Patrick Reynolds, Freelance Reporter

Preparations for Crossrail's major tunnelling and excavation works rest on years of extensive site investigation, studies and analyses to map the geology and geotechnical characteristics of the strata along the alignment of the twin tubes. As the second in a series of geotechnical features, this article examines the site investigation work carried out and joins Article 1, a project overview, and Article 3, examines the settlement control measures being implemented.

Crossrail has been gathering geotechnical information for the best part of two decades.

Site investigation (SI) work began in 1992 during the previous attempt to develop the east-west mainline underground rail scheme for London. Tunnels were envisaged then to run between Paddington and Liverpool Street mainline railway stations.

Mike Black, the project's Geotechnical Manager

When the attempt to develop Crossrail was shelved in the late 1990s the SI effort was also halted – until 2001-2 when renewed political and business interest began to build new momentum behind the scheme. The pace of the investigation work was initially slow but increased to be reasonably constant since about 2004. The major packages of SI work are now coming to an end having generated a good spread of data, particularly for groundwater conditions.

Mike Black, Geotechnical Manager for the project, said: "We are now in the late stages of ground investigation and we envisage that most fieldwork will be complete by October 2010."

Site investigation
When SI work resumed, in 2001-2, it covered again the corridor studied in the mid-1990s through central London, extending from west of Royal Oak near Paddington Station to the east of Liverpool Street Station. It also encompassed alignment changes, alterations to proposed station designs and where there had been changes in the city's built environment. In addition, the fieldwork also replaced some equipment, such as damaged or non-functioning piezometers installed in the early 1990s.

The fieldwork has also examined the new routes for Crossrail in the east of the capital; anything east of Liverpool Street is new alignment, extending beyond the 1990s vision. As such, the most recent SI focus has been in the eastern end of the route where the challenge is concerned with the Lambeth Group, as opposed to the classic favourable tunnelling medium of London Clay to the west.

Black said that many of the lessons learnt on the previous major tunnel projects were incorporated from an early stage, such as the need for high quality ground investigation with detailed testing and description. A number SI of contractors have worked simultaneously on different sections of the route.

Crossrail has undertaken or planned 33 packages of SI work, which have addressed most of the geotechnical issues although more could be done if and when necessary during the tunnelling phase. There have been about eight large packages costing approximately £1 million (US$1.56 million) to about £1.5 million (US$2.34 million) each, as well as many smaller packages.

A typical large package, said Black, could comprise up to 40 boreholes covering large areas with fieldwork and take three to four months and about the same again for lab tests and reporting. The packages included cable percussion and rotary coring techniques for the boreholes and a variety of tests, including insitu testing of the bores with permeability, cone penetrometer testing, standard penetration tests and both downhole and crosshole geophysics.

The main challenges, said Black, have been finding locations in which to fit a drilling rig on pavements which having utilities or other infrastructure below. This is even more critical for stations and shafts where there is less scope for finding alternative drilling locations. A further challenge is trying to find a site to house a SI compound in the dense urban environment.

A 3D view of the Tottenham Court Road complex where finding sites for SI work was complicated by the dense urban environment

The use of the techniques are independent of package size and are guided by design issues, such as specific soil parameters or behaviour, and also either identifying obstructions or establishing foundation depths of existing structures. For example, in west London, near the Royal Oak portal, Crossrail used primarily geophysical tests to establish that piles supporting a building were shorter than anticipated and therefore not an obstruction to the tunnel drives. The first TBM drive on the project will be launched from the Royal Oak portal in late 2011.

In east London, however, there has been an issue with deeper building foundations for a property developed in the 1990s before there was any safeguard of the project corridor. The challenge is not too difficult to solve from a technical perspective and would require working in consultation to re-design the foundations using smaller piles. It is not the first time such a change to building foundations would be required. On the London Underground Jubilee Line Extension (JLE) project, the tunnel alignment passed below the RAC (Royal Automobile Club) building in Pall Mall and planning works called for mitigation measures to take load of column away from a pile leaving it unloaded, and transfering it into two adjacent piles.

Other challenges for the Crossrail tunnelling works that have been identified by the SI fieldwork include:

• Man-made obstructions, such as wells.
• Ground conditions, such as variable strata, which could mean mixed face conditions along stretches of the TBM drives, or different permeabilities that could result in perched groundwater or inflow from sand lenses or channels.
• Structural features that have rapid changes in strata, such as faults or scour features that could lead to water ingress and/or ground instability.

Black said that, while the challenges are typical of any tunnelling works that might be carried out in the south east of England, they are likely to be encountered in various parts of the Crossrail tunnel drives. In general, both JLE and the Channel Tunnel Rail Link (CTRL) projects in London faced similar challenges.

Geology
Initially, explained Black, the ground investigation work concentrated on obtaining a broad framework of the geology along the route. Several phases of SI work have been carried out at increasingly closely spaced intervals to gain information for both projects tunnels and structures. The geology – soil and groundwater – is similar to that encountered on CTRL and JLE although the proportions of the strata differ with depth and location of each tunnel alignment.

Noting that many of the staff investigating the geological conditions for Crossrail, were also involved with JLE and CTRL, Black added: "We have compared the detailed geology between all the schemes and have improved the geological understanding for the London Basin, generally."

