Box jacking, also known as Tunnel Jacking, involves the advancement of a site cast rectangular or other shaped sections using high capacity hydraulic jacks. The structure to be installed is constructed, normally in reinforced concrete, on a launch pad at site adjacent to where it has to be installed. It is then thrust forward horizontally using advance support, open shield and jacking technology with excavation taking place from inside the box. This is frequently used where an existing road or rail track is on an embankment and space exists for the structure to be cast at the side. There are variations on this concept using short pre-cast units to form the box.
The main benefit of this approach is that it offers an effective alternative to disruptive open cut techniques, conventional tunneling methods being inappropriate so close to the structures above. Tunnel jacking can install the final structure in one go, it also allows the above infrastructure to remain “live” and active during the work. This obviously significantly reduces potential disruption to these services.
A number of installations have been made where the top of the box is immediately below the track. The system is designed for the track loads to be picked up by the box roof as it advances.