With a building to cover the Whitechapel shaft the only clues about the Crossrail work underway there are the lorries taking material to and
from the site.
Work on the Crossrail station tunnels at Whitechapel is well progressed but pedestrians walking along Whitechapel High Street could be forgiven for missing the major work that is taking place under their feet. The main focus of the construction is sandwiched between a multi-storey supermarket car park, a school and apartments and, with a building covering the site, there are very few visual clues to the hive of activity underway.
The work at Whitechapel is part of contract C510 and involves construction of the stations, platform tunnels and concourse escalator shafts for the stations here and at Liverpool Street. According to the site team, the job is technically challenging as it is all sprayed concrete lining (SCL) work and it is more advanced in terms of the volume of SCL undertaken than other Crossrail contracts.
The work is being carried out by BBMV joint venture which is a five way agreement between Balfour Beatty, Morgan Sindall, BeMo Tunnelling and Vinci Construction. Bachy Soletanche was brought on board as a subcontractor to undertake the compensation grouting work.
The C510 site team is working close to the site teams of C511 and C512 here and the C510 work is being undertaken and accessed via a temporary shaft that is adjacent to the permanent shaft in order to keep work flowing. The permanent shaft has been built under C511 and will be excavated under C512.
“The temporary shaft is covered with an acoustic shed to manage the noise levels and allow us to continue operating 24 hours a day over three shifts,” says BBMV C510 project director Nigel Russell.
The shaft is 12.5m in diameter and 30m deep with two openings at the base.
Ground movement is a concern at Whitechapel and the solution to prevent the Crossrail project adversely impacting on the area calls for a range of techniques. “We have 80 surveyors working on the contract and automated monitoring systems from Soldata,” says Russell.
Jacks have been placed under the Sainsbury’s supermarket car park so that the level can be maintained as tunnelling progresses.
The temporary shaft is covered with an acoustic shed to manage the noise levels and allow us to continue operating 24 hours a day over three shifts
Nigel Russell, BBMV
Compensation grouting is also being undertaken for another three buildings close to the site and is being carried out from a shaft between the temporary C510 access shaft and the permanent shaft at Whitechapel. There are 60 tubes à manchette (TAMs) in place at 16m below ground level which extend radially from the shaft for up to 63m.
“Grouting was undertaken before the tunnelling started to ‘tighten’ the ground and then grout is pushed into the TAMS in accordance with the predicted settlement as tunnelling progresses,” says Russell.
However, there are a number of sensitive structures that could not be stabilised using compensation grouting techniques.
“The tunnels pass under a block of flats called Kempton Court which is supported on piles and the tunnel construction will actually cut the bottom 2m off the piles,” says Russell. “The piles were designed on friction rather than end bearing and the cut off point is below the reinforcement level.
“The other structure that is sensitive to the work is the school where we have already carried out minipiling work and covered the large windows with film just in case the settlement causes them to crack.”
Excavation of the platform tunnels has been completed and work is now underway on the adits that will connect them.
The primary SCL lining is already in place where excavation has been completed and work is now progressing on the secondary lining, which in most places is another layer of SCL but in some higher stress sites, the secondary lining will be cast insitu.
“Excavation of the tunnels is being carried out using a 4m to 6m diameter pilot tunnel with 1m advances and a sloping excavation face,” says Balfour Beatty tunnelling director Paul Hoyland. “This is the safest approach and also helps to reduce settlement. The pilot tunnel is then enlarged after the primary lining is in place, in order to minimise the size of the unprotected face.”
The enlargement is undertaken in two stages with the crown excavation done first followed by the invert excavation.
The excavation is being undertaken by a Terex Schaff 312 with a central conveyor in the smaller diameter tunnels, but in the larger tunnels a Liebherr 944 excavator with dumpers working alongside is being used.
Excavation is being carried out using a 4m to 6m diameter pilot tunnel with 1m advances and a sloping excavation face
Paul Hoyland, Balfour Beatty
“We are also probing ahead of excavation as we are very close to the bottom of the London Clay at Whitechapel and it is possible that the excavation will encounter the granular, sandy bed of the Lambeth Group,” says Hoyland.
“The Lambeth Group is likely to contain some water which is under pressure so we are installing depressurisation pipes as the excavation progresses.”
The SCL mix for the primary and secondary linings is essentially the same with steel fibre reinforcement but the secondary layer also has polypropylene for fire retardancy. The SCL is produced on site to ensure there are no delays to the operation. The mix is an upper J2 which has been designed to have a rapid strength gain but the team are using retarders to give the material a life of three to four hours. The design aimed to give strengths of 30N, but Hoyland said that the strength was generally around 60N.
The excavation and shotcrete profile is being controlled by a laser total station system. The primary lining is around 75mm in thickness, whereas the secondary lining is 150mm and up to 600mm thick in areas where there are openings, such as close to the cross passages.
Work on the platform tunnels and crossovers is well progressed but the BBMV site team has a new challenge yet to deal with - excavating the escalator shafts from tunnel level upwards.
“An uphill excavator is being manufactured for the work and we think it is the first time such a machine has been used in the UK,” says Hoyland. “The GTA machine uses the principles applied in the mining sector and will hang from the crown of the tunnel.”
As well as having excavating capabilities, the machine is fitted with an SCL sprayer and will be used to excavate three escalator shafts at Whitechapel.
The tunnel boring machines will arrive at Whitechapel, having travelled from Stepney Green, before progressing on towards Liverpool Street.
They are expected to arrive in June next year so the pressure is on to ensure the tunnels are completed before then.
Managing the risk of movement caused by tunnelling under the City was one of BBMV’s first challenges. It needed to know the baseline levels and positions of buildings and streets above contract C510 and up to 100m around it, before construction started.
And of course, this information has to be recorded accurately and repeatedly before and during construction and after work has finished.
Total stations are widely used to measure and record positions on construction sites. More than 100 of these instruments were supplied for this contract by instrument manufacturer Topcon.
They automatically measure movement above ground and manually check the tunnel alignment below ground.
To monitor movement above ground, the challenge was in achieving the required accuracy and reliability of results, and in accessing the instruments and the information measured.
Many of the instruments had to be fixed onto the sides of buildings from where they collectively measured the exact location of thousands of prisms fixed to other critical buildings or infrastructure being monitored.
“Once we identified which buildings needed monitoring, prisms were located on these buildings and total stations installed on others,” says BBMV’s engineering survey manager Alastair Cruickshank.
“One of our main concerns was always logistics: ‘How will we reach that total station location? Will we require a road closure? Can the total station see the prisms we need to survey?’”
Crucially, once the total stations had been installed, Cruickshank and his team wanted to avoid having to revisit them due to the time needed to organise road closures, the need to notify building owners and the cost of hiring plant and trained personnel.
One reason for revisiting an instrument could be if additional prisms were installed on new structures to be monitored, requiring the instrument to be manually programmed to measure the position of each new prism.
An automatic total station manufactured by Topcon provided a solution to this problem. The MS05AX model has a function called “matrix detection”, which can routinely scan for new prisms.
A simple function, but one which was not standard on other total stations on the market.