Vertical services can be installed between infill blocks and the void made good with in-situ concrete.
These three basins are isolated from the North Falkland Basin, and have a completely different infill.
The work would have included removing the infill above Valley Aqueduct and carrying out repairs to that Aqueduct and the one over Crime Lane.
Larger span covers may require propping from below to prevent distortion caused by the weight of the wet concrete infill.
He said monitors would be installed to keep a check on how the new chalk infill was settling in.