Running tunnels are aligned mainly through London Clay and into the variable Lambeth Group to the east

Geology along the Crossrail alignment broadly comprises stiff clay, sandy clay and sandy gravel, sands and clays with subordinated limestone and sandstone, sands and some chalk – or, more formally, from below the ground surface and below made ground and river terrace gravels the sequence is London Clay, Harwich Formation, Lambeth Group, Thanet Sand Formation and Chalk Group. The characteristics of the strata are:

• London Clay – generally, relatively homogeneous stiff clay with cyclical variations in sand content.
• Harwich Formation – mainly in the east of the route, comprising three different members (Swanscombe, Oldhaven and Blackheath) with quite varied constituents and thicknesses, which range from sandy clay, through sand, to sandy gravel, and are 1m-5m in thickness.
• Lambeth Group – predominantly in the central section of the tunnels and up to 20m in thickness, comprising variable interdigitated sands and clays with subordinate limestone and sandstones.
• Thanet Sand Formation – relatively homogeneous fine sand, up to 16m in thickness and predominantly in the eastern section of the tunnels.
• Chalk Group – a soft white limestone in the eastern section of the tunnels.

The running tunnels are being excavated mainly through the London Clay and Lambeth Group. The elevation of the tunnels begins well within the clay in the west of central London and comes closer to, or meets, the interface with the Lambeth Group at about the centre of the tunnel route. To the east, the tunnels at Canary Wharf Station are in the Lambeth Group and then on the Woolwich arm the ground is more varied with wet gravels and sands expected.

Eight TBMs will excavated the parallel running tunnels from the west and the east towards Farringdon Station and under the Thames to Woolwich

The water table, as measured at station locations, is generally within the Lambeth Group, dipping to the Chalk in some central areas and In most cases it is below the Thanet Sands.

At Paddington and Bond Street Stations, the twin tubes are in London Clay, although for the former the water table is near the top of the underlying Lambeth Group while below latter Bond Street Station it is in the Chalk. Excavation is in the same strata at Tottenham Court Road, the next station to the east, with the water table also in chalk.

Moving further east to Farringdon, the geology is more complex. There is a major fault in the area and the elevation of the tunnels, about 25m beneath the surface, is at the interface of the London Clay and Lambeth Group. The water table is at about the interface of the underlying Thanet Sands and Chalk.

The twin tunnels are in London Clay below Liverpool Street and Whitechapel, although coming into contact with the Lambeth Group at Liverpool Street. Below both stations, the water table is in the Lambeth Group.

In Docklands at Canary Wharf, the water table is at the elevation of the running tunnels in the Lambeth Group.

The vertical elevation of the running tunnels varies, between the underground stations and as the alignment negotiates and weaves through the lacework of existing metro, road and utility tunnels.

Muck removal by barge from the Canary Wharf Station   (John Sturrock Photography)

A number of excavation challenges are associated with the different types of strata, said Black. The London Clay, Lambeth and Chalk groups contain nodules, concretions and limestone layers that can cause excessive wear on equipment leading to unexpected stoppages.

In addition, the Lambeth Group is variable, with sand channels, that can be water bearing and present a risk of a sudden influx of water with consequent stability of the face. The sands are predominantly glauconitic which, on previous tunnelling projects, have caused issues with removal of oxygen from the air.

The Thanet Sands are abrasive and can cause wear on the TBM face.

Potentially, the wettest conditions could occur in the south east arm of the route, along the section that stretches from the surface station at Custom House to below the River Thames and through to North Woolwich and Plumstead. There is the typical sequence of superficial deposits underlain by river terrace gravels, Thanet Beds in places, and chalk.

The lower and upper aquifers are effectively combined and groundwater is near the surface at about 5m below ground level for much of this stretch.

In dealing with Thanet Sands, CTRL had an approach of general dewatering which then resulted in high suctions and a relatively "hard" medium for boring east of the Stratford box. For areas where groundwater levels are not below the Thanet Sands, Crossrail will take a slightly different approach. Dewatering will not be employed routinely and only used at some cross passages, shafts and boxes. Localised dewatering is only needed for excavations not performed by EPBM or slurry shield TBM.

The Crossrail complex as it aligns with existing underground infrastructure at Liverpool Street Station

The main TBM drives will progress from the west and the east from the portals at Royal Oak and Limmo Peninsula - to terminate at the Farringdon Station area. The selection of the Farringdon Station location was made for geological reasons, but primarily for programming.

The arrangement of the biggest tunnelling packages to split either side of Farringdon is due to the logistics of spoil removal and supply of segments.

TBM drives in east London will also create the two branches, one towards Stratford but only partly in tunnel, and the other to Plumstead and mostly underground. The depth of cover for the running tunnels will range from 12.3m – to 43m, and the spacing of the parallel tubes varies from 3.5m – to 120m.

While the running tunnel excavations are expected to be fairly straightforward, the TBM drives will have to deliver efficient performance against the toughest settlement control regime for large rail bores in the capital.

This topic is explored in the next of the geotechnical article series.

References

Crossrail Geotechnical Series - 1: Preparing the ground for Crossrail - TunnelTalk, Aug 2010
Crossrail Geotechnical Series - 3: Settlement control measures - TunnelTalk, Aug 2010
Monitoring contract for Crossrail - TunnelTalk, June 2010
Crossrail management mobilized - TunnelTalk, May 2009

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