AS BEFITS the old solidly paternalistic Marks & Spencer, the retail firm's 1950s-built headquarters in London was almost a self-contained city. Four sevenstorey office blocks, and central interconnecting sections five storeys high, made up the complex giving it the largest frontage on Baker Street.
But M&S has moved on and the site is being redeveloped by London & Regional Properties into a swanky up-market office, retail and apartment complex. The old blocks remain but the design from London-based Make architects requires ripping out two of the old linking sections and rebuilding everything else. There will be a new high-level glazed atria linking the blocks, new entrances out onto Baker Street and even, it is rumoured, a swimming pool. The scheme is around £110 million in total.
While a main design-and-build contract was being let, now under way with HBG, pre-contracts were let for demolition by the developer's agent, consultant Tweed. And as demolition firm Erith was getting stuck in, there was also major work under way in the basement. The new structure requires fresh foundations and these will be minipiles, more than 1,000 in total, mostly installed from the cramped conditions of the old basement. Work began last autumn and is now winding up.
The job has been an excellent opportunity for Systems Geotechnique. Having recently bought Fondedile UK's piling division from Sir Robert McAlpine, the company was able to take advantage of a number of electric-powered, low-headroom minipile machines, ideal for working in confined conditions.
Even so, the groundworks contract was a chunky one and Systems Geotechnique teamed up in a joint venture with Bachy Soletanche for the bid. Bachy is managing the project and brought a couple of machines, but its main role is organisation of the project. It also takes design responsibility for the piles.
'You don't get more than one or two jobs like this in a 12-month period in present market conditions, which means you're not keeping the level of resources to do it on hand as a single company, ' says Bob Thompson, Systems Geotechnique's joint managing director. He used to work with Bachy, he adds, which was a great help setting up the JV since he 'spoke their language'.
The work has proved fairly demanding, not least because the basement conditions are cramped. The old single-level basement is not small, in fact it runs the entire length and depth of the site, around 138 m along the Baker Street frontage and 75 m back. Headroom was limited at around 2 m, though some areas of the former car park reached 3 m.
'Despite the size, the work is hemmed in, ' says Bachy Soletanche project manager Sue Saville. The piling contractors had seven machines working together at peak times. This required a fair amount of additional plant to keep them serviced, including miniexcavators and skid-steer machines to shift the spoil as it emerged, and to lift drill string sections.
'We also had some generators down there, though we were able to put the main unit at ground floor level, ' says Ms Saville. 'And the demolition teams were there too, both for work above and breaking out old foundations, ' she adds.
She was also able to put the main grout mixing unit at ground floor with a dedicated crew. With seven machines working, the demand for grout was relatively high ? some 44 cu m every day. But even with the unit above, there still had to be grout holding tanks and pumps in the basement for the machines.
All this activity could not spread out but had to be concentrated in one of four zones according to the activities, above and alongside, of the main demolition work.
'That means ventilation and lighting was a major issue, especially before the demolition works had opened up parts of the basement, ' says Ms Saville.
One advantage early on was that car park entrance ramps were still in place which could be used for bringing in the machines and materials. After a while these were taken out and access had to be by crane or long-reach excavator.
Tight planning and daily meetings with Erith were critical to the job's success, says Ms Saville.
But tying in with the main demolition contractor was not the only issue in programming the work in the basement. So too was the piling design itself, particularly the effects of obstacles in the ground.
Things did not go smoothly.
'The first job was test piling and the results were, how can I put it, a bit inconsistent, ' says Mr Thompson. The trials, done with hydraulic jack loading against three or four reaction piles, showed that they were going to need beefing up.
The problem was the nature of the ground. Like most of London this is clay, usually good firm blueish London clay. 'Any gravels had been removed long ago, ' adds Mr Thompson.
But so too, it seemed likely, had the clay which had then been replaced. 'Placed clay is re-worked clay, so its properties have changed ? essentially it is softer, ' says Mr Thompson.
The usual friction support, which in London is the main load mechanism on piles rather than end bear ing, is reduced. In most cases, this required an extra 2.5 m length being added to the 300 mm diameter piles.
The reworking may have been done because of the variety of footings across the site, both from the 1950s buildings and from older structures. 'Possibly the foundations were made in excavation, with shuttering put down for the concrete and then backfilling, ' suggests Ms Saville.
The sheer amount of foundations was an even bigger issue; there were far more than expected.
Nothing very excit ing was found , says Ms Saville, like perhaps old Georgian or Victorian brickwork, just a variety of 20th century concrete or reinforced concrete work. But it was a problem, especially as some were much larger than expected.
Tweed's Peter Ullmer adds: 'We had got some good photographs and information from McAlpine, the original 1950s contractor, but once on site discovered a lot of strange stuff underneath.' The old footings were mainly broken out, another exercise to be synchronised with Erith which carried out this work. The demolition firm was limited in the time it could work because of urban noise and environmental regulat ions, so this needed constant juggling.
All the old foundations were broken back to at least the base level of new pilecaps.
'We found some of the footings were quite deep, ' says Mr Thompson, 'going down 5 m instead of 1 m, for example. That was too much to break out completely and for those we actually cored the piles through using a tungsten carbide shoe on the end of the pile casing to get through.
'But you still have to get below the pilecap level or risk breaking the piles.' For most of the 1,068 piles, the work was done using Fondedile machines, FF4s and also a couple of German Klemm 701s. In most areas, the piles were drilled using a short temporary casing at the top, up to 7 m down, and then coring out with the augers to depth. In the clay, the holes are self-supporting and reinforcement and then grout can be easily installed.
But the process of adding 0.5 m casing sections and then auger string sections, and removing them again, takes time. A low-headroom rig with an operator and the 'spanner man' can manage about two or three piles a day.
And after hitting many obstacles some speeding up was needed, because an original end of March finishing date was running to the end of May.
Mr Thompson says his direct client, Tweed Cost Consultancy, which is co-ordinating contracting for the overall project, has been excellent in its cooperation and understanding of the problems, making allowance for delay and working essentially with a partnering ethos. It prefers to be flexible on programme rather than get into conf lict.
The client has made allowance for the additional costs on the original £900,000. It is now 'substantially more' than that. 'We have hardly taken the contract out of the drawer, ' says Mr Thompson. 'We like it that way, ' says Mr U llmer. 'And we pushed them around like chess pieces on the site to help progress.
We needed co-operation for that and we got it.' A way was found to pull back some lost weeks, using an opened up area of the basement on one block.
Here a bigger rig could be brought in, a CFA machine 24 m high. The 40 tonne CM48 was lifted in with a 300 tonne mobile crane and was then able to push forwards with some 16 or so piles daily.
'They were larger too at 350 mm diameter, which meant they could be shorter, ' adds Mr Thompson.
The opened area was on the critical path for the main demolition and the follow on construction which is being done by HBG, so the overall schedule has not been severely affected.
'We are pleased ? the concrete work is already well under way and all in all they have done an excellent job, ' says Mr Ullmer.