Jubilee line's plan to prop neighbours

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RESEARCHERS at the University of Bradford have come up with a way of stabilising the Treasury. Disappointingly, perhaps, this is not a system of economic forecasting, but a new type of grout to protect the building from subsidence when the Jubilee Line extension tunnels beneath its foundations.

Grout injection techniques and special chemical grouts developed by the university's civil engineering department will stop water getting into the tunnel during construction and offset subsidence to buildings directly over the route. Apart from the Treasury, these include the Royal Festival Hall and the Royal Automobile Club, which has a swimming-pool lying just 5m above the crown of the tunnel. Victorian brick railway arches at Waterloo and overground railway lines will also need protection.

Stuart Littlejohn of Bradford University is specialist technical adviser to Amec /Geocisa, the Anglo-Spanish joint venture responsible for the stretch of line between Green Park and Waterloo. He says some buildings will tolerate the subsidence that occurs because of tunnelling, but others will need to be protected.

Rather than underpinning buildings by deepening their foundations, they will be stabilised by compensation grouting, in which liquid grout is injected through pipes under the foundations using computer-controlled systems, and then sets, pulling the ground together. According to Professor Littlejohn, there have been no previous trials of compensation grouting in London.

'The scale of the grouting operation for the Jubilee Line extension is without precedent,' he said. There will be at least a 1 per cent loss of ground as the earth moves into the tunnel before it can be supported, which will cause subsidence at the surface.

'Many buildings won't be comfortable with this,' Professor Littlejohn said.

London Transport, which is responsible for the pounds 1.9bn project, has told the owners of buildings over the route that cracking will be kept below an agreed level, referred to in civil engineering as category 2 cracking.

Survey teams are working on forecasts of the area of movement and its magnitude, and assessing how much movement each building can tolerate.

The main difficulty in propping up sensitive buildings is the variation, from fine sand to coarse gravel, of the ground that the tunnel will pass through. Each type will need a different strength of grout to hold it together.

'The grout must not be so viscous that it cannot be pumped under the building, but it must be strong enough to stabilise the ground when it sets,' Professor Littlejohn said.

(Photograph omitted)