Geo-Integrity Ltd. were recently asked to undertake a halite risk assessment for a site located in Northwich, Cheshire which had been flagged as potentially affected by land instability due to the underlying geology. Cheshire like other places within the UK is underlain by rock salt which is present in Triassic and Permian bedrock. Rock salt, also called halite or sodium chloride (NaCl), is highly soluble and responsible for natural and man-made subsidence geohazards.
The Cheshire Brine Subsidence Compensation Board (CBSBC) requested the client obtains a halite risk assessment upon which the client contacted Geo-Integrity Ltd. to aid with their proposed development.
A halite risk assessment involves desk top searches of historical and geological information to investigate the risk of ground subsidence associated with the natural and artificial dissolution of halite. Here at Geo-Integrity Ltd. we produce clear and concise reports providing our clients with detailed information of the geohazards affecting their site.
For this project in particular the report identified the site is underlain by thick glacial deposits, overlying the Northwich Halite Member which comprised of beds of mudstone/siltstone and rock salt termed Upper Marl, Top Bed Salt, Thirty Foot Marl and Bottom Bed Salt.
The Top Bed Salt is known as the “Wet Rock Head” as this is the salt surface where dissolution has occurred from circulating groundwater enhanced by the last (Devensian) glaciation and it is here where the original brine extraction took place. The foundered ground above the “wet rock head” commonly has numerous collapse features including enclosed hollows 20m to 200m across to linear depressions up to several kilometres long. These depressions were called “brine runs” and it is into these and the brine springs that miners sunk their extraction wells (see diagram below).
The halite risk assessment identified the site was positioned between an area of known subsidence and a historic mine. The historic mine was one of four in Northwich which were stabilised as part of the Northwich Mines Stabilisation Programme between 2005 and 2007. The information gathered from the desk study indicated a high risk of ongoing subsidence associated with this site and therefore recommendations to aid with the proposed development were outlined within the report.
As part of the halite risk assessment it is necessary for the consultant to provide an interpretation of the ground conditions based on the desk study information collected. Here at Geo-Integrity Ltd. we have experienced/chartered engineering geologists who can provide detailed interpretations of the ground conditions and produce clear recommendations to aid with your proposed development.
If you require a halite risk assessment please give us a call on 01280 816409 or drop us an email on info@geo-integrity.co.uk. We can help you.
Write a comment