I have no details of this contract so here is my calculated guess. This layer of concrete appears to have been laid to a very high spec. I think it will be a temporary layer immediately under the roof slab. If so the next work will be to lay a membrane which will act as a debonding layer to the roof slab concrete. After the membrane will come the reinforcing bars, lots of them will be fixed. The roof slab thickness will be 800- 1000 mm thick. The roof slab will also act as a horizontal column holding up the piles. Piles, concrete or steel, can only hold up soil to about 3m deep freestanding. Over that depth you need propping, called temporary works. My guess is that this dig will be 7m deep. The structure being formed is a box or an n shape. Once the roof slab is formed excavation inside the box will occur including the concrete you can see today. A very efficient method of construction. The next few days will be interesting. Hope I am right.