Einstein’s general theory of relativity describes the gravitational effect of large bodies: spacetime curves due to the presence of matter and energy. A similar effect appears to apply to large rail infrastructure programmes. Writes Mungo Stacy
Often these seem to warp into a series of incomprehensibly large attention-grabbing statistics whilst absorbing vast quantities of resources.
Take Thameslink. Or rather, not the whole of Thameslink, but just Thameslink Key Output 2. Running over 93 months from 2011 to 2019, this £2.4 billion programme will, amongst other works, reconstruct London Bridge station and provide capacity for 24 trains per hour in each direction through the core route.
At The Rail Engineer, there’s no denying we like to cover the big stuff. But, as rail engineers know, complex programmes are the sum of their constituent projects. And the small parts can have their own difficulties and complexities, and be just as crucial to successful delivery of the overall programme as the major blockades.
One such project was signal gantry XTD 3736. This was successfully installed by Lundy Projects, working for Balfour Beatty Rail, on the night of Saturday 1 March. Whilst a small element in itself, it had critical links to the overall Thameslink programme and was subject to some interesting spacetime effects.
During Christmas 2014, the major LL09 blockade will reconfigure London Bridge station and commission the six low-level platforms. In order to de-risk the blockade, works to install gantry 3736 were brought forward. This was driven particularly by the weather risk, outside Network Rail’s control, which could force the abandonment of crane lifts.
Three of these large gantries are required to be in place by the Christmas blockade, and 3736 is the second. Once the decision was made to accelerate installation, Lundy Projects were given only eight weeks to design, manufacture and install the gantry and associated substructures. Time was therefore extremely tight.
The gantry is located on the 11-track viaduct approach to the east of London Bridge station. The tracks already occupy most of the structure width so there was limited room for foundations.
Therefore, the gantry spans eight of the tracks with a span of 31 metres. A cantilever over a further two tracks takes the total length to 38 metres spanning 10 tracks. The gantry is a Warren Truss arrangement with two support columns. The design was undertaken by Lundy Projects’ subcontract designer, Aspin Consulting.
There was room between tracks 3 and 4 for a regular pad foundation. At the south side there was no room for a new foundation between the tracks and the viaduct parapet wall. Therefore a new foundation had to be created at road level, some 12 metres below the gantry level. This foundation needed to be piled due to the large overturning moments caused by the height of the support leg and the span of the signal gantry.
Space was extremely limited for the new foundation. Part of an existing building needed to be removed to make room, and even then there was only a nominal three metres clearance between the viaduct and the remaining building.
Time was also short. The building’s tenants, Tower Antiques, only vacated the premises three weeks before the superstructure installation was due. This followed an extremely complicated property negotiation by Network Rail. It also created a key link to the foundation design as it precluded any ground investigation being carried out at the exact location until the tenants had vacated and demolition was complete.
Design and manufacture
Lundy Projects was in charge of the overall signal gantry sub- and superstructure conceptual and detail designs.
“We had to ensure that high level conceptual designs were realistic and achievable by all multi-disciplined parties involved in delivering this structure,” project manager Jamie Lundy explained. “Material availability, procurement of materials, manufacturing timescales, finishes, design approvals, curing times for concrete and so on all had to be taken into consideration to allow an achievable program to be developed, approved and ultimately implemented in such a short time”.
Within a week from starting the design, Lundy, working closely with Network Rail and Balfour Beatty, produced a conceptual 3D design for both the sub- and superstructure ready for structural and detailed designs to start. The procurement of materials and in-house manufacturing offered by Lundy commenced concurrently with detailing the signal gantry to ensure delivery timescales were still achievable and met.
The gantry incorporates some neat features to facilitate future stages of the wider programme. The gantry has been installed prior to re-signalling, therefore the existing signalling remains operational. The underside of the gantry was therefore set high enough to avoid blocking sightlines to the existing signals.
Likewise, the signal platforms are installed in a temporary position higher than their final level. This will allow the signalling contractor to install the signals and cables and undertake soak testing, before lowering the signal heads to their final positions. This will be undertaken using road-rail plant, avoiding the need for large craneage and reducing risk for this stage.
