Overcoming the challenges associated with decommissioning abandoned mines
23 October 2017
Mainmark: There is estimated to be approximately 50,000 abandoned mines across Australia with a significant number of these posing threats to both public safety and the environment. According to the 2012 NSW Auditor-General Report, derelict mines represent the largest category of contamination liability for the state of NSW.
While a proactive approach is recommended to address the ongoing risks associated with derelict mines, there are many factors to consider before a mine can be decommissioned. Usually, the decision to remediate or fill an abandoned mine is based on risk and stakeholder interest.
Finding a cost-effective solution to filling abandoned mines, particularly when groundwater and void geochemistry challenges are present, has been a major barrier. However, new and innovative fill technologies are now able to help overcome many of these issues, offering engineers a way to address the complex and technical challenges associated with decommissioning mines.
Ground engineering specialist, Mainmark, demonstrated its innovative approach to decommissioning abandoned mines using Terefil®, Mainmark’s proprietary cementitious grout solution. Terefil was used with great success for a technically challenging abandoned mineshaft in West Wyalong, NSW which saw Mainmark become a finalist for Project of the Year at the 2017 International Ground Engineering Awards in London.
The mineshaft, located beneath the car park of an operating regional motel, was 300 metres deep and at risk of collapse. A previous attempt to remediate it with 300 tonnes of concrete had caused it to partially cave-in under the weight and a 160-cubic-metre cavern had formed underground. This was causing the adjacent motel to subside and sink holes to form on the surface of the carpark. The mineshaft needed to be urgently plugged, capped and sealed under extremely challenging site conditions.
William Lindsay, Group Technical and Research and Development Manager at Mainmark said: “The shaft had been previously used as an illegal dumping ground and its original timber batten lining had partially collapsed, so it was heavily congested. It also had a high water level, which meant we were required to plug the shaft 30 metres under water at 3 bar pressure, and provide the client with visual confirmation that the plug had been successfully placed before pouring backfill materials.”
Mainmark undertook extensive research and on-site trialling to engineer a successful solution. It trialled a number of grout iterations, including expandable grout bags, polymeric expansive resins, and ultimately evolved the solution to deliver a bespoke, neutrally buoyant, cement-based grout.
“The neutrally-buoyant, non-dissolvable grout formulation and bespoke application method was an Australian first and a major achievement of the project. It involved extensive consultation with, and management of, external experts. To our knowledge, the solution is unrivalled. It remains cohesive in the presence of water and does not compress or expand, allowing it to form a self-supporting, floating plug,” said Lindsay.
“Mainmark delivered significant cost savings for the Derelict Mines Program. The neutrally buoyant plug eliminated the need to fill the shaft completely, from the bottom up, which could have doubled the cost of the project.
”It also delivered a solution for a problem that was unsolvable using conventional fill methods. The existing concrete cap that had been installed a year prior by another contractor for a similar project cost was unsuccessful, and had not prevented further collapse of the shaft,” said Lindsay.
Mainmark was the first contractor to successfully drill and pump grout into the shaft at depth. The mine shaft, which was heavily obstructed to a depth of 60 metres, required a sonic drill to successfully penetrate the obstructions. Three 76 millimetre casings were inserted down the main shaft, providing a conduit for 90 remotely-activated, self-expanding deflectors. These provided a temporary failsafe catch in the event that site conditions were different to laboratory conditions.
Seventeen cubic metres of grout was deployed to 60 metres below the surface to form the neutrally-buoyant plug. Importantly, CCTV cameras were also deployed to provide visual verification that the plug had placed before pouring backfill materials, despite the dirty water conditions and limited access.
The shaft was then backfilled using two separate grouts, one for the submerged portion of the shaft and one for the above water portion. A secondary pressure grouting program stabilised the adjacent ground around the site, by filling any latent sub-surface voids local to the shaft. The car park area was then re-surfaced, to complete the project.
For more information, visit www.mainmark.com.
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Source: Mainmark - www.mainmark.com
Contact: N/A
External Links:
http://theconversation.com/what-should-we-do-with-australias-50-000-abandoned-mines-18197
http://www.audit.nsw.gov.au/ArticleDocuments/255/01_Volume_Six_2012_Full_Reportv3.pdf.aspx?Embed=Y
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