Ref the above photos. As the station is underground the "bore holes" must be the first stage of creating a chamber that will be dug out. The station, while below ground, will not be fully enclosed but will have a cover, dare I say awning over it allowing natural light and probably rain to enter the platforms.
Wonder how the dual carriageway into the CTA will be affected if the road has to be dug up? Answers on a post card