Aspin Consulting designed both the foundation and the superstructure. Pile installation was carried out by Aspin Foundations while the pilecap installation was contracted to Terrawise.
Tom Keating, associate at Aspin Consulting, explained: “The south foundation was not at all straightforward. We were very restricted with the working space and this limited the rigs we could use.”
508mm diameter steel piles, which could be twisted into the ground rather than driven, were proposed to minimise disturbance to the adjacent viaduct. However, ground conditions proved to be worse than expected; it had not been possible to carry out boreholes at the site location due to the antiques building so conditions had been inferred from other investigations.
Problems were apparent as the first piles were installed. The recent severe rain had led to a high water table and there were concerns about gravels and fines being removed by pumping, potentially creating voids around the existing foundations. The smaller rigs also had limited power as gravels became trapped between the auger and casing.
Piling was abandoned and a new foundation methodology considered. Now with only two and a half weeks to go until the gantry installation, Aspin Consulting redesigned the foundation in two days. Again, close working with Network Rail gave a swift approval of the design.
The revised design used 150mm diameter grouted steel piles with closed end casings, which avoided the water ingress problems. These were installed by an in-pile Grundomat air-driven piling hammer. A larger number of the smaller piles were needed, and over four days the 24 piles were driven to set at around seven metres below ground level.
With the clock now at T minus 6 days, and Aspin Foundations having completed the foundations, the restricted site was handed over to Terrawise to install the pilecap reinforcement and concrete. The change in pile design meant the pilecap needed to be increased in depth by one metre, and this gave increased complexity around a brick culvert which had previously passed below the pilecap. C50 concrete was used to achieve the early strength required to install the gantry: the pour was completed at T minus 4 days.
Four hour possession
The possession on Saturday/Sunday 1/2 March was a planned 4-hour window to install the whole gantry. It was due to start at 02:30 but in the event, access was not available until 04:30 due to problems establishing the worksite.
Jon Andrews of Lundy Projects explained: “With a slight delay at the beginning of the possession we had less time than expected. Balfour Beatty were on site as the principal contractor, liaising with the Network Rail site representatives who were
in close contact with Network Rail Operations. We reviewed the situation and took a joint decision to proceed, as we were confident that we could install the gantry in the time available if all went to plan”.
With time being tight and the nearest road-rail access points being some distance from the site, mobile elevated working platforms could not be used for bolting the main boom to the gantry support legs. Therefore, scaffold access towers were designed and would be used as an alternative solution. Once the possession and line block was granted, the towers were lifted into position on track ready for the steel erectors.
The adjacent Druid Street was closed for 48 hours to allow the three sections of gantry to be assembled at ground level and a 1,000 tonne crane set up. Sarens undertook the contract lift.
Signal gantry XTD 3736 was installed on time, safely and the lines handed back within the agreed time scale.
Afterwards, Jamie Lundy reflected on the collaboration which had led to the successful installation: “I believe the hallmark of Lundy is not in the complexity of the projects we deliver, but rather in the simplicity with which we deliver them. This is credit to, not just Lundy Projects, but also to our client and preferred suppliers. Without close and open collaboration with Network Rail, Balfour Beatty and our supply chain the opportunity in bringing the installation of Signal Gantry XTD 3736 forward to de-risk the Christmas 2014 blockade would have been missed”.
Andy Hoffman of Aspin Consulting agreed: “Complex projects require experienced innovators; driven individuals with collective motivations and aspirations. Trust, empowerment and personal responsibility is liberating to both individuals and to organisations in succeeding on complex assignments. Lundy and Aspin Group shoulder these fundamental ingredients, which bring successes to such projects.”
Chris Ottley of Balfour Beatty Rail added: “Without doubt there were a number of occasions leading up to the possession where the obstacles appeared to be too much and the easier option would have been to delay the installation. Only through a true collaborative approach between all parties and sheer perseverance was this installation possible.”
Lundy Project’s contract includes a total of ten signal gantries, which in turn are just one element of Balfour Beatty’s signalling related minor civils contract at London Bridge. This one has been the most challenging gantry installed to date – but then, everything is relative.