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DUSTY ANSWER

Coal dust has been making a nuisance of itself in a Spanish coastal town. Damon Sch³nmann went to see the solution.

Santander on the Iberian Atlantic coast is the location of an elegant formal royal home, the Palace of the Magdalena. It was built in 1909 and became the summer retreat of Alfonso XIII in 1913. Although the royal family only used it until 1930, they must have admired the views from the peninsular it is sited upon.

More recently the views from Santander have been obscured by clouds of coal dust, staining buildings and getting up the noses of residents and tourists alike.

The townspeople decided it was time to sort the problem out, and the solution is claimed to be the largest structure of its type in Spain.

The dust comes from the nearby harbour where coal from China, Turkey and Egypt lies in open heaps awaiting transportation to power stations inland. In winter, the prevailing northerly wind exacerbates the situation, carrying fine particles into the town. Water spraying has failed to control the dust.

So a huge 44,714m 2 shed is being built to store the coal, as well as clinker from Poland, Russia, South Africa and Indonesia which arrives in rock form to be pulverised for concrete production.

Protection of the town is not the only reason for the shed; it will also automate the logistical process with conveyor belts delivering material to waiting lorries and railway trucks.

Once it is built, 1.1Mt of coal and clinker will move through the giant structure each year.

The scheme will cost about .50M (£34M) and while the main contract has not yet been awarded, a joint venture of Pennine Vibropiling and Spanish geotechnical contractor Geocisa is already at work on the £3.1M ground improvement package.

This will see 10,700 stone columns going into the ground. Pennine Vibropiling managing director Arwel Williams says: 'At 165,000m this is probably the biggest shed we've done in terms of column linear metres. They range in depth from 12m to 25m and go up to 1m diameter.

'We started with bottom feed but we couldn't get the right stone from the local quarry [it was too big to be used inside the fl ot]. So we converted everything to top feed wet.' This change presented a problem of its own because of the large amounts of silt coming up from the holes. Williams says: 'We use 60l/s of water to jet down and then reduce the pressure to just a positive flow for washing out the silts so we have a clean hole. [To deal with the silt] we're using a desander with a centrifuge which makes it the first job we've done wet using one.' Ground conditions are Keuper Marl under clays and sands with some silt under a soft granular material at the top. But in one corner of the shed's footprint the surface is harder because of contamination by a material formerly stored there.

It is unclear what the material was, though it was possibly calcium carbonate or clinker. It has resulted in what Geocisa group contracts manager Alejandro Segundo Gonzalez terms a 'natural concrete' and has meant rig crews need to pre-bore in this area with a down-the-hole hammer Ingersoll-Rand T4 rig.

At the time of GE's visit at the beginning of May, three crews were using two rig-mounted fl ots and one crane-suspended float for the deepest columns. The latter are going below the building's footings to refusal from 14m to 25m depth although most are 18m to 20m. For the rest of the shed, 12m columns will resist lateral movement in the ground.

Segundo Gonzalez says: 'This job is important because it has the most stone columns ever in Spain and has the greatest load on them.' Loads in the middle of the shed will be about 148kN/m 2. The train loading site will average 120kN/m 2 and in this area there must be zero settlement. After the stone columns go in, site workers will lay a 1m gravel platform and then a geotextile before building a 5m embankment to surcharge the ground for three months. This process will also be needed in a lorry weighbridge area.

The initial plan was to treat the ground of the shed site to different specifi cations as the clinker and coal were to be stored in specific areas.

But to allow greater flexibility, the stone columns will now improve the ground to the same standard throughout the storage areas so either material can go anywhere.

The first column went into the ground at the beginning of March and the last should be completed in September.

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