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At the geographic heart of this system are the world's first underground railway tunnels, opened in 1863 and built just below the surface of metropolitan London using the cut and cover method. Later, circular tunnels - giving rise to the nickname 'The Tube' - were dug through the London Clay at a deeper level.

COMPLEXITY SOLVED
Now celebrating 150 years of operations, the Underground is still growing: the current Bond Street Station Upgrade (BSSU) project, slated for completion in 2018, has been dubbed "one of the most complex tunnelling projects in the UK." As London's future Crossrail line intersects with the Bond Street Station, passenger numbers in the expanded interchange are expected to rise from 155,000 to 225,000 daily.

What makes the BSSU so complex is all the construction that is already there, located in London's busiest shopping district, the West End, and comprising a complex web of train tunnels, pedestrian walkways and escalators that include connections to the Jubilee and Central lines. "The new tunnels are located in close proximity to so many existing ones," says Dr. Ali Nasekhian, senior tunnel/geotechnical engineer with Dr. Sauer and Partners, London, the firm providing tunnelling expertise to the project. "As a result, the design challenges we faced included complex tunnel geometry and alignment, limited clearance to existing building foundation, restricted worksite and strict settlement criteria."

Dr. Sauer and Partners have been designing railway and road tunnels for over 30 years. In 2010 the company was subcontracted to a joint venture of Halcrow and Atkins (the main contractor is a Costain Laing O'Rourke JV), and has responsibility for preliminary-to-detailed design and construction on all BSSU sprayed concrete lined (SCL) tunnels.

These include two access shafts, one lift shaft, four construction adits (entrance passages), two binocular cross passage tunnels, four large concourse and connection chambers, three underpass tunnels, two over-bridge tunnels cutting through existing platform tunnels, two niches for electrical and mechanical equipment and four inclined tunnels for escalator barrels. The total length of tunnels, at widths varying between four and 10 meters, amounts to some 450 meters.

OPTIMUM DESIGN
Dr. Nasekhian came on board at Dr. Sauer in 2011 after finishing his Ph.D. in geotechnics at Graz University of Technology, Austria, and his eight years' experience with CAD was immediately put to work. "In such projects as the London Underground, where large tube systems are already in place and the impact of new structures on existing ones has to be carefully considered, comprehensive 3D analysis benefits both client and designer," he says. "In addition to 2D analyses, where time and budgets allow, 3D models can greatly assist in identifying the optimum design."

The Dr. Sauer design team of eight engineers used Dassault Systèmes' SIMULIA, Abaqus finite element analysis (FEA) software to perform all 3D numerical analyses ahead of the main tunnelling works. (Tunnel excavation began in the summer of 2013, with completion scheduled for 2015.) Dr. Sauer & Partners have employed Abaqus since the 1990s.

"We find SIMULIA's Abaqus solver to be very powerful, stable and speedy for plastic analysis of models with large numbers of elements," says Dr. Nasekhian, who led the FEA modeling effort. "It is well recognised that the quality of FE models is largely dependent on the quality of the mesh. Of course, finer meshes require a larger number of elements. I myself had not used Abaqus before joining the Dr. Sauer team, and I found that pre- and post-processing, and creating and manipulating large, complex geometries, is amazing with this software."

"If any changes need to be made in the geometry, which often happens when you are creating the most efficient design, Abaqus lets you modify them fairly rapidly. Such handy tools available in Abaqus/CAE reduce the risk of delay in delivering large 3D FE models."

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