Two disused 500ft deep mine shafts were found in Crusader Avenue, Glasgow, nearly a year ago.
Dating back to the Industrial Revolution, they caused five garages to collapse and residents from four nearby homes were evacuated.
Hoppers have pumped more than a thousand tonnes of concrete into the old shafts but work that was originally due to finish in March has now been extended, as one of the shafts is deeper than expected and still requires further filling.
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Matthew Rowton of the Coal Authority told a reporter at the Glasgow Evening Times that the work was dangerous and could not be hurried.
He said: "The works have got to be done properly to protect public safety. We can't speed it up, we have to make sure it's done right and the workers need to remain safe. The road closure has been extended until September 28, but it is envisaged works on and around the shafts will be completed before this. We apologise for the delay."
Residents complained that their lives were being disrupted by the works.
Soil Engineering Geoservices took on the work on behalf of the Coal Authority.
They established an exclusion zone around the ground collapse and conducted all works from outside it, drilling at angles so operatives did not have to walk on unstable ground.
Next they filled the holes with cement PFA grout, working from a platform over the mine entry.
(NE/GK)
