[shakman]

Aleks -

Those "poles" are temporary braces that are used to shore the ground around the site. With other structures nearby, the downward force on the ground in and around the Bishopsgate's excavation is quite high. An excavation this size and depth provides a large weak spot in the soil for that downward pressure to cave in. In order to mitigate that pressure from blowing into the hole, the poles you ask about are used. As the floors rise up to these braces and the the gap between the below grade perimeter walls is filled with compactable material(s), the braces are then removed upto that level.

There are other methods of shoring as well.

I hope I was able to answer your question.