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Living on air

Changing groundwater conditions have triggered a spate of foundations problems. Diarmaid Fleming investigates.

Emergency evacuation of a row of houses or block of flats due to fears about structural stability is a rare occurrence in the UK. But at a number of locations across the country, residents have been forced to leave their homes while engineers battle, not always successfully, to save their dwellings.

Uncanny similarities can be found at locations in Reading, Norwich and Edinburgh. Failure of old mine workings at each site due to changes in groundwater conditions has caused the collapse of ground beneath the structures, prompting emergency remedial action.

Other cases have emerged elsewhere around the country, including Taplow in Berkshire and Swanley in Kent.

Each incident has presented high pressure challenges.

As the ground moved, painful decisions - guided by engineering judgement - had to be taken to evacuate people from their homes. As well as the geotechnical engineering challenges to be overcome, the engineers involved have also been presented with the rare task of dealing directly with the concerned public, to translate detailed technical information into answers about the futures of their homes.

Unbeknown to residents in the Castle Hill district of Reading Ferniehill and Gilmerton in Edinburgh, and the Normandie Tower in Norwich, their homes were built on a geological time bomb.

At Reading, chalk mine workings dating from the late 19th century were designed on the traditional 'pillar and stall' technique, where vast caverns or galleries were excavated out, the roof of each void supported by pillars of unmined rock.

Ferniehill Terrace in Edinburgh was built on the site of old limestone workings around the village of Gilmerton dating from the early 18th century. Mining, also using the pillar and stall technique, was carried out to win lime, much of it ending up on the ceilings and walls of grand Edinburgh houses.

Much of Norwich is founded on a warren of mine workings, possibly dating as far back as the 11th century. Chalk was mined for a variety of purposes, ranging from building materials to quicklime used to bury victims of plagues between the 14th and 17th centuries.

At each location sudden ground movement forced the evacuation of local residents.

Movement in Reading began in January 2000 at Field Road when a crater 8m wide and 4m deep opened up, damaging two houses and prompting the evacuation of 17 homes. Another 18 would later be affected.

Some evacuated residents are still unable to return to their homes as work continues to make good the ground.

Consultant Peter Brett Associates was called in by Reading Borough Council. The cause of the collapse is unclear, but an old 75mm water main was discovered to have fractured. 'We exhumed it and it was in very good condition. It's not clear if the ground moved fracturing the main, or if a leak had led to ground movement which caused the fracture, ' says Peter Brett project engineer John Talbot.

The crater was filled as a temporary measure before ground investigations began. 'There was no documentary evidence of the mines, apart from an arcane record of a geology trip in the late 1850s, ' Talbot notes.

Just as some of the residents in Reading were coming to terms with their temporary accommodation, cracks began to appear in houses 640km away in Ferniehill, Edinburgh, in November.

The situation suddenly worsened when a 1m deep hole 70m long opened up, forcing the evacuation and subsequent demolition of seven houses. Consultant Ove Arup was called in by Edinburgh City Council.

'Around two weeks later there was another collapse, confirming that we were dealing with mine working failures, ' says Arup associate director Alan Richmond. Continuing failures led to alarm over the safety of three nearby four and five storey blocks of flats built in the 1960s.

In contrast to Reading, records of the mine workings at Edinburgh, worked into the 1940s, were relatively detailed.

They had been plotted by the British Geological Survey and in detailed council archives.

Records showed there had been a cast limestone quarry near the houses to depths of between 15m and 20m. The attraction for miners was a 3m deep band of 98% pure calcium carbonate at the bottom of the limestone. The 3m deep galleries spanned 9m-10m between pillars around 1.2m square from 15m30m below ground level.

It was discovered from records that the same mine working arrangement existed near the flats at Moredun Park View. 'The probability of collapse was similarly very high.

The risk was unacceptable, so the council decided to evacuate the flats, ' says Richmond.

Within weeks of the last collapse at Edinburgh in December, a leaking main led to the evacuation of the 16 storey Normandie tower block in Norwich. Completed between 1965 and 1966, the flats were built on an area known to have mine workings.

Investigations into residents' complaints about a drop in water pressure revealed that a high pressure main to the tower block had burst. Engineers now feared that with 600,000 litres having flowed from the main into the ground, soil underneath the building could have been washed away. Sagging pavements prompted dynamic probing which revealed that a cavity up to 9m deep and 8m wide had formed underneath one quarter of the building.

Inspections of archive drawings by Norwich City Council engineers days after the leak revealed that the building was founded on 170 Franki piles, 20m long and 600mm-700mm in diameter. Loss of ground around the upper sections of the piles prompted fears for the stability of the building and resulted in an immediate decision to evacuate.

Residents were told to leave the building with a few belongings and were brought to temporary accommodation.

