£28 million to clear town's salt legacy

October 1st, 2002

THE news that the town of Northwich is to receive the funding, £28 million, to stabilise four abandoned salt mines, is, unquestionably, the most momentous happening ever in the town's history. Never before has such a vast sum been handed to Northwich - the consequences will be far-reaching and simply cannot, should not, be underestimated. What is about to happen, what the townsfolk are about to experience first-hand in Northwich may be unique in the world.

It has been a long road since fears were first raised about the condition of the huge caverns, some 300ft below the surface around the town centre, a legacy of centuries from when the Northwich district was the epicentre of the salt trade. What remains is a landscape, particularly to the north towards Marston, pockmarked with the signs of former rock-salt mines and brine workings.

In Northwich itself, however, there are few tangible clues as to what may be going on deep below the surface, although so critical has the situation become that around180 pieces of monitoring equipment are currentlu dotted about the town to measure the merest change in ground conditions.

Extensive investigations have pinpointed four abandoned mines, Barons Quay, Witton Bank, Neumanns and Penny's Lane, which pose a serious subsidence threat to public safety and the long-term prosperity of Northwich. Together these four mines extend to over 40 acres, within an area, moreorless, from the Town Bridge to Station Road.

The grant to stabilise these mines has been secured through English Partnerships, a government agency responsible for a national Land Stabilisation Programme (LSP) which has been introduced to tackle problems outside of traditional coal-mining areas.

Vale Royal Council and English Partnerships estimate it will be October next year before the work can commence in earnest, with a 2007 completion date. Afterwards, monitoring of the mines will continue for a further ten years.

It will be a colossal and complex undertaking... and even now, despite years of studies and tests, there remain some crucial unknown factors and only time will tell if the experts have got their calculations right. Over two centuries of salt mining may yet throw up some surprises!

In broad terms, the idea is to remove millions of gallons of brine from the four mines and, simultaneously, replace this with a grout, of pulverised fuel ash (pfa), cement and salt. The pfa will arrive by rail, the cement and salt by road, and the grout will be mixed alongside the Oakleigh sidings, at Winnington, from where it will pass, via a pipeline, specially laid beneath the bed of the River Weaver, to the mines.

The displaced brine will travel in the oppposite direction, via a pumping station at Barons Quay, back to Oakleigh sidings from where it will be taken by train to British Salt, Middlewich. Despite some earlier fears after heavy metals and contaminants were found in a number of borehole samples, British Salt has undertaken its own extensive sampling and anticipates using the brine in a wide range of products, from road de-icing to water softening.

Over the years, Vale Royal Council, with financial support from English Partnerships, has commissioned numerous geological engineering studies and the latest estimate is that there is three-quarters of a million cubic metres of brine in the four mines.

Or to put it in good old imperial measure, approximately 161 million gallons...equivalent to almost 2,000 Olympic-size swimming pools.

Some knowlegeable townsfolk have questioned the wisdom of tampering with the mines at all. Others insist a lack of monitoring and inspection during the past thirty years has contributed to the accelerating deterioration of the mines.

Whatever has gone on in the past is now irrelevant, however. It is a fact that the mines were abandoned as rock-salt workings about a century ago and they were subsequently flooded with brine to aid supporting pillars and ensure stability.

The combination of brine and supporting pillars have served Northwich well, but time has, undoubtedly, run out. It is, principally, the supporting pillars that are causing the most anxiety. In a modern salt mine, such as Winsford, the practise is for around 30 per cent of the available salt to be left unworked as pillars. In the old days it was quite different. The mine owners wanted every last scrap of rock salt extracted and the supporting pillars amounted to, at best, eight per cent of the working area, sometimes as little as five per cent.

Slowly, but surely, these pillars in the Northwich mines are beginning to buckle and, depending upon which of Vale Royal Council's commissioned studies are examined, the possibility of some form of serious subsidence varies from "imminent" to thirty-five years, or even 150 years.

One report, six years ago, into the condition of the Witton Bank Mine, identified six supporting pillars as being "unstable " and having "exceeded their limiting strain". In particular, Pillar 1 was said to be in a "critical condition" and "close to, or has commenced failing".

Indeed, Witton Bank Mine was, until recently, considered the most dangerous of the four mines. Now, however, Vale Royal Council and English Partnerships insist that the Barons Quay Mine is causing the most concern and needs to be tackled as a matter of urgency. Closest to the River Weaver, and partly mined beneath it in one place, Barons Quay is the largest of the four mines. It is butterfly-shaped and covers approximately 26 acres.

The plan is to first stabilise Neumanns and Penny's Lane, followed by Barons Quay and then Witton Bank. The smaller mines will allow the engineers to fully trial the stabilisation work and iron out any snags before they turn their attention to the much more potentially dangerous larger mines.

It is impossible to guage precisely what is going to happen during the stabilisation process, not least because Northwich has what is described as a "complex hydrogeology regime", i.e. the unpredicability of the water courses essentially connected with the River Weaver, Witton Brook and the Flashes.

This was highlighted during the drilling of test boreholes when a significant inflow of brine occurred in the Barons Quay Mine. By the time the borehole was sealed, levels in Barons Quay had risen by 25 metres, whilst, at the same time, the Witton Bank Mine levels had dropped by 10 metres and, at Marston, ICI monitored a drop of almost four metres.

