Councillor Geoff Lyons moved the motion to adopt the option of filling the entire shaft with a fine quarry product and then reinstating the yard and stabilising the iconic headframe and skyshaft structure.

Works at the site were deemed necessary after the council became aware of the risk of a 35-metre sinkhole opening beneath the headframe when high rainfall in June caused a collapse in the Hart Shaft.

Engineers Pitt and Sherry estimated the cost to fill the shaft would be about $1.2 million, in addition to the $200,000 already spent.

West Tamar Council infrastructure services manager Ian Howard said the total cost of the works could reach $1.5 million, 20 per cent more than originally expected.

“We did look at the risk assessment of every [remediation] option and this is actually the lowest risk option, which was originally unaffordable at $2.4 million, and with the modifications that Pitt and Sherry have done they have come back into the realm of affordability,” he said.

General manager Rolph Vos said he believed the $1.2 million estimate the engineers had given the council was likely to be correct because savings would be offset by additional, unknown costs.

“The do-nothing option has been looked at but the current thinking is that over time, as we get into a new wet season, the material will wet up again, more will run down the hole and more subsidence will occur,” he said.

“In all likelihood a cavity will appear, the headframe will collapse ... I don’t believe the do-nothing option is an option because in this day and age you can not allow uncontrolled failure.

“We would probably have to shut the council offices [in Beaconsfield] because the road would be impacted.”

One cost saving measure discussed by the council would be to have Boral trucks carrying the fine quarry product using the West Tamar Highway instead of back roads but exact figures had not yet be calculated.

Mayor Christina Holmdahl said she expected the mine yard to reopen by November in time for peak tourist season. Once work begins the engineers predict it would take a month to fill the shaft but additional time would be required as the ground settled before the yard was deemed safe.

REPAIR PLAN FOR THE BEACONSFIELD MINE

Two existing steel pipes, which were once used to carry water out of the mine, will be used to fill the shaft with a fine quarry product.

The pipes extend 180 metres into the Hart Shaft and will be cut open at the minus 100 metre level, at a structure called the Kingset.

The shaft will then be filled down to minus 415 metres, where the decline begins.

Engineers have determined that the soil will not shift significantly from the bottom of the shaft into the decline.

Once the quarry product being sent into the mine reaches the minus 100 metre level, a thick concrete plug will be poured, again via the pipes.

The rest of the shaft will then be filled to the surface to prevent surrounding soft clay material from entering the shaft and allowing further collapses at the surface.

The West Tamar Council’s infrastructure services manager Ian Howard said small voids would still exist in the mine and would collapse over the next few decades. 

As the ground settles daily inspections of the mine yard will be necessary.

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