'It was a big decision to evacuate but we could not take risks with safety, ' says Norwich City Council project manager David Hebborn.

Different challenges faced the engineers at each site. At Norwich, monitoring of the building revealed no movement, while cutting off the mains supply arrested the problem, essentially making it a matter of making good an isolated area of ground.

But in Edinburgh and Reading decisions were less clear cut.

Assessment of how far the old mine workings extended and their potential effect on other buildings prompted detailed site investigations at both locations.

A decision on whether to make good the ground or demolish the structures will then have to be made.

Extensive bore holes were drilled at 70 locations around the houses and flats in Edinburgh.

Further information was gleaned using an underwater cavity sonar device, which rotates through 360degrees to enable sections.

The sonar output was used to draw up plans of the ground.

'This allowed us to determine how much of the ground had been removed during mining and the size of the pillars remaining, plus make comparisons between the area around the flats and where the collapses under the houses had already occurred, ' says Arup project manager Peter Stevenson. 'We found the extraction rates similar, around 90%, and the pillar sizes similar.

Loads on each of the 1.2m square pillars were estimated to be anything between 2,000t and 3,000t.

'In the 1940s, the mines were largely dry, but cessation of pumping from nearby coal works meant they have become flooded, ' says Richmond. Pure, soluble calcium carbonate in the pillars dissolved in the flooding groundwater, creating discontinuities in the pillars, which, it is believed, then caused the collapses.

Different options were examined, but stabilising the ground was eventually ruled out. 'There would be a huge cost in grouting this size of mine at that depth underwater. Plus, if you grouted up the mines, the groundwater would have to go somewhere else. We've done assessments of where the water would go, and because of the effects elsewhere, it was not a solution, ' says Stevenson. This left demolition the only option, with the council finding alternative housing for those affected.

In Reading, detailed investigations were carried out with up to 1,400 dynamic probes, 30 light cable percussion bore holes, two geophysical surveys, 50 window sample bore holes and CCTV.

This revealed made ground on Reading Formation clay and sand. It also showed an irregular mine working arrangement, with floors of the 4m high galleries at depths between 10m and 16m and pillars of varying size.

'It was not mined to a rectangular grid - instead it was random, burrowing off in all different directions and leaving pillars behind, ' says Talbot.

Some pillars were 'robbed' of material afterwards before galleries were backfilled. But a more positive outcome for the residents emerged.

'We looked at the different options. Demolition of buildings and then abandoning the site would have blighted the land.

Then there was the cost of disposing of contaminated waste.

So it was decided the ground would have to be stabilised before rebuilding, ' says Talbot.

'The houses were in remarkably good condition. Some of this was just good luck, with party walls resting on pillars, some walls on ground that had not collapsed, with walls holding each other together.'

Demolition and reinstatement of the land would have cost up to four times the amount needed to make good the ground and repair the houses. And that is without counting the cost of the far greater social disruption that demolition would entail. Although groundwater movement had ruled out grouting in Edinburgh, this did not apply in Reading.

Keller Ground Engineering came to the rescue at both Reading and Norwich.

Work is still proceeding at Reading, where a combination of bulk fill grouting with sand and pulverised fuel ash (PFA) is combined with pressure and compaction grouting. Pressure grouting uses sand, cement, PFA and bentonite at pressures up to 20 bar, followed by compaction grouting of a thicker mix.

'This squeezes the ground and stiffens it up - almost like forming big footballs or a string of conkers made from grout, ' jokes Talbot. Over 10.6km of drilling has been needed, with 3,300t of grout material placed so far. Up to two or three times as much again will be needed before the job is complete.

At Norwich, a combination of vertical grouting at 20 bar and higher pressures to form a grout wall around the void and inclined tubes a manchette were used to grout the ground.

Around 46 tubes were used to compaction grout a cement/ sand/bentonite/PFA blend. The inclined tubes to grout between the tower block piles applied a cement/bentonite mix at very low pressure to avoid putting any lateral stresses on the piles.

'The challenge at Norwich was the speed - we had to get the work done as quickly as possible to allow the residents back in, and work as a team alongside others on services. At Reading, we are working in tight spaces and have to ensure that the ground we are working on is solid, ' says Keller contracts manager Bernard Reed.

The residents of Norwich are now back home, while some in Reading have returned. Talbot says that further work remains to see if other areas are affected, although the mine works are believed to be localised and not a threat to Reading overall.

In Edinburgh, however, the extent of the problem could be wider. With 30 houses already demolished and the three blocks at Moredun Park View condemned, 141 families in another block of flats at nearby Hyvots have received notices to quit by the council because of fears of subsidence.

Arup is working to determine other areas of risk while the council pursues re-housing options. The Scottish Executive has also set up an inquiry to determine why the buildings were built on areas with old mine workings in the first place.

Similar questions are also being asked in Norwich.

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