Hanger Lane Junction on the Piccadilly and District lines of London Underground was last completely renewed in 1984. Thirty years later, it was due for renewal and modernisation. Considering the volume of traffic over it, the junction had performed well.
This junction – curiously named as it is actually quite some way distant from Hanger Lane itself – provides the facility for District line trains to Ealing Broadway to branch off from the Piccadilly line between Acton Town and Uxbridge. It is a conventional double junction.
However, an unsatisfactory feature of its location is that part of the switch area lies on the major truss bridge carrying the Underground lines over the Network Rail Great Western lines. This has meant that differential support conditions existed between the part of the junction on the bridge and that on the embankment formation, aggravating difficulties for maintenance of good geometry and contributing to frequent points failures.
While renewing the junction, London Underground took the opportunity to improve the support conditions by laying geotextile membranes and regularising the depth of the new ballast as far as possible. The overall design of the new layout allows a 5mph speed increase through the junction and incorporates new switch power units compatible with the forthcoming ‘Endstate’ resignalling and modernisation project. The work has also increased capacity to 16 trains per hour into and out of Ealing Broadway.
All these improvements looked good, but the estimated 76-hour possession to renew the double junction would not be available. Therefore, a novel possession strategy was devised, one that would allow the desired renewal to be done very efficiently and with very little disruption to the normally available weekday services. Inevitably there would be some effect on weekend services.
The work was carried out by Track Partnership, a strategic alliance between London Underground and Balfour Beatty. Owen Stratford, engineering manager for Balfour Beatty, and Louise Allaker, assistant project manager for London Underground, explained that it had been decided to adopt a modular approach to the junction renewal.
By designing the switch and crossing (S & C) layout so that it could be prefabricated in modular units, it became possible to use the more readily available 52-hour closures.
With this approach, the westbound modules could be installed independently on the first weekend, and the eastbound on the second with the junction only being coupled up throughout at that stage. The works were carried out between Saturday 24 October and Sunday 1 November, using two weekend 52-hour possessions along with a week long closure of the District line between the junction and Ealing Broadway. The only station to lose its service during the week was Chiswick Park, Central line services being available from Ealing Broadway.
Two prior 52-hour closures of the District line had been taken earlier in the year to renew 450 metres of the eastbound and 525 metres of the westbound plain line in preparation for the junction renewal. These works included a significant realignment, five-metre track slews, to prepare for a revised junction alignment.
The timing of the actual double junction renewal work during the October half-term week was selected for two reasons. Firstly, the passenger numbers are slightly lower overall and so there would be less disruption to the travelling public. The second reason was that it had been negotiated for the 14 major S&C modules to be stockpiled in the grounds of the Ellen Wilkinson School for Girls, just adjacent to the northern side of the railway embankment. From there they were readily moveable by road-mobile telescopic crane to their installed position, but it was desirable that this should happen whilst the school was on holiday.
One weekend at a time
On the first weekend, the westbound part of the junction was removed, reballasted and renewed using 10 prefabricated modules. The new switch on the westbound and the old switch on the eastbound were secured for through running by Piccadilly line trains from Monday to Friday. On the second weekend, the reballasting and renewal of the eastbound part was completed with the installation of the remaining four modules.
Starting on Saturday 24 October, after removal of the old trackwork and ballast on the westbound portion of the junction, new geotextile membranes – TriAxial Geogrid TX190L from Tensar and Geofabrics’ Tracktex – were laid. The exposed part of the Network Rail bridge was re-waterproofed with Wolfin and Tiflex membrane laid. Reballasting was completed by 01:00 on Sunday morning.
Sequentially with this, the new switch and crossing modules were progressively lifted in between 20:30 Saturday and 04:30 Sunday. Two 250-tonne cranes supplied by Ainscough lifted the 10 modules from their storage positions in the school compound.
At the planning stage, consideration had been given to the use of a single 500 tonne crane. However, accessing this crane to site through the school would have required the removal and reinstatement of some lighting columns and other school infrastructure. So, although no tandem lifts were needed, the two smaller cranes were there because of lifting radii.
