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Above ground, nothing of the original 1960s, 11-storey monolith remains. Foster and Partners鈥� sparkling design, now under construction, features two eight-storey wings of offices fanning out from a central service core. A glazed roof spans these wings to form a magnificent full-height atrium opening onto the Circus itself.

Below ground, however, is a different story, with much of the Mirror building鈥檚 original substructure retained. The project鈥檚 structural engineer, Yolles Partnership, has taken the unusual step of reusing the old building鈥檚 columns, foundations and retaining walls and incorporating them into the structure of the Andersen building.

Reusing the foundations offered significant advantages. 鈥淚t shortened construction time and saved the developer money,鈥� says David Cuckows, an associate at Yolles. It was a decision that was arrived at only after careful consideration, and one that affected the whole construction programme.

Difficult site

As with many city-centre developments, access to the Holborn site was always going to be difficult. The plot is defined by busy, narrow side-streets, a traffic roundabout and an adjacent building, Maxwell House, to the south.

Refurbishment of the existing building was not an option, however. Andersen Consulting鈥檚 requirements for its corporate headquarters were incompatible with those of a 1960s newspaper office, and so the architect decided to demolish the superstructure. This would be relatively straightforward, but the design team knew that demolishing the basement and building its replacement would be less so. For a start, this would mean the loss of internal floor area as new walls would have to be constructed inside the existing walls, and might also lead to the temporary closure of adjacent roads 鈥� works that would significantly increase the project鈥檚 on-site time and construction costs.

Removing the basement floors and columns was also going to be difficult. The four original reinforced concrete basement floors were strong enough to support the Mirror Group鈥檚 printing presses and other heavy machinery, and the 23 basement columns had been designed to support the load from the 11 floors of offices above.

Faced with these problems, the structural engineer made the unusual decision to retain the existing foundations. 鈥淭he advantage to the client in terms of cost and time far outweighs any disadvantages,鈥� says Cuckow.

However, the client still had to be convinced, because the foundations would not be covered by warranties on the new construction.

鈥淲e took the developer through our reasoning step by step,鈥� says Cuckow. 鈥淭he developer was aware, for example, that the waterproofing of the existing basement was not going to be 100%, but that could be designed out.鈥�

Once it was convinced that the plan made commercial sense, the developer circumvented the warranty problem by taking out a separate insurance policy on the whole building, which involved independent consultants checking the design.

The next job for the engineer was to integrate the new structure with the old. A way had to be found to stop the existing foundations and basement columns imposing the Mirror building鈥檚 layout of supporting columns on Foster鈥檚 new building. Yolles took its inspiration from Terry Farrell鈥檚 Embankment Place, above London鈥檚 Charing Cross Station, and designed a huge reinforced ground-floor slab structure that would transfer the loads from an entirely different column layout above ground to the existing columns below. This gave the architect freedom to align the above-ground floors differently to the basement.

Having finalised the structural design, the engineer then had to work out how to construct the new building around the retained foundations. Even though the basement walls, the basement raft foundation (the basement floor) and the columns from the raft to ground-floor level were all that were to be retained, careful programming would be needed if construction was to proceed smoothly.

The engineer was particularly concerned about the possible failure of the ground-retaining wall while the basement floors were being demolished. Investigations carried out by Yolles showed that the wall would need propping while the two lower basement floors were demolished and the new ones constructed.

Planning

A sequence of demolition and construction was devised. The two lower floors were demolished first and replaced by a single intermediate floor. Temporary props were then installed on the new floor to support the retaining wall while the upper basement floor and ground floor were demolished. Finally, the new ground-floor transfer structure was constructed.

Things did not quite go to plan when it came to creating the new basement floors. These were to be supported on the existing columns. Yolles had originally proposed using a high-pressure water jet to cut into the columns to create a recess on which the new floor slabs would sit.

However, the columns were found to be so densely packed with reinforcing bars that the jets were ineffective.

Swift Structures, the concrete subcontractor, together with pre-stressed specialist Matthew Consultants, Yolles and the main contractor, Bovis, opted instead to clamp the new floors to the columns by post-tensioning the concrete. This involved casting a small square of floor slab complete with two horizontal, U-shaped loops of reinforcement set at right angles to each other around the top of each column. The ends of the loops were left projecting from the concrete square. Once the concrete was strong enough, the reinforcing loops were tightened to 鈥渃lamp鈥� the concrete to the column. The surrounding floor slab was cast around this section once the concrete had achieved adequate strength and initial shrinkage had taken place.

Demolition of the old building started in October 1998 and construction of the new basement floors began a month later. The below-ground works will be completed this month. Despite the problems of programming the works, Cuckow is convinced the decision to reuse the Mirror building鈥檚 foundations was the right one.