The redevelopment of London Underground Victoria station was first covered in January 2013 (issue 99). In that report, the comment was made that The Rail Engineer would revisit the project to keep track of how work was going. Writes Chris Parker
So arrangements were made to meet with Keith Ramsay, section manager – south ticket hall, at the project offices at 25 Terminus Place. He was only too pleased to give an update on the progress that has been made by the Taylor Woodrow / BAM Nuttall joint venture.
The project is focussed primarily on providing more space and better Victoria Line access for the very large and rapidly increasing numbers of customers seeking to use the station each day. It is already regularly necessary to control access to the station for safety reasons during peak periods due to congestion, which sometimes requires holding customers outside the entrances.
It was predicted that by 2020, if nothing had been done, the station would have been closed more often than open. In addition, there is no step free access to any of the LUL platforms at the station at present.
The project addresses these issues in a number of ways. The first is to open up access to both ends of each Victoria Line platform, there currently being access only to the southern end of each. Next is the construction of an entirely new ticket hall to the north of the station, providing a new large circulation area at the head of the new northern accesses.
The existing south ticket hall is too cramped for today’s needs, and so this is being doubled in size. Nine new escalators and eight new lifts are being provided. An emergency access shaft is being built, with lifts and stairs to provide separate access for the emergency services and for emergency evacuation purposes. The existing stairs to the main line station are being widened and new lifts are to be provided here too.
Work in progress
Transport & Works Act authority for the scheme was granted in late 2009, and work began on site on the Mott MacDonald designed scheme in May 2011.
The Taylor Woodrow / BAM Nuttall JV contract is a significant part of the overall £700 million programme and the new works are due to be completed and opened to the public in full in 2018. Some 400 people are currently working on the site, about half are employees of the JV and the remainder of sub-contractors which include Cementation Skanska (piling), Keller (jet grouting), T Clarke (electrical installations) and Bailey Rail (mechanical installations).
Around the station site, the enabling works for the project have been demanding in themselves. The area involved is large and is one of the most intensively occupied areas of London. The usual service diversions were complex. Then there is the King’s Scholars’ Pond Sewer close by the site of the new evacuation shaft. Carrying the former River Tyburn, this is a large, old and sensitive structure. New switch rooms for the station needed to go in and be commissioned too.
The north ticket hall is going below ground at the junction of Bressenden Place and Victoria Street. The new access to street level will be from Cardinal Place. Constructing it has meant moving the Bressenden Place highway bodily eastwards, partly onto the roof slab over the new works.
Water, water everywhere
Excavations are mostly in sands and gravels as the London Clay beds here are below the required level of most of the new works. Ground water levels are high; indeed the existing station remains open only by means of the continuous removal by pumping of huge volumes of water.
The new ticket hall structures will be water- tight, but that means that they have to be tied down by piles to prevent them floating when groundwater levels are high. All this water has its uses though – it is proposed to be used in a cooling system for the station as it is significantly colder than the ambient temperature in the tunnels.
Because of the generally wet conditions, the ground is first prepared using jet grouting. This entails the insertion of plastic sleeves in a carefully designed pattern. A drill sleeve is inserted into each one in turn through which the grouting is deployed under high pressure. The grout and pressure are both carefully selected to ensure that the groundwater in the soil is displaced and replaced by the grout, creating a roughly cylindrical zone of strengthened soil. The design of the grouting pattern ensures that each column joins with its neighbours to form an impermeable barrier of material through which tunnelling or excavation may proceed easily without water penetrating into the works.
Getting the necessary pattern of grout tubes into a congested site like this one is tricky. Above the zone to be grouted is a mass of utilities and services which must not be damaged or disturbed. On this site, extensive use has been made of 3D computer modelling (the BIM of which we hear much nowadays, and for which this project won a BCI Award in 2012) to map the positions of these obstructions, model the grouting positions and visualise the expected grout penetration. By this means, it has been possible to avoid conflicts and see where there might be gaps in the effectiveness of the grouting and avoid these occurring.
The introduction of Skanska’s piling rig and handling crane for the secant walls into the site of the south ticket hall extension was tricky. This site is quite small, and the ramp required if the rig was to be driven down into the hole on its own crawler tracks would almost have filled it. In addition, it was estimated that around 280 vehicle movements, equivalent to approximately 3.6 tonnes of carbon and 4,500 litres of fuel, would have been required to get the rig into and out of the site this way, since the ramp would have needed removing once therigwasinplacesoastoallowittodoits work. That would have meant building and removing the ramp twice.
