The salt deposits in question were formed 220 million years ago in the Triassic era. In the human era, it is believed that several settlements sprang up due to the existence of brine springs. A settlement, Condate, was built during Roman times at the current location of Northwich in northwest England. It is believed that the Romans built this settlement due to the strategic river crossing of the Weaver and the presence of the brine springs. The Romans used lead salt pans to extract the salt from the brine.
The salt beds beneath Northwich were re-discovered in the 1670 by employees of the local Smith-Barry family. The family were actually looking for coal.
In the 19th century it became uneconomical to mine salt, and so solvent extraction using water as a solvent was used. Hot water was pumped through the mines that dissolved the salt and the resultant brine was pumped out and the salt extracted from the brine. By 1897, Winsford had become the largest producer of salt in Britain. All the Weaver valley was used for salt works and salt was shipped to Liverpool to be used all around the world. Trees in the immediate area were cut down and burned for evaporation processes.
The mining operation was originally used for salt licks and to enrich the brine in the evaporation process.
However, over extraction in Northwich left as little as 8% rock left to hold the roof up and numerous new “flashes” were created with “apple coring” taking place in the pillars. At one time there were 90 mines under Northwich, all flooded and still used for pumping brine. Flooding the mines and pumping brine had a disastrous effect on Northwich (see Figure 1) and in the 1980’s to 1990’s, it cost GBP 27M to stabilise the area.
Modern operations
The mining operations by Compass are at about 170m below ground with the mining areas based on controlled and regular shaped room and pillar mining (see Figure 2) with the crushing and screening taking place underground. The crushed salt is then transported to 10t skips which are then transported through 170m vertical shafts to the surface. After transport to the surface the salt is then stockpiled and dispatched as required. Though the terminology of ‘grit’ and ‘gritting’ is referred to when this is being placed on the roads, this is actually crushed salt with no further additives.
Demand for the salt is obviously highly weather dependent and the colder the better for the demand of the mined product, however regular production is maintained throughout the year and the peaks and troughs of demand are managed by stockpiling or drawing down supplies on the surface.
As the severity of UK winters can be notoriously unpredictable, managing this stockpile is a delicate process (see Figure 3).
For mining the salt, a continuous automated mining excavator is employed with potential for more excavators to be brought online in the future.
The current excavator in the mine is a JCM HM36 with the shift patterns in the mine working on five days production with two days for maintenance. The production rate of the machines averages 500t/hr but are capable of up to 1200t/hr when cutting from on the down cut (see Figure 4) The mine is looking to use a JCM HM46 in the future
Though the current areas of salt extraction have yet to be mined out, the area in Stanthorne to the south has enough reserves for the next thirty to forty years. A long heavy duty conveyor is being installed along a 339m long, 1 in 8 inclined tunnel (see Figure 5).
Deep store
As well as mine production, the old mine workings also act as storage for longer term secure storage, or for environmentally sensitive items. Notable for tunnelling, this includes storage of the geological core samples for the Crossrail project in London. (see Figure 6).
The ‘unique’ space is known as Deep Store, an ideal facility for documents as it has consistent temperature and humidity and is naturally free from ultraviolent light, rodents and flooding. It could feasibly house every piece of archive material in the UK. Deep Store currently stores more than 1M assets and documents, and is being used for a portion of the storage needs for the UK’s National Archives.
Looking as though it is straight from a spy film, there are currently 170 miles of tunnel storage space available though only 10% is currently full. This provides an interesting secondary source of revenue for the mine. The infrastructure includes two-hour and four-hour fire walls, additional air filtration which removes all dust, dehumidifiers and fire detection with electric vehicles being used for transportation (see Figure 7).
