are simulated to better understand the structural integrity of the pipe. The model considers loadings such as internal pressure, the weight of soil and pipe. It also considers pipe components such as concrete core, steel cylinder, prestressing wires, saddle steel, and mortar coating.