The JV has worked with heavy lifting and transportation specialist Mammoet before, so approached that company for options . The proposal was for a lifting gantry that employed two 100-tonne strand jacks to lift the rig. Once lifted, the rig could be moved laterally by means of carriages running along the gantry beams. This permitted the rig to be lowered into the excavation and for its subsequent removal after Skanska had completed the eight week piling task.
Moving Bressenden Place
The site for the new north ticket hall is larger so Mammoet’s gantry wasn’t required, but there was a different complication here instead. In this case, as already mentioned, there was a road over the top, Bressenden Place. This meant that the ticket hall structure had to be constructed in two parts so that the road could be shifted laterally onto the first completed section before work could commence on the second. The road move from east to west was undertaken over one weekend.
An innovation of this project was the form of protection installed over the roof slabs of the new ticket halls. Instead of the tiles usually installed on top of the waterproofing, the JV proposed a new system which was agreed by the client. After the application of sprayed ‘Pitchmastic’ waterproofing, a 100mm thick layer of red fibrous concrete was added. This is more economical to install and arguably gives better protection against damage by future excavation.
The new emergency access shaft goes down past the King’s Scholars’ Pond Sewer to connect to the existing Victoria Line platform tunnels, bottoming out in between the two of them. The shaft passes so close to the tunnels that its cross-section has to change from circular above them to a narrower elliptical shape as it passes between them. Even so, the cast iron tunnels segments were exposed in places as the shaft was excavated.
There was much concern about this exposure because these sections of each of the tunnels were lined with experimental unbolted cast iron segments when the Victoria Line was built in the 1960s. Due to the care taken by the construction team, close monitoring of the tunnels detected movements of no more than 3mm, so the trains inside were able to continue to run undisturbed throughout the works. The shaft was dug in one metre lifts, and each of these was lined with sprayed concrete before the next could commence.
The connections into the station tunnels of the Victoria Line necessitated digging and lining a 15 metre long passageway from the shaft base parallel to and in between the station tunnels, then a cross-passage to complete the actual connections into them. All of this was hand mined, exposing the tunnel linings either side of the main passageway, and requiring these to be propped temporarily until the permanent works were placed. In this job alone some 1,000 tonnes of clay were excavated while 114 tonnes of steel and 250 tonnes of concrete were placed during 40,000 man hours of work.
Extensive tunnelling is required for access passages and escalator tunnels, and jet grouting was judged to be the best way to allow this to be done safely in the conditions, with the water table only seven metres below ground level. Over 2,000 jet grout columns are being installed by sub-contractor Keller to create a safe zone for the tunnelling to pass through.
Piles of challenges
For the ticket halls, the method of construction is to install secant piled walls around the perimeter of each worksite and excavate the new voids ‘top down’. By creating these perimeter walls down into the underlying London clay, the water is cut off from entering the excavations. The necessary internal columns within the site are driven from the top by piling rig, as are the plunge columns intended to support the planned over-site developments. Casting the roof slabs of the new ticket halls will prop the tops of the piles. As excavations below this proceed, lower props will be required, but casting the floor slabs level by level as the correct depth is reached will meet that need.
This, the largest jet grouting project in the UK to date, is nearly complete. The grouting plan was designed in 3D software and the actual results are monitored closely and are similarly modelled in 3D to produce an accurate as-built record. Probing into the grouted area is undertaken to detect ungrouted permeable zones so that, if any are found, they may be rectified. The project won a ‘Ground Engineering’ award in 2013 for this use of the jet grouting technique.
The tunnels, generally six metres in diameter, are being dug through the jet grouted zones and given a primary lining of sprayed concrete impregnated with steel fibres which is applied by robotic equipment. After the installation of a waterproofing layer, an in-situ concrete secondary lining will be applied.
The site is very congested and surrounded by heavily used roads, footpaths and, of course, railways. The theatres have matinees as well as the usual evening performances and are particularly sensitive to noise and vibrations while they are taking place. This all means very careful attention to planning and sequencing of the works themselves and the associated movements into and out of the sites.
The project team is very proud of its good record in terms of relationships with neighbours, environmental performance and in respect of health, safety and welfare. The JV partners each have safety and quality programmes and co-operate to ensure that the JV delivers on all of these on this project. BAM Nuttall’s strap-line ‘Don’t walk by’ has been taken to heart across the JV in the pursuance of safety, and Taylor Woodrow’s ‘Defect Free’ programme ensures that quality is delivered. The project itself also runs a safety and quality programme called ‘Beyond Zero’ and an extensive community liaison programme provides regular information to locals and travellers on the trains and buses using the area.
The project has won a number of awards for safety and quality as well as National Skills Academy awards in 2012 and 2013. In October 2012, the project reached a million man hours without a RIDDOR reportable accident, and went on to achieve 1.4 million hours (285 days) before the first reportable incident.