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May Gurney turns t the tide

PILING - A hotel on London's South Bank will sit on hundreds of piles forming a wall to control groundwater movement, all put in without vibration equipment.Adrian Greeman reports

JUST a stone's throw from the London Eye is the old white concrete 'pyramid' building, once an extension to the Greater London Council's Thames-side offices. Since the 1980s GLC abolition, it, and the roundabout where it sits, have become a rundown eyesore.

But the spreading renaissance of the South Bank is coming this way and the pyramid is earmarked for demolition, to be replaced by a new office development.

In the meantime another part of the roundabout, an old car park next to the rail viaduct into Waterloo's international Eurostar station, is being built up.

'We are putting a new hotel on what was a fairly scrappy piece of land, a single storey factory and then just a ground level car park space, ' says Gus Wright, project manager with main contractor Ardmore Construction.

Ardmore is building the shell and core of the building for client Galliard Hotels.

The hotel will be 14 storeys high with just under 400 rooms - service apartments for buying or leasing longterm.These new types of hotel are just arriving in the UK after successful introduction in the USA over some years, and already seem popular.

'We are just two weeks on site and already a third of the rooms are sold, ' says Mr Wright.

The structural work is not unduly complex - conventional reinforced concrete frame slab floors mainly - but it needs to make full use of the site.This is a triangular space on the south side of the roundabout with the rail lines and station at the back and the south London 'lying-in' hospital on one side, with more of the rail viaduct.

Structural designer Yolles Partnership decided on a secant pile wall bounding the perimeter of the site to act both as a retaining wall for a one-storey basement, and to support the foundations. Some additional bored piles will provide bearing support for columns both at the edge and within the site.

A major function of the wall is to cut off groundwater movement in the site.

'It is fairly close to the river and it is amazing how much water movement there is with the tidal flow, ' says Nick Sharp, southern manager with piling firm May Gurney Geotechnical, which is installing the cutoff.

The ground here, as along much of the river, comprises 2-3 m of made fill with a gravel layer beneath it.

'This simply lets the water through, ' says Mr Sharp, 'and fairly quickly too.'

Below the gravel at around 9 m beneath the surface is London clay, somewhat fissured but firm and essentially watertight.

Most of the piles being driven will be 11 m long, long enough enough for the wall to key into this layer.A number of much deeper piles at intervals along the wall and inside the site will be bearing piles for columns and these will go to 26 m.

'Driven' is the wrong word, however. Pile hammers and vibration equipment were specifically ruled out on this job on account of the nearby hospital and residential and office buildings.Train passengers on the Eurostar platforms might not appreciate the noise either.

All of this suits May Gurney, which specialises in continuous flight auger piling, which will be used exclusively for the Waterloo site.

May Gurney has to form some 390 piles for the wall - females 11 m deep and the intermediate reinforced 'male' piles 26 m deep for the cantilevering strength.All of these are 750 mm in diameter.Then there are a further 203 load-bearing piles needed around the site for column foundations, most of them just 600 mm in diameter but 64 of them at 750 mm.

'We use CFA for speed and programme reasons and have become pretty experienced in using the method, ' says Mr Sharp.'I think we are very competitive with them and can construct at twice the speed of bored piles.'

It could be argued that the load-bearing piles would be better done by rotary boring, he says, because they are mainly formed in the area of the basement which will be excavated down some 5 m and therefore require the piles to be cut off at that level. Because CFA is a one-off process - the whole of the pile bore is removed in one go - the reinforcement cage comes right to the top and has to be cut back.

'But it is still quicker to do it with CFA, ' he says. It also means only one type of machine is used.May Gurney has brought two rigs to site, a Spanish-made Llamada P150 and a P140, both capable of around 29 m maximum depth and therefore working within their range.'Much more equipment could be counterproductive anyway because of space constraints.'

Getting the machines onto site means bringing them down a ramp road from the entrance onto the site, which of course also has other activity under way.May Gurney also needs space to make reinforcement cages with up to 14 m-long rebar - overlapped for the longer piles - and for the various smaller excavators it uses for removal of the risings.

'We also have a concrete pump and holding drum for the pile concrete, ' says Mr Sharp. Concrete is delivered from a local ready mix plant - there is no shortage in central London and within a close enough range to avoid any traffic difficulties. But just in case, the drum provides a reserve and means trucks do not have to wait for the next pile to go in.

'That saves on delivery truck standby time, which can be an expensive extra on concrete cost.This way we just need a concrete wagon every half an hour, ' says Mr Sharp.

The piles use a high slump mix for easy pumping through the central auger flight core as they are withdrawn.Machines make two diameters of pile, mostly 750 mm for the wall and some at 600 mm for the loadbearing inner site piles.

'Our first job was to form a guide wall in the initial two weeks on site, ' says Mr Sharp.The 1 m-deep concrete wall contains cutout voids for the piles, which provide position and direction guidance for the operator when he begins boring.

Working on the outside wall needs some care with spoil to avoid muck dropping onto the pedestrian footpath running just round the outside of the site; the CFA rigs are quite high and lift the auger to a full height when they extract.May Gurney has a strange multipaddle device which fits within the flight and scrapes any stubborn material out.There is also a netting guard to catch any dropped material.

Pile reinforcement bars are fitted on the top 2 m with a foam plastic sleeve.This allows the pile tops to be broken out more easily, which is necessary for the installation of a capping beam to complete the wall.

Ardmore does this work using a hydraulic breaker mounted on a mini-excavator.'You need such a machine these days because of what we know about white finger vibration damage to health, ' says Mr Sharp. Breakings and spoil are also dealt with by Ardmore, which takes material to a licensed dump.

May Gurney's work on the wall should last about 10 weeks.

'After that we come back to do some of the piles in the middle for load bearing, ' says Mr Sharp.

Ardmore will need to do more on the wall because, although the secant piles will cut off most of the groundwater movement, they are not totally watertight.

'We will need to put a bit more waterproofing there, ' he says.

Ardmore will be on site for a total of 90 weeks, carrying out work valued at £20 million.

And then comes the roundabout itself. But that will be another story.