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Jet grouting for secure financial foundations

KELLER GROUND ENGINEERING has just finished a major jet grouting contract which included infilling abandoned tunnels below the new regional headquarters for financial services company Merrill Lynch in London.

The offices are being built on the site of a former post office. Abandoned tunnels and accommodation under the site had to be infilled to prevent the possibility of future collapse. Jet grout underpinning was then used to allow deepening of the existing basement and to protect a listed facade.

Keller, working for consultant Ove Arup & Partners and construction manager Mace, carried out a detailed sequence of grouting to ensure that the maximum volume of the tunnels was grouted.

This consisted of a staged bulk infilling operation with cement PFA grout, followed by injection of more than 840t of grout through steel pipes with ports at 500mm intervals to achieve pressurised grouting of the final void.

The basement had to be deepened by about 2m for the new building. This meant extending the existing perimeter retaining wall, which was left in place and supported by two levels of raking props. Before the new basement slab could be joined to the retaining wall, the main body of the slab was cast, allowing transfer of the propping loads.

Keller used jet grouting to underpin 68m of the retaining wall, stabilising the underlying Thames gravels and allowing excavation to the new basement slab level. Operations had to be designed around the props and waling beams which complicated access.

Part of the site also has a listed facade supported by a steelwork frame on large concrete pad foundations.

Jet grouting around and beneath each pad foundation transferred the load to the underlying London Clay.

Because the limited access prevented the jet grout rig reaching all the positions, a supplementary programme of hand lancing using a chemical grout was also carried out.

Keller says that the support of the facade was successfully transferred to the basement slab without incident.

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