The blockade started last weekend, with activities this week focused on stripping out track, power supply and signalling from the tunnel.
Joint venture contractor Corber (Costain, O'Rourke, Bachy Soletanche and Emcor) has already installed 12m to 13m deep contiguous piled walls for the station box either side of the Thameslink tunnel.
Precast reinforced concrete roof beams were installed above the tunnel in areas where there was enough clearance.
The next step is to complete construction of the station box roof, which will act as a propping strut, preventing movement of the walls while the station box is excavated.
A 1m thick concrete slab will be cast to form the station box floor.
One of the new 600m long, 6m diameter Thameslink tunnels has already been excavated using a backhoe mounted within a steerable shield and equipped with a face support system.
The machine is now well advanced on the second tunnel drive, about 400m north of the station box.
This tunnel will be skewed directly underneath the existing Thameslink tunnel, which in turn runs below the Midland Main Line railway.
The contractor has installed piles either side of the Thameslink tunnel over the length of the affected area to ensure that there is no ground movement which could potentially interfere with mainline train services Next, Corber will install ground anchors through the old tunnel's invert to improve integrity of the underlying soil between it and the new tunnel. Steel rings will also be erected within the old brick tunnel to prevent collapse.