WORK ON the UK's second tallest building is literally overshadowed by its tallest building. For foundation work on Hongkong & Shanghai Banking Corporation's new UK headquarters at Canary Wharf in London Docklands is being carried out with the spectacular backdrop of Canary Wharf tower.
The new HSBC building, one of the tallest structures being built in Europe, is also the scene of some of the highest capacity pile load testing ever carried out in the UK.
Geotechnical contractor Keller recently started work on the foundations for the 44 storey structure in a contract let by Canary Wharf Contractors. Working from within a 12m deep cofferdam, the tower is being founded on large diameter bored piles designed by Ove Arup & Partners. The contract, which includes the piled foundations for an adjacent 22 storey building, is to be finished in a demanding 18 weeks.
Ground conditions are recent terrace gravels overlying the Tertiary clays and then sands of the Lambeth Group, with the dense Thanet Sand Formation at depth. Piling is being carried out from around the level of the original dock basin.
Working pile loads for the tower are very high, each typically around 13,700kN. Keller Ground Engineering is installing the 332, 1,500mm diameter piles to lengths up to 33m under bentonite support.
'We installed the preliminary test pile on site towards the end of last year,' said Keller special projects manager Steve Longdon. 'The 900mm diameter pile had to be test loaded to 21,000kN in compression. We used working piles for reaction and then subsequently tested another 900mm pile for uplift loads of 15,000kN.' Keller claims it is one of the highest capacity pile load tests of its kind yet carried out in the UK.
'The results were excellent,' said Longdon, 'providing Ove Arup with a great deal of valuable data for working pile design.'
The settlement criteria for the test piles was a maximum of 6mm at design verification load and 15mm at design verification load plus 50% safe working load.
Ove Arup also set extremely tight settlement criteria of just 6mm for working piles. With more than 70% of working load designed to be carried in end bearing, the performance of the sand founding stratum is particularly crucial to the success of the project. For this reason Keller is base grouting every working pile.
'The base grouting induces a base reaction and therefore a pre-stress into the pile,' said Keller senior project engineer Mark Connolly. 'This is necessary to control settlements and improve pile performance.'
To carry out the base grouting operation Keller has mobilised one its Soilfrac modules to site. These self-contained units house powerful high pressure grout pumps and on board computer monitored instrumentation with immediate hard copy printouts. There are also two bentonite mixing and de-sanding units, five boring units and six handling cranes on the busy site.
Keller was preparing to test load a working pile as Ground Engineering went to press. 'This test will be equally demanding,' said Longdon. 'We will be testing a 1,500mm diameter pile to 27,000kN using anchor piles to provide a reaction.'
An added complication is dewatering currently under way on the site which could induce settlement and drag down in the piles.
To proof load the effects of this negative friction, a 30% increase in working load will be added to the pile for the test. Nevertheless settlement criteria remains the same, making it, like the structure, a very tall order.