As the project progresses, the length of the tunnel increases and there is more space to walk along the inside of the TBM to complete various jobs. Currently, about 70m of tunnel has been excavated. Inside, it looks like a long, slightly mucky corridor:

When you reach the cabin and boring head, it gets more interesting. In general whilst drilling is taking place, two workers are required in this area.

1. TBM driver. They sit in the small cabin and steer the path of the TBM and mechanical arms via various levers and buttons. They can see what is happening at the very front via a screen connected to a camera.
2. Someone to watch the placement of the concrete rings. They stand in front of the cabin, where the rings are transferred through, and instruct the TBM driver where the next ring must be placed through a microphone. This can be as simple as, for example, ‘left a bit, down, stop’.

Each concrete ring weighs about a tonne and is approximately 1.2m long.

These thick cables are about 150mm in diameter and carry a high-voltage current way down the power tunnels that extend through south London. They are similar in design to those carried by power lines that stretch across the countryside. High voltage, but low current, results in minimal power loss and overheating.

Six cables run through the tunnel, as part of two circuit systems.

The total tunnelling length is about 32km, which means about 200km of cables must be installed during the project. This requires heavy lifting machinery, with the cables weighing about 50kg per metre.

Installation of cables will be via a monorail that runs along the tunnel ceiling.

The tunnels at London Power Tunnels 2 use a spray of reinforced concrete to stabilise the lining whilst thick concrete rings are inserted.

The material chosen consists of cement, sand, and steel fibre reinforcements. . I imagined these to be thin and easily bent, but on inspection of the sprayed wall in the tunnel I saw they were firm: of about 1mm diameter, hard and spiky. Although you might expect the spray to form a smooth-ish façade on the inside of the tunnel, in fact it is very rough-looking, with spikes of steel fibre.

Before the deep, horizontal tunnels can be constructed, the project needs stable shafts, which look like vertical tunnels and are about 12m in diameter. From the bottom of these, horizontal tunnelling will then begin. Formation of the vertical shafts consists of this process:

2. Shaft underpinning

This technique works ring by ring. About 1m deep, a circular section is excavated with machinery. Segment rings of the same 1m height can then be inserted. Then they are stabilised by filling behind them with grout. This is a course, cement-like substance that is pumped behind the ring via groutholes designed specifically for this purpose.

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At each excavation, the first and last rings to be inserted are angled, to enable the last ring to slide in.

The process continues to build about one ring a day until the shaft is completed.

3. Contiguous bored piling

Approximately one third of the way down the shaft, about 15m deep, the type of piles used changes. Contiguous bored piles are similar to secant piles, but the previous ‘female’ piles are replaced by grout fill instead. This is a because the ground changes from ‘made (build-upon) ground’ and Thanet sand to mostly chalk, which is firm enough to hold its shape with this new piling, and we pass the water table.

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:

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