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CROSS Safety Report

Stability of existing structures impacting construction sites

Report ID: 693 Published: 1 January 2018 Region: CROSS-UK

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Overview

A reporter discusses the clients and principal contractors duties regarding the stability of existing structures on site, particularly those scheduled for demolition.

Key Learning Outcomes

For clients and construction professionals:

  • Good collaboration can ensure the relevant information is provided to the principal contractor / contractor to allow them to develop the Construction Phase Plan and a safe system of work

  • Where demolition works are required it is good practice to provide knowledge and history of the structure, including form, materials of construction, structural interactions, and location

  • If this information is not known, consider specifying compliance with BS 6187:2011 in the tender documents to create an item (including assessing stability) that the principal contractor / demolition contractor can cost for and deliver on

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A reporter discusses the client’s and principal contractor’s duties regarding the stability of existing structures on site, particularly those scheduled for demolition. There is no regularly accepted procedure for principal contractors to assess in detail the stability of structures when they take possession of a site, says the reporter.

If an existing structure is so unstable as to represent a risk, then should there not be a duty on the client to point this out to the incoming principal contractor? Duties in regard to other types of existing hazard attract reference in the CDM Regulations 2015 (CDM 2015).

If an existing structure is so unstable as to represent a risk, then should there not be a duty on the client to point this out to the incoming principal contractor?

While structural surveys are a normal part of due diligence if a structure is to be preserved, such a survey seems unlikely if the structure is scheduled for demolition. Perhaps this is a general omission on the part of the industry says the reporter. The precautions taken by structural survey experts when approaching a structure, in particular historic structures, could be shared.

Expert Panel Comments

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This type of risk is considered in the Construction Industry Research and Information Association (CIRIA) report Guidance on catastrophic events in construction (C699) published in 2011 when the failure of a structure can have serious consequences on an adjacent structure.

Insufficient information leads to fatality

CROSS is aware of a fatality arising from the collapse of a partially demolished wall in such circumstances. The demolition contractor had left the site having carried out most of the demolition but could not complete the task because there were concerns that the remaining construction was tied to another structure.

It was agreed that the main contractor would complete the demolition. However, it transpired that this remnant section of the existing site was not tied in, and subsequently fell onto a member of the contractor's staff during the final demolition. It was clear that there was insufficient information made available between the parties which then led to the problem.

Given the dangers associated with instability it would be thought that any contractor taking over a site ought to be informed over any existing structure’s stability system and its status, not least so that inadvertent removal of any part of the system does not trigger disaster.

On the other hand, a competent contractor would be expected to ask questions if they took over a site with a partially demolished structure in it and the paperwork, he was provided with was deficient. As ever on interfaces, both parties have some responsibility. Indeed, in the submitted report it is probable that the contractor has more technical competency than the client.

Providing the relevant information can improve safety

The provision of relevant information to help inform the development of a safe system of work is fundamental; the nature and extent of which would be dependent on the size and complexity of the project. The principal designer should assist the client in determining what information the principal contractor / contractor will require in order to develop the Construction Phase Plan. Unfortunately, as the reporter has highlighted, this doesn’t always happen.

The principal designer should assist the client in determining what information the principal contractor / contractor will require in order to develop the Construction Phase Plan

BS6187:2011 Code of practice for full and partial demolition specifically mentions gaining knowledge and history of the structure, including form, materials of construction, structural interactions and location. It states the chosen methods of work should be such that demolition activities can be carried out in such a way that the unplanned collapse of any part is avoided by maintaining the structural stability of the remaining parts at all times. 

Whilst BS 6187:2011 isn't mentioned explicitly in CDM 2015, there would be an expectation that demolition contractors would follow BS 6187 (or something similar) in producing the demolition plan as this represents good industry practice.

The benefits of carrying out a structural investigation

Whatever the contractual position, it is essential that a full structural investigation is carried out by competent persons in such circumstances. The condition and nature of the structure must be ascertained, and the deconstruction procedure risk assessed. This can assist with identifying problems and to eliminate, mitigate against, or control the remnant risks.

If the client cannot provide such information from records in the Pre-construction Information, then it needs to be otherwise established. Perhaps the recommendation could be that the client specifies compliance with BS 6187:2011, and this will create an item (including assessing stability) that the principal contractor / demolition contractor can cost for and deliver on. As can be seen from the comments, this report generated considerable interest amongst the Expert Panel.

The problem is not just limited to demolition but also includes alterations as part of refurbishment work highlighted in the 2017 CIRIA report Structural stability on site (C740).

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