Portals for Connington
The Connington area, on the East Coast main line south of Peterborough, experiences relatively high windspeeds and, as such, is prone to dewirements, resulting in significant train delays over recent years. Network Rail identified this area for conversion to portals to help build resilience into the OLE system. Arup was commissioned to prepare a design study to look at replacing several headspans in the area. Again, the provision of installing new structures was rejected and a detailed analysis of the possibility of reusing the existing support masts was undertaken.
Using Arup’s well-developed tools for assessing geotechnical issues and ground conditions, a close study of the foundations was made, to discover whether they would cope with the varied stresses and loads from the new portal geometries.
Point cloud surveys of the existing structures were undertaken.
Economy would best be achieved by the reuse of the existing masts, but these would have been installed in a manner that facilitated the cross-track wires. Using point cloud surveys, the precise positions of the masts had to be recorded, along with any skew or twist as had been seen at Paddington, which would have an impact on the loading of the finished portal.
Ground engineering studies of the foundations were essential as the original foundations would have been installed to take cross-track stress rather than the new loadings imposed by the beams. This involved a detailed structural analysis to determine the existing foundation loads and compare them with proposed portal loads. Arup’s proprietary software was used both to model the new loads imposed on the foundations and to check the stresses in the Series 1 boom and connection angle to the masts due to the loads of the UK1 OLE, a design first used on the West Coast main line for higher train speeds, and the OLEMI (OLE Master Index) equipment which continued to support the OLE in the conversion.
Installing a portal beam while the headspan remains in place.
In summary, the emphasis was on the strength of the concrete, reinforcing bar cover and the general suitability of the foundation for the portal conversion. However, without the cross wires, the bending stress on the vertical structures is reduced. In all cases, the foundations at Connington were side-bearing concrete – no piles were involved.
Following these initial design considerations, a detailed design for thirty structures was prepared, covering this high-risk area on the East Coast main line. Performance aspirations would suggest that all headspans in a tension length should be changed, but the complexity of replacing items such as mid-point anchors and neutral-section supports drove the decision to convert only the simpler, multitrack headspans within the tension length. Mid-point anchors, booster transformer structures, and switching structures were among some of the structures deemed too complex for this phase of the headspan-to-portal conversion projects.
Installing the Portal booms
Many design and construction meetings were held with the Network Rail’s works delivery team, accompanied by drawing revisions, such as extra dimensions, to suit the construction team’s needs on site. Installation was carried out on site by Works Delivery, acting as principal contractor. Design acceptance was similarly eased by working with the E&P (electrification and plant) and structures route asset managers, and the crane provider was brought in at an early stage.
Staging of the work was very important, particularly on such a heavily used route. The design, therefore, detailed all of the stages of the conversion process, not just the finished result, taking into account the analysis of both mast and boom orientation, the detailed construction methodology and a view of simple versus complex structure types.
First, after the headspan wire was removed, the new boom was landed on the two main steel masts, with the two cross wires retained. After this interim stage, the SPS was modified, in a staged process, to fit in with site availability and line possessions.
Lifting in the new boom was a complex procedure, each one weighed in excess of a tonne and had to be manipulated into the final position with existing wires in situ, but, once it was in place, the processes became more self-contained. An initial stage-by-stage approach could have led to one road being upgraded at a time, but Jonathan Ridley pointed out that, in practice, all four roads were completed at once.
If an incident occurs, damage is now usually limited to the single track involved and the equipment can be returned to normal operating condition in less time than if the failure occurred on a headspan structure.
With the conversion of 30 headspans completed, Arup can look to the future. The next step is to convert a complete tension length, including all of the complex structures that were left out in this conversion project. Further evaluation of the complex structures will be required as part of a new feasibility study and the performance improvement gain is expected to be considerable.
