As was highlighted in the accompanying article on Crossrail’s Thames tunnels, one of the challenges involved was agreeing a way to construct the portal box at Plumstead.
The box was to be some 300 metres long and, at the deepest point adjacent to the portal itself, around 25 metres deep. Its south side was to lie parallel to the railway out of London Bridge towards Woolwich and Dartford and was to be only about six metres from the nearest track. These tracks carry South Eastern Trains’ services and freight traffic, at very frequent intervals, and disruption just could not be tolerated.
Falling foul of standards
Network Rail standards concerning works like these alongside the railway were very restrictive, following the same principles as had applied for many years – back to British Rail days. Cranes and similar plant were not permitted to work where they might fall foul of the running lines. Detailed provisions applied, but in essence, no such equipment was to work facing the line or at an angle to it such that the jib, load or other part it would foul the line should the machine fail or overturn.
The portal box sidewalls were to be constructed in a combination of secant and diaphragm piles. The secant piling rig was considered and deemed acceptable to work even in this tight spot, since it was a hydraulic machine which could have the check valves and hydraulic fall arrest systems that Network Rail required in order to agree to its use in such circumstances. The diaphragm wall piling rig was a different beast. It is considered a crane as the grab it carries, three metres long and one metre wide, is suspended on ropes – just like a crane. The sort of constraining equipment used on the secant rig was just not applicable to this machine. Further, the piles required steel reinforcement cages, around 20 metres long, to be lowered into them using an auxiliary crane.
With 150 piles along the side of the box, constructing the track side wall under night time track possessions was not a feasible solution to this problem, and some other ideas needed to be considered.
Working through the problem
Fortunately the Crossrail team, led by John Kinnear and Simon Chittenden, foresaw the problem in advance, and in September 2010 discussions began between them and Network Rail’s principal engineer – Crossrail, Steve Brame. He involved the Network Rail asset protection team led by Geri Quinn.
When the Hochtief/Murphy Joint Venture (HMJV) was appointed in April 2011, the discussions began in earnest as the HMJV was able to suggest realistic, practical options for mitigation of the problem. Andreas Raedle, HMJV technical risk manager, and Paul Baker, HMJV rail interface manager, represented the JV while engineers George Christou and Tom Smith took the technical lead for Network Rail.
It was quickly established exactly how the HMJV preferred method of working would be non-compliant with the relevant Network Rail standards. Collaborative, open discussions ensured that all parties understood the options that had been considered in arriving at the preferred method of working, the reasons why certain of these had been rejected, the resultant non-compliances and the mitigations that might be adopted.
Everyone was clear that the mitigations adopted must not just transfer the risk somewhere else, they had to achieve genuine and significant risk reductions. An iterative process developed the final derogation that was proposed to Network Rail for their acceptance and ratification.
It was clear early on that the two key aspects were the diaphragm wall rig and the auxiliary crane, either of which might fall foul of the line.
Proposals and derogations
Consideration was given to relocating the portal box away from the line, but the constraints imposed by the listed status of White Hart Depot precluded this option. Construction of such a massive wall would have meant so many night-time possessions that the disruption to the rail service would have been unacceptable, whilst the delay and cost implications for Crossrail would also have been intolerable. Risk assessments also suggested that so much night time working would have caused a very significant increase in risk compared with predominantly day time working.
The suggested mitigations to support the proposed derogation included:
- Additional inspection and maintenance regimes for the diaphragm rig and its auxiliary crane, to reduce the risk of failures;
- The use of heavier machines downrated to the required capacity, again to reduce the failure risk by ensuring that the machines were significantly less stressed than normal;
- The construction and use of substantial reinforced concrete platforms for the machines to stand on to work, to reduce the risk of overturning;
- The provision of attendant rail safety staff to monitor the works and carry out emergency procedures in the event of an incident;
- The production and implementation of detailed, comprehensive emergency procedures and communications systems in case of an incident;
- All working to be restricted to daylight hours.
The HMJV found this last mitigation to be too restrictive given that the work was going on in winter, when the days were very short. Evidence was produced which actually showed some increases in risk resulting from the daylight hours restriction, meaning that it was not as effective in reducing risk as had been thought. For example, there was the risk that pile excavations could be left incomplete overnight because of the short working day, resulting in the additional risks of excavations left open. It was agreed that the restriction to daylight working be removed with the installation of a reactive warning system which triggered both audible and visual alarms should the asset protection barrier be breached, alerting the attendant safety staff.
After several iterations between the parties, an agreed draft derogation against the Standard was finalised. This was submitted to Steve Brame, who accepted it on Network Rail’s behalf.
Implementation involved the documentation of the derogation and the necessary revised working methods, a significant effort devoted to the review of things like crane operation diagrams and a comprehensive programme of briefing staff about their responsibilities. The latter gave particular emphasis to the agreed emergency procedures, and included regular auditing and reviews to ensure that everyone knew their roles. Paul Baker was heavily involved in this element on behalf of the HMJV.
The outcomes were good. The work was completed on schedule in mid 2012, a pretty good result in itself. There was only one occasion when the emergency procedures were implemented ‘for real’. This involved a minor spill of bentonite onto the track. Fortunately, no train was in the vicinity and the spill was not great enough to require any suspension of the train service. HMJV reacted very responsibly, suspending work for 36 hours whilst the causes were reviewed and for revised procedures to be agreed and put into place.
The negotiation and successful implementation of this derogation to the Network Rail standard was a very significant step. It was the first time that such a derogation had been attempted, and it showed that such a change in approach was achievable by agreement and in action on site.