CROSS Safety Report
Failure of precast concrete 'L' shaped retaining wall
This report is over 2 years old
A recent incident occurred on a site, where a temporary precast retaining wall failed leading to a pile of clay approximately 3m high spilling through the wall.
Key Learning Outcomes
For construction professionals:
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
A suitable risk assessment is vital especially where there is the possibility of a small failure leading to major consequences, such as a train derailment
For civil and structural design engineers:
Consider what reasonably foreseeable loads could be applied beyond the code minimum values on safety critical elements such as retaining walls adjacent to railway lines
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A recent incident occurred on a site, where a temporary precast retaining wall failed leading to a pile of clay approximately 3m high spilling through the wall. The fill was fortunately prevented from falling further by a site boundary hoarding. This was located immediately adjacent to an operational railway, and had it failed the hoarding and the fill could have fallen into the path of an oncoming train, potentially injuring railway staff and customers.
The reporter says that failure was at the bend in the L shape, suggesting that the overturning moments caused by the clay back-fill exceeded the capacity of the precast units. The applied moments could have temporarily exceeded the design moments due to the recent periods of sustained heavy rainfall leading to saturation of the stored clay material. The reporter says that similar incidents have occurred on other sites.
Expert Panel Comments
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There have been failures over the years where temporary stability has been inadequate. Here, whether the designer was fully briefed, or whether the design limitations were spelt out, but ignored, we do not know.
Both are vital to a safe design as is a design check. Again, a suitable risk assessment is vital especially where there is the possibility of a small failure leading to major consequences, such as a train derailment. The Health and Safety Executive publication: Preventing catastrophic events in construction (2011) examined these ‘low probability but high-consequence’ safety hazards by looking at:
The types of catastrophic event which have occurred, or which might occur during construction
The reasons for occurrence when there have been (or could have been) catastrophic events during construction, including an examination of the underlying factors
The controls which should contribute to an avoidance of a catastrophic event and where the UK construction industry could improve
Sometimes structural elements simply topple under unexpected load. One case involved tall slender I bridge beams that fell over under wind before a concrete deck was added. In another, in a shopping centre, something being temporarily leant against a wall just toppled over and killed a child. CROSS has reported on posts being lifted into position and at risk of toppling before being fixed to holding down bolts.
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