Within the confined space available, it was not possible to stack all the modules in positions such that they could be transferred from their stored locations to their installed track locations from only one crane position.
Final lining and levelling, welding, top ballasting and tamping was completed by 17:10 Sunday. Conductor rail replacement and point fitting work was all carried out by 18:00 with the signalling work and testing finished by 02:30 Monday morning.
Between Monday and Friday, with the spur of the District line closed, the opportunity was taken to carry out significant additional work between the junction and Ealing Broadway. Over 100 metres of plain line was renewed on each of the eastbound and westbound lines.
Redundant sidings were removed and the track layout at Ealing Broadway was simplified, also in readiness for Endstate. A ‘spring toggle’ switch unit at Ealing Broadway was removed and replaced with a like-for-like fully-powered version using electro-pneumatic points operating equipment in the four-foot.
The second main possession weekend tackled the switch and crossing renewal work on the eastbound line with a similar sequence of operations as the previous weekend’s work. In this case, it comprised four S&C modules only. The last module was in place by 20:15 on Saturday 31 October and the overall works were complete by the very early hours of Monday morning.
The 14 switch and crossing modules had been trial erected by Progress Rail at its Beeston depot, (Issue 132 October) with the timing of the build being planned to allow the modules to be delivered to the school compound within the school half-term holidays. The design of the individual units had to be such that the 10 modules installed on the first weekend could be placed in their final positions whilst leaving the eastbound trackwork unaffected. All ‘through’ concrete bearers, those that support both westbound and eastbound lines as one structural unit, had to effectively consist of two sections longitudinally. For each of these through bearers, the junction between the two sections falls either within the four-foot or the six-foot, depending on its location within the layout.
After accurate positioning, lining and levelling of the eastbound modules, they were ready for making good with the previously installed westbound modules. All the through bearers were coupled up by the installation of modular tie-plates. Four coach screws fastened into the end of each bearer secure these tie-plates, the coach screws being torque loaded at 340Nm into the pre-formed housings. The dimensional accuracy of the location of these housings is of paramount importance.
Whilst this technique and methodology has been previously used on Network Rail layouts, it is believed that this project is the first to have used it for a double junction on London Underground. The use of modular tie plates on the Underground infrastructure gives rise to an additional complication, in that the fourth-rail negative electrification equipment in the four-foot has to pass over them and there is limited electrical clearance. To overcome this challenge, the tie plate design was developed to include the mounting of a specially designed insulator pot and Brecknell Willis No.6 conductor rail was installed. This has a shallow section and, for use in this application, incorporates an additional foot shroud that maintains the required 75mm electrical clearance.
Some noteworthy safety features are worthy of comment. During the lifting and placement of the switch and crossing modules an ‘exclusion zone’ was established. Once the bottom ballast had been accurately profiled and compacted and was ready for receipt of the modules, the whole area of those operations was cordoned off with red/white boundary tape. Whilst the modules were being lifted, the only personnel permitted within the zone were crane staff – slingers and supervisor – and those Track Partnership staff responsible for positioning the modules.
A comment overheard from someone watching from outside the exclusion zone was: “It looks like we’ve got a UFO up there”. This was a reference to the flashing red ‘trolley lights’ strapped onto the panels at several places. These bright LED lights, flashing once every one or two seconds, and normally used on rail trolleys, were a very effective warning device to maintain awareness of the moving loads. Apparently, the batteries can have a life of up to two months.
This simple precaution was in popular use elsewhere to warn of hazards. For example, the LEDs had been attached to a kerb on the walking route between the site briefing office and the site entrance, undoubtedly preventing likely trips that would otherwise have occurred.
Project cost was £3.2million for the double junction renewal and the District line plain line renewals.
The unusual sight of large sections of permanent way stacked neatly in the grounds of a school was intriguing. However, Rail Engineer suspects that the school is relieved to have their premises back for their exclusive use and that London Underground is delighted with its brand new junction, having promised passengers a noticeably more comfortable ride.