It is in place from the usally short time from when the tunnel is excavated until it can be lined with a permanent support structure made up of (depending on the period) bricks, concrete, cast iron, or steel.
It is in place from the usally short time from when the tunnel is excavated until it can be lined with a permanent support structure of (depending on the period) bricks. It is in place from the usally short time from when the tunnel is excavated until it can be lined with a permanent support structure of (depending on the period) concrete. It is in place from the usally short time from when the tunnel is excavated until it can be lined with a permanent support structure of (depending on the period) cast iron. It is in place from the usally short time from when the tunnel is excavated until it can be lined with a permanent support structure of (depending on the period) steel. This permanent support structure is made up.