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CROSS Safety Report

Embankment slip during excavation works

Report ID: 572 Published: 1 October 2016 Region: CROSS-UK

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Overview

Concerns were raised and strengthening works required after vertical movement was observed on the safety critical surface at the top of an 8m high embankment.

Had there been a failure of the embankment the consequences could have been severe.

Key Learning Outcomes

For construction professionals:

  • Consider appointing a competent temporary works coordinator (TWC) on site who should be able to ensure all temporary works are carefully considered and planned
  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

For civil and structural design engineers:

  • Consider how the design intent can be effectively communicated to contractors on site, particularly on high risk or complex projects

  • Ensure that the requirements of CDM Regulations 2015 (CDM 2015) for site specific hazards (such as earthwork instability during excavation works) have been clearly communicated to the principal contractor during the project planning stage

Full Report

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Vertical movement was observed on the safety critical surface at the top of an 8m high embankment. This occurred over a distance of approximately 25m directly above a length of rockfill shear key being installed at the embankment toe. Had there been a failure of the embankment the consequences could have been severe.

The design intention had been to perform the construction in short bays. However, a continuous 130m length of toe of the embankment of had been excavated during the previous week and cut back with a face of 1-2m high at an angle of approximately 70 degrees (Figure 1).

Image
Figure 1: cross section through embankment

After investigation, it was concluded that the surface deformation above this section was a result of movement within the embankment caused by the method of excavation. The excavation was later stabilised and strengthened by the installation of a rock fill berm with a shear key and the slope was reprofiled.

After investigation, it was concluded that the surface deformation above this section was a result of movement within the embankment caused by the method of excavation

Recommendations from the reporting organisation:

  • The principal contractor should clearly document the scope of enabling works and clarify the authorisation process to allow these to commence on site

  • Include a requirement for the contractor to document, prior to completion of the design, the findings of buildability reviews for similar works that have been categorised as higher risk

  • Ensure that the requirements of CDM Regulations 2015 (CDM 2015) for site specific hazards (such as earthwork instability during excavation works) have been clearly communicated to the principal contractor during the project planning stage

  • Review arrangements for the storage and retrieval of findings of meetings or investigation reports that contains risk information and lessons that can be learned

Expert Panel Comments

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The root cause of many failures is lack of clarity on design constraints to those who build, aggravated by poor communication. Well intentioned decisions taken on site can compromise design intent and the safety of the works. Designers should be better at identifying critical constraints, especially in higher risk situations. Contractors should further mitigate the risk of misunderstanding by actively seeking confirmation that their chosen method of construction does not compromise the design.

Construction methodology needs careful consideration

Parties to a project have to be sure how that finished state will be arrived at safely and consider the implications of any proposed construction methodology on safety at all intermediate stages. Whatever procedures were in place it might have been obvious to those on site that an excavation face 1- 2m high at a slope of 70 0 was inherently unstable. Also, a design team has to consider how its design will be implemented on site having regard to potential risks; in this case a clear risk of instability both by length and height of excavation.

Also, a design team has to consider how its design will be implemented on site having regard to potential risks; in this case a clear risk of instability both by length and height of excavation

Put more formally it is a statutory requirement for pre-construction information, which should contain details of significant residual risks, to be passed via the principal designer, to the contractor. Even though the parties may be from the same organisation, this should be done through nominated individuals who have authority for design and construction issues.

If temporary works are to be involved then a temporary works co-ordinator (TWC) should be engaged in the process to give advice as to the practicality of the design proposals, and secondly, when on site to ensure control as envisaged by BS5975:2008+A1:2011 Code of Practice for Temporary Works Procedures.

The important role of method statements

There should be a method statement for the works which has been agreed with the designer and communicated to those carrying out the work. The method statement is not there for paper trail protection; it is there to be implemented. In safety-critical situations, such as this, it is important that the risk management process is controlled by someone who is capable and experienced. This process should include ‘lessons from past experiences’.

There may be occasions, in high-risk situations, where an independent reviewer should be engaged to provide oversight as recommended by CROSS for structures: see Independent peer review through peer assist - form of agreement Jan 09.  

Projects require an intimate relationship between design and construction. It is prudent for the design team to visit the site during the works when possible and liaise with the site staff to assure safety is being achieved.  This might include the need for a competent geotechnical engineering presence to ensure embankment conditions of a variable nature meet with the designer’s intent.

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