CROSS Safety Report
Control of temporary works excavation
Poor, high-water content excavated material, slipped into a 15-20m deep borrow pit. The material had slipped to the bottom of the pit partially burying excavation, crushing, and screening equipment. If this slip had occurred during working hours, the impact could have been very serious.
Key Learning Outcomes
For geotechnical, civil and structural design engineers:
- Assess what temporary states may exist and provide information via the principal designer
For resident engineer’s staff:
- Assess what temporary works and temporary states may exist during the works
For contractor’s supervisory staff:
- Adhere to the requirements of BS 5975:2019 – TC Code of practice for temporary works procedures and the permissible stress design of falsework
- Be aware of responsibilities in your organisation to identify temporary works and temporary states
- Ensure a temporary works coordinator is appointed
- Be aware of TWf guidance TW17.037 Principles for the management of temporary loads, temporary conditions and temporary works during the construction process
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The construction project was a substantial dual carriageway scheme in the UK. Borrow pits were used to win quality rock material for embankment construction with the pits then used for disposal of unsuitable poor excavated material - this was done on a phased basis where the pit was filled in from one end as the excavation of rock progressed. Design requirements and compliance on the project were generally very good, both for permanent and temporary works, e.g., bridge structures, embankments, and materials. However, neither the client, the client's advisor, the contractor, nor the contractor's designer paid proper attention to geotechnical design for the filling of borrow pits with poor materials.
One morning says a reporter, it was discovered that poor, high-water content excavated material, had slipped into a 15-20m deep borrow pit. The material had slipped to the bottom of the pit where excavation, crushing, and screening equipment was situated. The equipment was partially buried. The reporter confirmed that if this slip had occurred during working hours, the impact could have been very serious, potentially with fatalities or serious injuries. The incident was reported as a dangerous occurrence.
The underlying safety issue was the lack of sufficient attention paid to the stability of the excavated materials. The reporter argues there was a requirement for the contractor to undertake all temporary works design, however adequate geotechnical design and risk assessment of the borrow pits was simply missed by the contractor, their designers and the client's project manager. Procedures for the handling of the material should have been in place and monitored, continues the reporter, particularly because high water content materials are inherently unstable. However, as a temporary works operation, it was missed.
The reporter concludes that even with the large professional team of people and several organisations involved in this project, the risk was not properly raised; the lesson would be to ensure that temporary works are designed and risk assessed as importantly as any permanent works.
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By bringing forward this report the reporter has very helpfully shone a light on an important issue that can be missed – that is, what temporary works will exist on a particular site? Had the excavations been recognised as temporary works, then control measures would likely have been in place and the incident prevented.
Excavations and adjacent materials must be adequately controlled. Legal safety requirements relating to excavations and stockpiles are clear. The Health and Safety Executive webpages Temporary Works and Excavations set down requirements that are relevant to the management of excavations, stockpiles and adjacent materials. BS 5975:2019 – TC Code of practice for temporary works procedures and the permissible stress design of falsework sets down requirements that should be followed for the design of any temporary works which includes excavations and other earthworks. Risk assessment of all excavations is required. The risk assessment would highlight a requirement for geotechnical design. In cases similar to that reported, a geotechnical engineer would perform as the ‘designer’ of the works. A design check as required by BS 5975: 2019 would also follow. The design and check would contribute to the risk assessment required for the adequate management of the excavation and associated operations.
risk assessment of all excavations is required
The Temporary Works forum has published much relevant guidance including TW17.037 Principles for the management of temporary loads, temporary conditions and temporary works during the construction process. This guidance states, ‘One of the most important questions in temporary works is simply: ‘When and where are temporary works needed and how long do they need to be in place?’ Many failures occur because it is assumed that structures, excavations, stockpiles and other features on construction sites will stand up on their own, at each and every stage of construction, when in fact they won’t. Engineering analysis is needed to back up such judgements.’ The guidance examines the fundamental issues that those persons (including those at head office) responsible for construction projects which include any form of temporary works, should understand and act upon.
All involved in projects, including but not only, principal designers, designers, temporary works coordinators, works co-ordinators and all contracting parties should be asking the question ‘what temporary states may exist as part of the works to which I am contributing?’ Ultimately, of course, contractors should have in place appropriate systems of work that ensure all temporary works, and importantly temporary states, are identified. BS 5975:2019 requires the use of temporary works registers to capture all temporary works and temporary states – control measures would then follow.
ensure all temporary works, and importantly temporary states, are identified
Where earthworks are identified as temporary works or a temporary state, then the specification of materials, working and management can be given due consideration. The following may be considered matters of importance:
- Excavations, stockpiles, slopes, embankments and similar should be risk assessed by a competent person.
- Stockpiles next to any excavation are likely to be higher risk.
- Encourage those preparing or checking risk assessments to adopt a ‘what if’ scenario and have contingency measures in place.
- Understand the impact of changing environmental conditions on temporary works/states, particularly where earthworks are concerned.
- Recognise the importance of continual monitoring and formalising the checking of temporary works or temporary states.
- Ensure that inspections are undertaken by staff with the appropriate level of experience and competency who have the requisite power to stop works, if they feel the situation so demands.
No method statement, no work
Clearly whilst this report concerns earthworks, the same need to identify temporary works and temporary states applies across all engineering and building activities. A good process for controlling all works may be via method statements – ‘no method statement, no work’. Such a control can be taken right down to grass roots with all control measures passing through the appointed temporary works coordinator.
The TWf has published guidance upon excavations including Information Sheet No 5 which covers good practice for the management of stockpiles.
The Construction Plant-hire Association has published guidance (including input from the HSE) on the management of shoring in excavations. Part 1 covers management processes including assessing the levels of excavation risk and design scrutiny. The appointment and competency of duty holders is also considered.
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