By Ceri Sansom (Environmental Manager, Groundsure) and Rebecca Paul (Chief Officer, Cheshire Brine Subsidence Board)
Cheshire has a long and fascinating history of salt extraction, which needs to be considered when developing in the area, as it can cause ground stability problems. Here we take a brief look at this history and explore the effects this has and how the Cheshire Salt Search can help.
Salt has been valuable since Roman times and the Cheshire area is a rich source. A large number of towns end in ‘wich’, identifying them as having salt connections. Initially, brine was taken from salt springs, before extensive underground mining began and later, water was flushed through abandoned mines. This had the undesired effect of dissolving the supporting pillars and mines started to collapse. (1) Uncontrolled brine pumping like this ceased at the end of 1986, although mining continues at Winsford under the control of the Planning Authority.
Compensation for subsidence
Catastrophic collapses in Northwich, and a series of subsidence incidents in the wider Cheshire area, started a chain of events that resulted in the Cheshire Brine Pumping (Compensation for Subsidence) of Act 1952. This Act created the Cheshire Brine Subsidence Compensation Board (CBSCB) and Cheshire Brine Subsidence District (CBSD) to provide a means of providing compensation for subsidence caused by brine extraction. As the name suggests, the CBSD is within Cheshire, extending from the south of Warrington and Manchester, to the east of Newcastle under Lyme. Nantwich self-excluded itself at the time of the Act, as it was concerned it might blight the area. However, the unfortunate side effect is that this area cannot now claim compensation as it is not within the CBSD.
Rock Salt is found in high concentrations in the upper surfaces of the Wilkesley Halite Member and Northwich Halite Member and these are interbedded with low permeability mudstone. These are the most vulnerable strata to dissolution (removal of salt by dissolving in water) and subsidence is often found where they are nearest the surface, which can result in complications as natural salt related subsidence is exempt from compensation.
Human made brine related subsidence can result from several actions, of which compensation can ONLY be claimed as a result of brine pumping. The main forms of brine pumping that could result in subsidence are:
- Uncontrolled (wild) pumping;
- Controlled pumping; and
- Bastard brine pumping (dewatering of abandoned mines).
Uncontrolled brine pumping specifically caused accelerated dissolution of salt, often at some distance from the pumping, as it pulled fresh (unsalty) water into the rock. The results were uncontrolled dissolution channels with potential for settlement that could lead to subsidence, which were recorded in some cases up to 5km from the pumping site, meaning such subsidence can occur at quite a distance from where the pumping took place. Interestingly, urbanisation appears to reduce the rate of the subsidence, possibly due to the reduced flow of fresh water into the rock from rain (infiltration). This can mean that during development water management needs careful consideration to meet the opposing need to control salt dissolution by reducing general infiltration and to at the same time control flooding by increasing infiltration using SUDS (Sustainable Urban Drainage Systems) which often makes use of soakaways.
Brine Risk in areas outside of the CBSD
Whilst there is a mandatory conveyancing search for the Cheshire Brine compensation area, there are areas outside the CBSD that are also impacted by brine extraction. Broadly, these are in the Midlands and North West, however, they do not have the same degree of protection provided by the CBSD. This includes areas excluded from the CBSD within Cheshire (particularly Nantwich) and around Droitwich, Pressall Saltfield, Haverton Hill and Salthome and Stafford.
Recent changes between the 24th and 25th Editions of the Conveyancing Handbook mean that Droitwich or Greater Manchester are no longer identified ‘affected areas’ that require a mining search.
However, the risk to clients still exists, so every Groundsure residential and commercial report still includes information over and above this minimum requirement – using the BGS Non-Coal Mining Hazard dataset for indicative mining risks for not only brine but many other types of mining. This provides a 5-fold scale of likelihood of risk sufficient to identify whether it is at a level of concern to require further consideration.
How the Compensation Acts work
Cheshire Brine Compensation was introduced in two phases. The first Compensation Act, enacted back in 1891, provided compensation in the Northwich area, for those affected and not profiting from the extraction. In 1952 (amended 1964) the Cheshire Compensation Act generally set out the current approach, managed by the CBSCB. The CBSCB is a private statutory consultee which is set up with a balance of pumpers and councillors, with a maximum of 15 members. Five members are elected by the brine pumpers and ten by the Unitary Authorities; five from Cheshire West & Chester Council and five from Cheshire East Council.
The Board has two main duties:
- To provide compensation for specified categories of damage to land and buildings caused by subsidence due to brine pumping.
- To respond to consultations from the Planning Authorities and Building Control regarding new development on properties within specified areas, the ones which may have been affected by historic brine pumping (Consultation Areas).
The 1952 legislation created seven, smaller, Consultation areas within the wider Compensation Area, where it is necessary to consult on foundation design when building. These are Northwich, Knutsford, Winsford, Crewe-Sandbach, Middlewich, Alsager and Lymm.
The Board is able to make recommendations for the strengthening of foundations in order to mitigate the effect of possible future brine movement. This is typically done with either raft or jack point foundations.
The claims process
Anyone who wishes to pursue a claim against the CBSCB must first lodge a Prescribed Notice of Damage (PNOD). This form covers only damage which became apparent in the six months prior to lodging the PNOD. Provided that the property has not been commuted (meaning that the Board previously severed liability) and the applicant is permitted to make a claim, CBSCB will accept and file the Notice. Where possible, to avoid blight, the Board offers a discussion with the potential claimant prior to submitting a PNOD. Claims against historical subsidence, meaning subsidence over six months old, are unlikely to be compensated, so it is important that buyers are aware of previous problems before making a purchase, which is where the required Cheshire Salt Search comes in.
Currently approximately 4,300 PNODs have been recorded. In some cases many claims have been made against individual PNODs, although the current approach is to accept one claim per PNOD.
After the claim has been lodged a CBSCB Surveyor will visit the property to carry out an internal and external inspection with a recommendation to either accept or deny liability, or that further investigatory work or survey is required. The claimant is then notified of the CBSCB decision. If the claim is not upheld the PNOD still remains in place, highlighting the recorded subsidence at that property. If the Board has accepted liability, a structural engineer will visit the property so that a Schedule of Damage & Repair can be prepared. The repair work is then carried out, and the Board meet the cost upon satisfactory completion. Should the occupant be required to vacate the property whilst it is repaired, the Board will also cover reasonable relocation expenses and storage of contents for the duration of the work
Should CSBSC judge either that further significant ground movements are likely to occur or that the property is beyond economic repair, CBSCB can elect to commute the property. This severs the Board’s liability for any future damage at the property upon payment of an agreed sum to the owner, usually equivalent to the market value of the property. However, there have been examples where the buildings have been demolished and new buildings with approved foundations have been constructed. These new buildings could access compensation should they experience eligible problems with ground stability due to approved settlement.
Additional uses of the Cheshire Salt Search
Further useful questions are answered within the Cheshire Salt Search which are not related to CBSCB claims. These include records of wells and shafts, lines of weakness and current extraction activities. Most relate to ground stability issues that are worth considering as part of any due diligence or divestment.
The Cheshire Salt Search is a step forward in providing clarity to this issue and is recognised as a Law Society requirement when purchasing or developing property in Cheshire.
Click here to order the Cheshire Salt Search.
Click here to find out further information about the history of Cheshire brine and salt extraction, or make a claim at the Cheshire Brine site.
- An interesting history of the Cheshire salt extraction can be found at http://www.cheshirebrine.com