Collapse of brick arch overbridge during demolition

A reporter describes a project where the steel girder centre spans of two multi-span bridges over rail lines were to be demolished and re-built. Both bridges were of similar construction; a steel girder centre span supported by flanking brick arches. Brick jack arches spanned transversely between the girders of the centre spans. The brick piers and arches of each bridge are believed to date back to the 19th century, whilst the steel centre spans were of later construction.

The planned reconstruction involved removing the steel centre span of each bridge and replacing it with a new concrete beam structure; the existing brick substructure and arches would be retained and refurbished. Shortly after work began to break out the brick jack arches between the steel girders, a crack was observed in one of the supporting brick piers. All work on this bridge was immediately stopped and the on-call engineer attended site. 

In the meantime, similar demolition work continued at the other bridge and had reached the extent where the steel centre span had been completely removed. Shortly after the steel centre span had been removed, one of the flanking brick arches collapsed. Fortunately, there were no injuries. As a result of this incident, and due to concerns regarding the stability of the remaining structure, all of the flanking arches and supporting piers on both bridges were subsequently demolished, leaving only the abutments in place.

According to the reporter, the underlying causes for this incident include:

Many of those involved in the project had worked together on previous projects and knew each other well. As a consequence of this, activities seem to have been undertaken as a matter of routine, with a confident assumption that “everybody understood what was required”.

The design history of the bridges was not adequately understood, and the condition of the structures was not fully assessed prior to the planning of the demolition work. Section 7 of BS 6187:2011 Code of practice for full and partial demolition states that understanding the state and condition of the structure is “fundamental to the completion of a successful project”, and contains detailed guidance on how this understanding should be gained. Section 9 of BS 6187 discusses structural condition in the context of avoiding instability

In accordance with the CDM Regulations 2015, demolition should be carried out in a manner that prevents risk or reduces risk to the lowest reasonably practicable level. The method during this demolition relied on the positioning of plant and removal of fill over the flanking brick arches as a means of avoiding instability once the centre span was removed. This method had a high level of residual risk.

Once the demolition contractor had submitted a preferred method of demolition, all efforts were focused on demonstrating that this method was workable. There seemed to have been no objective consideration as to whether this was the most appropriate or safest method, or whether it respected general engineering principles, or the good practice set out in BS 6187.

Throughout the design development and review, no fundamental challenge was made to the method of demolition. Additionally, during the checking of the calculations, it was identified that the arches would not support the weight of an excavator unless all four rings of the brickwork arches were intact. This knowledge, and the narrow margin of safety, were not communicated to the principal contractor, the client or the demolition contractor. As a result of this, the level of residual risk was not understood on site.

The formal design review of the demolition methodology was only undertaken in the last few days before work commenced and was fragmented between various individuals. The review focussed on the detail of what had been submitted and did not challenge whether the method being proposed was actually appropriate for a structure of this type.

The management of the works on the night of the incident fell below the standard required to ensure that the planned method of demolition could be delivered safely. As a result of this, the stability constraints specified by the designer were violated, increasing the risk of structural damage and failure.