LPT2 horizontal tunnel construction: an explanation
At the London Power Tunnels 2 site at New Cross, the primary rock type is chalk. This rock has desirable properties for tunnelling, since it tends to be very stable. However, issues arrive when the rock is weathered and/or fractured:
- The existence of fractures, or gaps in the rock, increases its permeability; it becomes more likely to crumble and absorb water.
- This effect is exacerbated at depth, which is a particular problem here, where we’re working at depths of between 40 and 60m below ground level.
- Central London has a high water table – the Thames is never far away
- It’s important then to both fill the gaps in the rock and pump water away accordingly to ensure tunnel stability.
The horizontal tunnel construction begins with the digging and stabilisation of a vertical shaft. It is stabilised and then construction of the horizontal tunnels can begin. According to the properties of the site just established, the method of horizontal drilling (after the digging of the shafts) is this:
the reinforced concrete ring segments, ready to form the perimeter of tunnels at New Cross 
View of the fully constructed shaft from its bottom
1. Saw-cut the initial frame of the tunnel from the bottom of the shaft.
- Segments of rock and the concrete shaft wall get broken away ready to grout behind the newly-formed gap
2. Fissure grouting
- Fractures in the rock get filled with a grout to block pores, decreasing the material’s permeability around the perimeter of the shafts
4. Construct the horizontal shaft
- The excavation consists of step-by-step ‘advances’ using a ‘Blue Badger’ excavator machine. This creates a hole (of depth a few metres) with curved sides, a stable shape.
5. Fore/back-shunt using SCL
- At each advance, the section is stabilised with fast setting concrete. This is applied with a spray-concrete lining (SCL) process, using a sprayer machine called the MEYCO Oruga, which runs along tracks in the shaft.