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Fortune is following fame at last with the redevelopment of Liverpool. Marcus Brierley went to Merseyside to find out more.

Headlines like 'Welcome to the capital of cranes' in the local press leave little doubt the landscape of Liverpool is undergoing a major overhaul.

Inner city building plots are appearing as if from nowhere.

Developers as prominent as the Duke of Westminster are investing hundreds of millions, if not billions of pounds. Transport systems are being touted and road systems routed, as if Liverpool had been declared a future Olympic host.

What it will be is European City of Culture 2008, a designation that is helping to drive a long overdue renaissance. The famous waterfront is begging to be redeveloped.

Despite its charisma, history, attitude and heart, Liverpool seems to have been forgotten during the last quarter of a century in comparison with the big regeneration spending of its northern neighbours Leeds and Manchester.

Its many approach roads are lined with steel shop shutters and graffiti while the inner city, particularly the financial quarter, still operates from old buildings.

One indication of better times ahead is the £58M Unity Sentinel Building under construction for developer Rumford Investments just 200m from the River Mersey.

The project consists of a 27-storey residential tower and a 16-storey office tower. Both will sit above an eight-storey podium of apartments, workspace, retail and restaurants. A basement and semi-basement will provide car parking.

The residential tower features a pair of penthouse apartments - three-storey homes built in what is described as a 'penthouse pod' that cantilevers out from the tower. This will be a steel frame construction while the tower itself will be a cast insitu reinforced concrete frame.

The main contractor is Laing O'Rourke and FaberMaunsell is the structural engineer. The foundations were installed by Expanded Piling.

Foundation design had to consider the neighbours, which include the ship-shaped Atlantic Thistle Hotel. Liverpool's main waterfront, its six-lane riverfront road and the old Mersey Tunnel lie next to the site on two corners, 1m below ground level as it nears the river.

The geology of the river bank is well known: strong Bunter Sandstone dipping gradually towards the river, overlain with sand and made ground.

Basement walls from the site's former occupant, the now demolished Richmond House, were left in place as they were retaining the ground and surrounding roads. This allowed the new, deeper, basement to be built using a contiguous piled perimeter retaining wall, without disturbing neighbouring structures.

Investigations centred around the condition of the sandstone, with mapping of its quality, jointing and strength across the site. Work revealed the presence of some narrow bands of weakness in the rock which had to be removed.

Expanded Piling designed the contiguous piled basement wall and FaberMaunsell made contributions on stresses and cage strengthening.

The wall was designed unpropped; however during construction it became clear that some propping would be needed at the corners of the site where it comes close to the Mersey tunnel.

'As we dropped the foundation level down, we had to introduce additional props at the corners to ensure that the contiguous wall was not overstressed, ' says Peter Lloyd, regional director of Faber Maunsell.

'The critical element was not that the piles themselves were the cause of potential stress, but the piling rig was, particularly the downward forces created when extracting the temporary steel casings, ' he says.

Expanded Piling installed 175, 600mm diameter bored piles at 750mm centres for the residential tower basement wall with a further 185 piles for the commercial tower.

These piles averaged 12m long, including 3m sockets into the sandstone. Cages were full length and boring was carried out using temporary steel casings down to rock head.

Piling did encounter some underground obstructions - the brick foundations that supported Richmond House. These were broken out and removed by Laing O'Rourke.

The towers are completely independent buildings and the foundations have been designed so either tower can be demolished if only part of the site is redeveloped in the future.

Laing O' Rourke decided pads were more suitable and economic at the rear of the site, taking advantage of the higher level of the Bunter Sandstone, which slopes rapidly upwards away from the Mersey.

Bored piles were specified nearer the river, where the sandstone is deeper. FaberMaunsell calculations ensured differential settlement was managed correctly.

All the residential tower columns are on piled foundations. Expanded Piling installed 92, 900mm diameter and 15, 600mm diameter piles for the residential building which were for lighter loads not directly beneath the tower block.

The residential building was designed to resist lateral loads via cores and frames. A particular structural challenge, it includes two levels of load transfer. The lower load transfer level occurs over the basement car park, while the second load transfer level occurs at the eighth floor of the podium.

Within the depth of one floor, transfer walls transmit the load from the tower above onto two columns below. This means 1,500t is transferred to every pair of columns at the lower levels.

For the commercial tower, the contractor installed 74, 900mm diameter and 12, 600mm diameter piles. Expanded Piling were on site with two rigs and a service crane for 11 weeks.

The scheme is due to be finished by October 2006. The basement is retained by a bored pile wall.

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