CGI of London Olympics stadium

Olympics Watch - The main site

The 2012 Olympics may seem like a long way off, but preparations are well under way in London. In the first of an occasional series, E&T looks at how work is progressing, and what engineering challenges are being overcome.

On 6 July 2005, the day when London discovered that it had won the race to host the 2012 Olympic Games, the proposed Olympic Park was a brownfield, urban landscape in the Lea Valley. It comprised 321 hectares of light industry, disused workshops, some houses and allotments, partitioned by canals, roads, railway tracks with 52 pylons carrying power lines across the site.

But it was not always like this. East London's Iron Age population would have lived in a valley of lakes and rivers and fished in today's River Lea. Archaeologists from the Museum of London have found that the Olympic Park bordered Neolithic and Bronze Age wetlands. In their evaluation trenches they found Mesolithic flints and Bronze Age crannogs, dwellings on piles driven into the wetlands. On the Aquatic Centre site, four prehistoric skeletons, one thought to be 3,000 years old, were discovered in separate graves in an area of an Iron Age settlement.

In the 3rd century AD, when the Olympic Games were being held in Ancient Greece, Romans or Romano British lived on the site, which is shown by a coin with the Emperor Constantine II (337-340AD) on one side and Caesar on the reverse.

There was textile printing there in the 17th century, and later petrol factories, and bone, varnish, soap and tallow works, as well as distilleries, engineering and chemical plants. From the end of the 19th century, the site was used for landfill. It has strong electronic associations too - in 1904, Professor Ambrose Fleming developed the first diode valve; Britain's first radio valve factory was established there in 1916, and the first television tube factory in 1936.

Choice of the Olympics site

It is easy to see why the site was chosen. It had to be in London; it is closer to central London than Earls Court and larger than Hyde Park. The Eurostar rail station is at Stratford, and other rail and underground stations are close by; it was not an extensively built-up area and few people lived there. For the planners, it was a site of great potential for social and economic redevelopment.

The London Development Agency (LDA) was the body chosen to oversee the first stage of the grand project, and it seized the opportunity with zeal. It had to acquire the land, rehouse people who would be displaced and relocate businesses. It had to remove pylons and clear the site before handing over to the Olympic Development Authority (ODA), which would then run the operation until the conclusion of the Games.

Although most of the land was acquired through private negotiations, the LDA had to apply for two large and complex compulsory purchase orders to secure the whole site. This provided an inevitable downside to the general euphoria: a number of unfortunate people lost their homes and businesses.

Some 450 residents were displaced and relocated in neighbouring boroughs, but several were unhappy that their community was dispersed; 35 traveller families had to be resettled elsewhere; businesses were relocated; most moved, but some owners chose early retirement. Allotment holders on a 100-year-old site were offered alternative plots and the opportunity to return after the Games have closed. All were given compensation for the upheaval.

Power cable tunnel

It might have been a daunting site, with buildings that had to be knocked down, pylons to remove, rubble and landfill from centuries to be crushed, treated and carted away. But they had a plan of course.

Before the pylons could be taken down, power cables had to be rerouted through tunnels under the site. Two tunnels, 6km long and up to 4.5m in diameter, were driven between Hackney and West Ham at a depth of up to 30m below surface level. These ran beneath Hackney Marshes from substations in Bidder Street, West Ham, to near Millfields Road in Hackney.

Four Lovat Earth Pressure Balance tunnel boring machines (TBMs) from Canada and one Iseki slurry machine from Japan drilled through layers of mainly Lambeth Group clay and Thanet sands; soil that had lain undisturbed for 50 million years was tunnelled through at a rate of up to 32m in 12 hours.

All the tunnelling was done in the water-bearing level. The TBMs were sealed behind the cutter head at the front by a water-tight steel bulkhead. The lining was sealed on to the back of the TBM by rows of dense steel wire brushes, injected with thick grease to prevent water getting in. As the drilling progressed, the tunnel was continuously lined with pre- cast concrete segments that had to be formed into circles. All the joints between segments had to have gaskets incorporated into them for water tightness.

And then 9,000 brackets were installed to carry the 200km of cables, monitoring and ventilation equipment. The whole operation took 424 days, and about 250,000t of soil were removed and used for banking on the site. Then the 52 pylons, with their 130km cables, were cut down and their 1,300t of steel recycled.

Clearance

When the pylons had been removed, work to clear the site began in earnest.

Some 192 buildings had to be demolished and more than one million cubic metres of soil excavated. But the soil was not just dumped or carried away. It was taken to the soil laboratory for testing. Sixty scientists, technicians and other specialist staff worked in a laboratory on site to work out the best form of treatment. Five soil-washing machines then cleaned and shook the soil. Contaminates were removed from coarser particles of sand and gravel and mixed with the finer silts and clay; large lumps of metal were also extracted by magnets from the contaminated soil. The coarse gravels were separated at this stage and cleaned sand was filtered out for reuse. The remaining mixture was pumped through sieves, mixing blades and water sprays. Chemicals were sometimes added to remove contaminated material. The water was then treated and used again.

Remediation

If soil was found to contain only petrol-related substances, it was usually treated by remediation. This process was to store the substances in concrete tanks and to add oxygen to break down the contamination, rather like composting. Other contaminants were left where they were after being stabilised chemically to stop them leaking into the ground water system.

The scientists also monitored ground and surface water and air across the site to check for contamination.

Over 800,000m3 of soil, contaminated with substances such as oil, petrol, tar, arsenic and lead, were cleaned and then reused to raise some areas of the Olympic Park by 5m.

Waterways

A feature of the Olympic Park is its waterways. It is divided and partitioned by about 8km of canals and rivers. Some have to be dredged, some cleaned and some walls have to be repaired. Seven permanent bridges are planned to span the River Lea; the first, in the north of the park, is now in place.

Prescott Lock

Previously, the waterways were tidal, which meant water levels rose and fell by several metres according to the state of the tide. Tide tables had to be used to calculate when and where a barge could be moored. So the Olympic Delivery Authority, working with British Waterways, developed a new lock and water control system at Prescott Channel, just to the south of the Park. The project, costing £20m, will have twin water control gates, a 62×8m tidal lock and footbridge, a lock control building, and a fish pass and weir gates. This will stabilise the river level at just over 2m deep and allow 350t barges to transport materials to and from the construction site. It is expected to replace hundreds of lorry journeys a week from local roads and save thousands of tonnes of carbon dioxide as well as linking the Olympic Park into the Thames Gateway. The lock will open up the rivers in the area for transporting materials by water. In the long term, it will provide access for barges carrying construction traffic and recyclables between Stratford and the Thames. It will also create new opportunities for leisure boats, water taxis, tour boats and floating restaurants.

Construction has not always been uneventful. In June 2008, workers dredging for the lock struck an unexploded bomb. It was a German Hermann type 2,200lb bomb, dropped during an air raid in 1941. It took bomb disposal units four days to make it safe before detonating it in a controlled explosion.

Completion

Now that the Olympic Park site has been fully cleared, the construction of the Athletics Stadium, the Aquatic Centre and Velodrome, buildings for Handball, and other events and the Athletes' Village can go ahead at full speed.

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