LPT2: vertical shaft construction
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:
- Secant piling to cut off groundwater
- Secant piles consist of reinforced concrete column foundations that interlock, driven into the ground. They have a greater stiffness than traditional sheet piles, and are essential when such a high water table is involved. These form a concrete ‘wall’ around the vertical shaft for the top half of its vertical height
- The piles consist of harder, reinforced concrete ‘male’ piles, and softer, unreinforced concrete ‘female’ piles
- Secant piles form a temporary support while shaft underpinning takes place
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.
the first excavation and rings (taken at another site) 
an example of the last, angled section
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.
