Farringdon Station will be “the heart of the heart” of Crossrail. It is going to be the only station in London with interchanges between Crossrail, London Underground services and the Thameslink network. Writes Chris Parker

It will also be the only station in the city to have direct connections to three of London’s five airports, Heathrow, Gatwick and Luton. There will be up to 140 trains per hour through the station by the time the Crossrail services are in full swing, and it is forecast that these will carry up to 150,000 passengers/day.

With this in mind, The Rail Engineer was very pleased to be invited to go and see how the works are progressing. The day included some unusual sights such as Farringdon Thameslink Station from below track level and a grade 2 listed brick arch viaduct suspended some distance in the air! But more of those later…

Bored not excavated

Jose Garcia, construction director, described how, in addition to this station contract, the BFK JV (BAM Ferrovial Kier) has been responsible for all of the running tunnels west of Farringdon, right through to the east end of the new station. In total, on these and other Crossrail contracts, the JV and its suppliers are employing some 2,000 people and training 100 apprentices (almost one third of the total number of apprentices on the entire Crossrail project).

Looking at Farringdon itself in more detail, Jose said that it will have the longest platforms on Crossrail, some 400 metres in length, stretching between two ticket hall complexes at the east and west ends of the station. Developments above each ticket hall will be constructed, creating commercial and retail space.

Farringdon is unique among Crossrail stations in that the platform tunnels are being constructed in a different manner from the others. Whereas the others are being excavated as underground caverns, constructed from shafts driven down from above, at this site the running tunnels are being used as pilot tunnels.

The difference was driven (sorry for the pun!) by programme issues. At other station sites it suited the programme to begin to excavate the station caverns before the running tunnel TBMs arrived, allowing them to break into the voids already created. Where they needed to continue beyond, they were then hauled through the station area to continue their drive on the other side.PA1A3756  BFK Crossrail C435 Farringdon 240714 www.johnzammi [online]

At Farringdon this order of construction would have been too disruptive. Instead, it was decided to allow the tunnel boring machines (TBMs) Phyllis and Ada to complete their drives from the western portal through to the east end of the station at Farringdon before work on the station platform tunnels commenced. There was another advantage in this sequence of  work, in that this methodology reduces the likelihood and magnitude of settlement affecting the buildings and structures up above. Given the close proximity and sensitivity of infrastructure like the London Underground’s Barbican station and the Network Rail Farringdon station, this was no bad thing!

Opening out the station

The eastern end of the station was the end of the line for Phyllis and Ada and so they were diverted to the side and buried. Once they were clear, the excavations could begin, to create ten-metre diameter tunnels from the seven-metre diameter running tunnels over the required length of each to accommodate the platforms.

The platform tunnels are being lined with fibre- reinforced sprayed concrete (FRC), so the work involves careful removal of the precast segmental tunnel lining that was installed by the TBMs, the excavation of the ground behind to the required profile, and finally the robotic spraying of the FRC lining to the enlarged tunnel bore (pictured). Clearly, this is a sensitive business and so it is carried out in carefully managed stages. Normally these are threefold, first the crown, then the benches either side and finally the invert at the bottom. This ensures the minimisation of ground movements and the consequent settlements of structures above.

Despite the care taken in planning and executing the works, some settlement was still to be expected, and so mitigation measures were provided, in the form of compensation grouting. This technique can be applied after settlement has occurred to restore the ground to its original level. Alternatively, it is feasible in some circumstances to raise a structure slightly in advance, so that the works below cause it to settle back to where it came from.

Which method is used depends upon the nature of the structures and the type and magnitude of the expected movements. Some structures, for example, can tolerate heave better than settlement and those would probably be best treated by lifting ahead of settlement. For others the converse may apply.

The compensation grouting is being applied using “tube a manchettes” that have been installed from five 14 to 15 metre deep grout shafts so as to cover the whole area above the tunnels. Some 18,000 linear metres of grout tubing have been installed and the area covered is about 27,000 square metres in extent. One of the grout shafts lies in the adjacent corner of Charterhouse Square, and it was during its construction that several intact ancient skeletons were discovered.

One further benefit of the pilot tunnel technique is the reduction it means in the quantity of spoil to be taken out of the site by road. Since the spoil from the seven-metre running tunnel bores was taken away through the tunnel by conveyor back to the west portal, then something approaching half the spoil went away from the station tunnels without needing to be trucked away from the site. 90,000 wagon loads of spoil were therefore removed from London’s congested roads.

Halls at each end

Jose was assisted by his colleague Tom Moore in describing the construction of the two ticket hall complexes. These lie at the east and west extremities of the platforms. The differences between the construction methods employed at these two sites illustrates once again the care that has been taken to select the most appropriate engineering solution at each and every location on the Crossrail project.

110476_chief secretary to the treasury danny alexander and commercial secretary to the treasury lord deighton given tour of first complete crossrail tunnelThe western ticket hall structure is being constructed from the bottom, eight levels below ground, upwards. Segmental piled walls surround the shaft, forming its sides as the spoil is dug out from between them. As the construction proceeds temporary props are required to ensure that deflections of the piles are restrained appropriately.

When the right level is reached for a floor, this is shuttered and cast with ties into the wall piles. Once the concrete in the floor slab has reached the required strength, the floor is able to act as the prop to the wall piles, and the temporary props can be removed from that level. At the time of the visit the props had just been removed from level -6, two floors up from the base of the works.

