Keeping track of Crossrail
RAIL continues to document the progress of Crossrail with a detailed look at the high-tech track underpinnings currently being installed
RAIL takes a look at the high-tech track being installed under central London.
Crossrail’s surface sections hit a key milestone this month (May 2017) as the first TfL-operated services begin running from Shenfield-Liverpool Street, resulting in the first Bombardier-built Class 345 Aventras entering traffic.
Until December 2018 they will have to terminate at Liverpool Street main line station, however, while Crossrail’s central tunnelled section is fitted out and commissioned. This will pave the way for the 66-strong Aventra fleet to serve destinations further west, and also in south-east London, on what will have by then become officially known as the Elizabeth Line.
Finally, the westernmost sections of the route from Paddington to Reading will open in December 2019, completing the 73-mile pancapital railway.
To facilitate this phased introduction of services, Crossrail Ltd is currently focused on fitting out the tunnels, and stations within them. Track laying, electrification, communications and signalling works are all due for completion later this year by joint venture ATC (comprising Alstom, TSO and Costain), so that testing and commissioning of the operational railway can begin in early 2018.
80% of the track within the tunnels will be standard track slab (STS) secured directly to the tunnel floor. This is delivered on site as individual track panels by four giant multi-purpose gantries (MPGs), where they are clipped into place, welded together and then concreted into position by a 465metre concreting factory train, stabled near the Plumstead tunnel portal in south-east London. More than 80% of the 25.6 miles of STS needed for the project had been laid at the time this issue of RAIL went to press.
The remaining 20% of the tunnelled section track comprises four distinct types. 2.6km of Direct Fixed Track has been laid within Connaught Tunnel in Docklands, due to clearance restrictions presented by its 1878built bore, while the Crossrail Act (2008) legislated that high-attenuation sleepers and two different types of floating slab track were to be installed in order to mitigate the vibration of passing trains beneath several isolated and noise-sensitive areas of central London.
In June 2016, RAIL was invited to a tour, hosted by track engineer Juliet Murray, of the 1.97km section of the route between Tottenham Court Road and Bond Street, where lighter floating track slab was being laid due to the proliferation of hotels, recording studios and editing suites overhead ( RAIL 803).
Nine months later, RAIL went to visit a 1.34km section of tunnel near Liverpool Street, where a heavier and more energy-absorptive variant of floating track slab (FTS) is being installed in Crossrail’s two running
tunnels, 40 metres beneath the Barbican Theatre.
The process for laying both light and heavy FTS is very different from STS, explains our guide, Crossrail track manager Adam Ersser.
FTS sits above the tunnel floor on a bed of rubber bearings and heavy duty springs. To start with, steel rebar is laid to provide reinforcement for 30-metre floating slabs, upon which the track will sit. Unusually heavy Magna-Dense concrete is then poured to form the slabs, which are then jacked up, one by one, into position. Small holes are left within the concrete for the heavy-duty springs and bearings to be inserted beneath the slabs once they have set. These have to be flexible enough to absorb the energy of passing trains, yet sufficiently rigid to bear their weight and prevent the slabs from being forced out of position by the horizontal forces exerted by fast-moving traction.
During RAIL’sL visit on March 3, work to lay a 500-metre stretch of heavy FTS between Liverpool Street and Farringdon was over 50% complete; 15 slabs had been fully concreted and jacked into position out of the required total of 25 in the western running tunnel, and 11 out of 25 in the eastern.
Having started laying the heavy FTS in January, this section was due for completion by the end of April (when this issue of RAIL went to press), with 200 staff deployed on eight-hour shifts, and three shifts per day.
RAIL presents a series of photos taken during its latest site visit, documenting the heavy FTS track laying process.