CROSS Safety Report
Culvert roof slab failure
This report is over 2 years old
A culvert, which carries a river under a busy main road, partially collapsed.
Key Learning Outcomes
For asset owners:
Regular inspection and maintenance can help to keep an asset safe and help to identify any obvious safety issues that may need to be addressed
Good record keeping of inspections can assist with future inspections and maintenance
For the construction team:
When making alterations or repairs be made aware of the potential structural consequences of your actions
It is good practice to carry out your own inspections and assessments of aging infrastructure elements before disturbing them
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A culvert, which carries a river under a busy main road, has an internal span of 2.4m and headroom of 1.0m. The reinforced concrete roof slab was 300mm deep and had, from the time of its construction, four 120mm metal service ducts cast into it. Some years later two additional ducts were cut into a channel on the top of the slab. There had been a regime of inspections and key dates are:
1989 inspection satisfactory
1995 inspection found some damage due to the installation of new cable ducts which was repaired
2001 inspection generally satisfactory
2007 inspection found that a section of the roof had partially collapsed (Figure 1)
Learning Points (from the reporter):
The inspection regime was helpful in that problems were identified in 1995 and 2007 which allowed repairs to take place before a critical collapse in which a vehicle could have fallen into the culvert causing danger to both people and property
The damage on both occasions was only seen by an internal inspection of the structure, thus suggesting that inspections should always be as complete as possible
Damage can be done to a structure and not display itself, or affect a structure's serviceability for a substantial period of time
Any structure which is known to have been damaged must be monitored closely thereafter
The roof slab did not have a red-sand asphalt carpet over its top surface. If it had this might have alerted contractors that they were working over a structure and prevented damage to it.
Whilst demolishing the slab it became apparent that the only reinforcement was the longitudinally spanning tension bars. There was no transverse reinforcement and no reinforcement on the top of the slab. The bars were unevenly spaced, particularly where they had been grouped together so as to not clash with the original ducts.
The presence of the newer ducts evidently impacted adversely upon the performance of the slab. The probable cause of the failure of the slab was that at some time the slab had cracked but as this was not visible it was not repaired. Over time and under loading, the cracks opened up allowing ingress of water causing corrosion to the reinforcement.
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What is also concerning is that the Utilities should have been issuing NRSWA notices to the Highway Authority. They should have then identified that the route included a structure and classified it as an engineering difficulty requiring additional caution and (sometimes) submission of method statements to the Highway Authority for approval.
The extent of surveys is not always clear and here there are several points worth highlighting: good records must be kept, the relevance of defects must be understood, and the ownership of, and responsibility for, structural modifications must be clear. Those making alterations or repairs must be made aware of the potential structural consequences of their actions. It is important that access is available for inspection purposes. Utility companies should make their own inspections and assessments of aging infrastructure elements before disturbing them.