Top down at the other end

At the East ticket hall, Tom and his colleagues are involved in top down construction works for a trapezoidal shaft going down to 42 metres below ground. A slab was built two storeys below ground level, once again inside an area walled by segmental concrete piles that extend down below the intended bottom floor level. This first slab provides the permanent propping to the piled walls at that level. There are both permanent and temporary apertures in it, the former for escalators, lift shafts and so on, the latter for use during construction. Temporary steel props above the slab provide restraint to the pile walls pending later construction of floor slabs at level -1 and at ground level.

 

PA1A4487  BFK Crossrail C435 Farringdon 240714 Site Vsit to ETH [online]

Five plunge columns have been driven within the shaft to foundation depth below the level of the bottom floor. These will act as internal columns to support the permanent floors, several of them are also serving as supports to a partial temporary slab at ground level which is being used by construction plant and vehicles. Once the station works have been completed, the plunge shafts will form a significant part of the foundations for the steel framed over-site development which will sit above the ticket hall.

Excavation within this shaft was initially by means of a clamshell grab on an excavator sited on the temporary ground floor slab. This worked through the apertures in the slab to raise the spoil and deposit it into tipper trucks for removal from site. With this form of construction, the floor slabs are constructed on the excavated base of the hole once the correct level is reached. Each slab then provides the requisite propping to the wall piles at that level, permitting the excavation to continue beneath the slab to the level for the next floor down. This repetitious process has now passed the point where, at -30 metres, the excavator could no longer reach from ground level. Having peaked at a productivity of seventy-eight 20tonne tippers full of spoil in one day, the machine handed over to a mobile crane and skip for the rest of the job.

New techniques

The crane/skip operation is the first in the UK to use “hook-cam” technology to permit the safe operation of the system down through the apertures in the floor slabs to the working level far below. Cameras on the hook, connected wirelessly to screens in front of the crane driver, permit the driver to direct the skip on the crane hook safely to where it is needed, and bring it safely back to ground level when full. Slinger/Signallers are still employed at the working level, in radio contact with the driver, but the camera technology ensures that they need not stand below the hook to guide the driver during the lowering or raising operations.

On completion of the excavations, when the floor slabs are all cast in place, work will commence to fill in the temporary apertures in the slabs and to construct the sloping structures upon which the station escalators will sit. Screwed starter couplers have been connected into the reinforcement of the slabs to permit the necessary connections between the slab steel and the reinforcement in these further structures.

The segmental piled walls of the shaft are being lined with reinforced concrete as the floor slabs are completed, to hide the rough piled faces.

Five plunge columns have been driven within the shaft to foundation depth below the level of the bottom floor. These will act as internal columns to support the permanent floors, several of them are also serving as supports to a partial temporary slab at ground level which is being used by construction plant and vehicles. Once the station works have been completed, the plunge shafts will form a significant part of the foundations for the steel framed over-site development which will sit above the ticket hall.

PA1A4527 BFK Crossrail C435 Farringdon 240614 Tunnelling the Cross Passages [online]

Another interesting feature of the works is the interface with the Network Rail Thameslink line and the Farringdon Thameslink station. The East shaft lies on the line of a pair of sidings that run through Farringdon Thameslink into Moorgate. These have had to be severed to allow the Crossrail scheme to be executed. At present, this gives some interesting views of the Thameslink station from points not normally available. The severed tracks are to be reinstated upon completion of the Crossrail station.

Assorted oddities

At the opposite side of the worksite from the station lies Lindsay Street Viaduct, a Grade 2 listed structure built many years ago to carry the eponymous street over the Network Rail lines. Unfortunately some of its foundations lie over the site of an escalator shaft for the new Crossrail station. In consequence it has been necessary to underpin several of the piers of the bridge and make provision for them to be lifted with hydraulic jacks should any settlement occur during the construction of the shaft. The bridge now appears, at first sight, to be hovering slightly above ground!

As mentioned, one of the grout pipes runs below Charterhouse Square, which lies across the road from the station. The Charterhouse began as (and takes its name from) a Carthusian monastery, founded in 1371
and dissolved in 1537. It was known to have been on the site of one of the large graveyards from the Black Death plague. One skeleton had been found previously elsewhere in the Square about one metre below ground. However, the discovery of several neatly stacked intact skeletons, all aligned east/west about two to three metres below ground as the grout shaft was under construction was a bit of a surprise. Crossrail had been prepared for some findings, but not quite this.

The site at the time of the plague lay well outside the City of London in rural surroundings. Staff from The Museum of London and local volunteers from the community have been investigating two pits within the Square, hoping to find remnants of the monastery and to discover the reason for a linear geophysical anomaly that crosses the square. They are rather hampered by restrictions on where they may dig, caused by the requirement to pay due respect to the ancient and protected plane trees in the square. However, they are persevering – assisted by the Crossrail and BFK teams who are very proud of their close collaboration with the Museum and of their very good relations with the communities in the area more generally.

These good relationships were exemplified for us later during a tour of The Charterhouse itself. Here, it was explained how the project has been liaising with the Brothers and other local communities to ensure that there is minimal harm done to local interests whilst the Crossrail project passes through the area. The Charterhouse is worth a visit – it is open to the public at certain times, though there is a small entry fee payable.

So, with Ada and Phyllis buried for good, and several ancient skeletons uncovered, a lot has been going on at Farringdon. And there is a lot more still to do.

Many thanks to Mabel Garcia Aranda, BFK JV corporate responsibility co-ordinator; Jose Garcia, BFK JV construction director; Tom Moore, agent for the JV for the Farringdon East Ticket Hall; Nisrine Chartouny, Crossrail project manager for the C435 Farringdon Station contract; Jay Carver, Crossrail’s lead archaeologist; Nick J Elsden, project manager of the Museum of London and Brother Philip of The Charterhouse for their help in preparing this article.