Mainmark announced as a finalist for 2017 Ground Engineering Awards

Mainmark decommissions one of NSW’s most challenging abandoned mineshafts

Ground engineering firm Mainmark has been announced as a finalist for the International Project of the Year Award at the 2017 Ground Engineering Awards for its work in successfully decommissioned one of New South Wales’ most technically and physically challenging derelict mineshafts.

The $1.1 million fixed-price project involved using a bespoke grout formulation and self-releasing grout deflectors to place a neutrally-buoyant plug 30m underwater at 3 bar to support 270-cubic-metres of backfill material.

Located in the car park of an operating motel in the regional town of West Wyalong, the mine shaft is 300 metres deep and was at risk of collapse. A previous attempt to fill 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 hotel to subside and sink holes to form on the surface of the carpark.

The mine shaft needed to be urgently plugged, capped and sealed under challenging site conditions. Mainmark was commissioned by the Derelict Mines Program, which is administered by the New South Wales Department of Industry, to complete the project.

William Lindsay, Group Technical and R&D 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 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 60m, required a sonic drill to successfully penetrate the obstructions. Three 76mm 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 by filling any latent sub-surface voids local to the shaft. The area was then re-surfaced to complete the project.

Kate Maddison, Derelict Mines Project Manager, who led the project said, “I’ve never seen a method like this. It was a novel approach that successfully solved an extremely complex engineering challenge. Unlike most of our other mine shaft remediation programs, this project required a unique approach, sensitivity to the surrounding urban environment and management of risks to the public.

“Mainmark went above and beyond to deliver results. What began as a 10-week program of work ended up taking months because it was much more technical than anticipated and required extensive site trials. The Mainmark team was intuitive, easy to work with and demonstrated a strong understanding of documentation requirements for government