CROSS Safety Report
Near collapse of masonry highway bridge
This report is over 2 years old
Overview
Works adjacent to a masonry arch bridge caused the foundations to subside and damage the bridge beyond repair.
Key Learning Outcomes
For the construction team:
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Be aware that before starting any process in the ground; excavation, underpinning, trench digging, piling or as in this case de-watering, consideration must be given to the stability of adjoining structures
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Approval from the local roads authority prior to carrying out works such as in this report should be obtained
Full Report
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A contractor working for a water utility company when laying a watermain along an unclassified road, had installed a new 315mm diameter water main across a masonry arch bridge without prior consent from the local road authority. The new pipe sat directly on the barrel due to the lack of sufficient cover to the road surface.
About 15 months later a second contractor, again working for the water utility company, laid a 750mm large diameter water main approximately 6m from the bridge structure in land adjacent to the bridge structure. This required the ground to be de-watered and it is believed that the process caused the foundations of the bridge to subside which resulted in large cracks appearing in the arch barrel, right through to the road surface following the line of the first water main.
The bridge was damaged beyond repair. It is clear that the water utility company should have contacted the local roads authority prior to installing both the first and the second water mains, and most likely approval would have been refused due to lack of cover to the arch barrel. The water utility contractor/s had wrongly assumed that placing their work on an electronic notification system was sufficient warning.
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Expert Panel Comments
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Before starting any process in the ground; excavation, underpinning, trench digging, piling or as in this case de-watering, consideration must be given to the stability of adjoining structures. There have been numerous cases where excavation has led to the collapse of adjacent structure. Arch bridges rely on thrust support from the abutments to keep the arch in place and the barrel in compression.
De-watering will cause significant ground movement and consequent abutment movement. It is surprising that a public utility did not recognise the consequences of such actions or the need to contact the owners of the bridge. As with other reports in this Newsletter there has been, at some stage in the process, a lack of basic engineering input.