Last month the temporary works for Northern line Adit No.3 were successfully completed, one week ahead of programme. The space provided will facilitate the construction of a 4.0m wide new passage between platform tunnels.
As reported previously, DSP redesigned the entire temporary works approach by using a sprayed concrete lined (SCL) access tunnel and canopy spanning between the two platform tunnels. This negated the need for small timber box headings and significant amounts of hand mining.
During excavation for the adit the platforms were kept operational and no propping was required inside the tunnels. Movements to both platforms were as predicted.
To limit impact on the platform tunnels DSP designed a clever system of walers and struts which did not need to be bolted to the cast iron linings. The lean ground support system also maximised the space between the back of the platform linings. At the springline the gap was less than 1.0m.
The design and its execution were well-received by London Underground. Tunnel Inspector Padraig Kilgannon stated: “The DSP approach was safer and quicker than typical handmining. The design was easy for the miners to follow. This was a really good first-time design.” The mining gangs also expressed appreciation for the greater working room the SCL solution gave.
To form the passage the next stage is to place the reinforcement and cast the concrete invert slab, sidewalls and roof slab.
When the station is opened there will be three new passages at Northern line level between the existing Northbound platform and the existing Southbound platform tunnel which will be converted into a concourse. A brand new Southbound platform is being constructed to the West.
Fig. 1: Working from the SCL canopy
Fig.2: Ramped SCL access tunnel
Fig.3: Looking down between the platform tunnels
Fig.4: Lean support system