Looking for Clues: Hamilton Demonstrates Value of Condition Assessment Program During Investigation of Critical Watermain
In late 2015, the City of Hamilton, Pure Technologies (Pure) and Robinson Consultants worked together to perform an external investigation of a pipe section in the Woodward Greenhill Transmission Main (WGTM) to gain understanding of what was causing degradation in a specific geographical area.
A 2014 condition assessment performed by Pure had identified a cluster of damaged pipes on the WGTM, which is comprised of prestressed concrete cylinder pipe (PCCP). Pure used electromagnetic (EM) inspection technology to identify which pipes in the WGTM had broken prestressing wires, a sign of deterioration in PCCP. When prestressing wires break, the pipe loses compression. When enough wires break the pipe becomes at risk of failure.
Understanding the root cause of why pipes deteriorate
Pure’s condition assessment of the WGTM included finite element analysis (FEA) to determine the number of broken wires a pipe can safely operate at given the pressures of the pipeline. Although each pipe reported with broken wires had total wire breaks well below the threshold, and not in any immediate need of rehabilitation, further investigation of the pipeline was undertaken to understand the root cause of the degradation.
The City of Hamilton’s pipeline management plan is comprehensive, going beyond identifying and managing damaged pipe sections. By understanding why the pipes are deteriorating, expectations on future degradation can be made.
The investigation included testing the soil and mortar to determine if aggressive soil conditions were contributing to the distress, and confirming the pipe properties and actual number of wire breaks, factors key to the structural analysis of the distressed pipe.
External EM scan confirms correct pipe excavated
On October 20, 2015 Pure performed an external investigation on one of the pipes identified with wire breaks. Pure’s external investigation included an external EM scan to confirm the correct pipe was excavated. This was accomplished by comparing the data from the external EM scan to the data from the internal EM data collected from the 2014 inspection. The comparison confirmed the correct pipe was found.
The next step was an external and visual sounding inspection of the exposed pipe. The mortar was inspected for cracks, spalls, corrosion staining and other signs of distress that would indicate advanced deterioration. None of these signs were found.
Process for verifying number of broken wires
The process of verifying the number of broken wires on PCCP involves removing an approximately 2-inch wide strip of mortar across the length of the pipeline to expose the prestressing wires, and using a multi-meter to measure the electrical resistance from wire to wire (this process is specific for PCCP without shorting straps). If an electrical discontinuity is identified, it confirms that a break exists between the two points of contact. Upon completion of the tests, the mortar is patched to protect the wires and prepare the pipe for burial.
The electrical continuity measurements on the exposed pipe section in the WGTM confirmed the two regions that were identified by the electromagnetic inspection. No other wire breaks were found across the pipe length.
Results used to recalculate FEA analysis
The results of the investigation were used to recalculate the FEA analysis, adding to a slight increase in the number of wire breaks that the pipe can withstand under pipeline pressures. Ultimately the findings concluded that the pipe can remain in service with no repair or changes to operating pressures.
The City of Hamilton’s condition assessment program for PCCP pipe is an example of their comprehensive approach to pipeline management. Beyond locating the damage, the City strives to understand the root cause. The use of the EM inspection technology allows the City to pinpoint damaged pipe sections. FEA analysis provides a means for determining if a pipe requires rehabilitation.
For this project, further investigation through external testing will provide insight into the root cause of pipe deterioration (the results of the soil and mortar testing will be presented by Robinson Consultants at a future date). This project showed how collaboration between the City, Pure Technologies, and Robinson Consultants resulted in a comprehensive condition assessment of one of Hamilton’s critical watermains.