This suggests a possible maze of connections and is one of the many "unknown" challenges which will face engineers who will be appointed to undertake a project that has never been tried before in a single salt mine, let alone four beneath a town centre.

In their quest to secure government funding, Vale Royal Council, understandably, painted the bleakest scenario as to what could happen:

"... failure to deliver funding in the immediate future, given current assessments of the likelihood of progressive subsidence and collapse of the Witton Bank area would have immeasurable consequences - the loss of homes and businesses over an area of 6.4 hectares, irrevocable destruction of the town's strategic highway, the ultimate loss of a vital and viable town centre etc."

It was known in the late 1940s that all was not well with the Northwich mines. Everyone was, and is, aware of the town's history of localised subsidence which has been an ever-present restraint on development. A mine collapse, however, is potentially far more serious and as long ago as 1950, the Ministry of Town & Country Planning and Cheshire County Council considered the possibility of relocating the town centre.

The idea was not taken up, but it is understood in the early days of the current round of negotiations, Vale Royal Council was also asked to consider the feasibility of building a new town centre, away from the mines. It is not difficult to appreciate, of course, that this was always a non-starter... the cost would have dwarfed the £28 stabilisation grant.

The first major study of Northwich's salt legacy was in 1987, by Peter Wharmby, a mineral surveyor and, at the time, head of the Cheshire Brine Subsidence Board. The Wharmby Report revealed that the town, and a square mile tract of land towards Marston and Wincham, is honeycombed with around 90 former rock salt mines and over 250 shafts, brine wells and boreholes.

The report's conclusions, concerning the mines was unequivocal:
"It seems likely that most or all of these uncollapsed cavities were particularly unstable at the time of abandonment and until further investigation proves otherwise, it must be assumed that this condition persists, or may have deteriorated."

For some reason, which has never been fully explained, the Wharmby Report was not made public at the time of its publication, though, following an independent investigation by private developers (around Leicester Street and Tabley Street), Vale Royal Council was galvernised into action.

As a result, in 1994 , all development of land above the town's abandoned mines was suspended and, later, the council warned that without stabilisation a collapse could occur with 15 years.

Land valuations, particularly around Barons Quay, were hit dramatically and one major casualty was the long-established firm of Moore & Brock which quickly went into liquidation when its land value collapsed from £2 million, in 1990, to £500,000, in 1996.

This important site was subsequently purchased by Vale Royal Council which now owns most of the land above the Barons Quay Mine and, in the fulness of time, will become part of the prime redevelopment area for Northwich town centre.

The 'Vision for Northwich' is still many years away...but at least the "key" , in the form of the mines' stablisation grant, is now in the lock!

Important industrial heritage areaIT is not without justification that Northwich and Mid-Cheshire is considered by many to be the second most important industrial heritage site in Britain after Ironbridge.

The brine springs of Northwich and the manufacture of salt in the area brought prosperity, great wealth, the canals and the chemical industry. It created work, harsh conditions, and a legacy!

When the Romans invaded Britain they discovered the natives of Cheshire manufacturing salt by pouring brine on faggots of charcoal and scraping off the crystals as they formed. From this the invaders perfected an "open pan" method which has essentially survived for 2,000 years.

Salt mining is, by contrast, a much more modern development. Rock salt was discovered at Marbury in 1670 and over the next two centuries scores of mines were dug in the area.. The earliest were top-bed mines, i.e. approximately 100 ft beneath the surface, and then followed the bottom-bed mines, 295ft-325ft down.

Mining from the bottom-bed took place continuously from 1781 to 1928 when the Adelaide Mine, at Marston, was flooded and had to be abandoned.

Possibly the most famous mine, and one of the largest, was the Great Marston which was worked from the 18th century. It attracted many important visitors, including, in 1844, the Emperor of Russia, Tsar Nicolas I, and the British Salt Trade Association, in 1854. On these occasions the mine was brilliantly lit by thousands of lamps and candles suspended along the walls of rock salt.

The Barons Quay, Witton Bank, Neumanns and Penny's Lane mines are all former bottom-bed mines. The following information is taken, principally, from Vale Royal Council's latest study, by geotechnical engineers W.S.Atkins.

The date of sinking the Barons Quay Mine is unknown but it was being worked by the Verdin Brothers in 1856. The west wing lies to the north of Witton Street, between the Weaver and Leicester Street; the east wing extends northwards for roughly 400 metres from Albion Road. It is supported by approximately 80 pillars and was flooded with brine in 1913.

The Witton Bank Mine is just under 13 acres, sunk in the 1860s and had several owners, including the Salt Union and, later, Brunner Mond. It is located between Old Warrington Road, Wade Brook, Chester Way and Albion Road. It was maintained "dry" until 1929 when it was flooded with brine and left supported by 36 pillars.

Neumanns, just under one acre and located to the east of Chester Way and south of Station Road - last worked in 1904 and flooded in 1932.

Pennys Lane, approximately 1.7 acres, located to the north of Station Road and the east of New Warrington Road. Mining abandoned in 1889 and the workings were left "dry" in order to permit the pumping of brine by Brunner Mond, later ICI, from the nearby Marshall's Mine. It was flooded with brine in 1939.