Underground station boxes

How does Underground station boxes work, step by step?

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   Step 1: Fix the operating concept before the box is sized

   The station is designed from the passengers outwards. The designer works from forecast flows, the train configuration, the interchange arrangement and the evacuation strategy to determine how many platforms are needed, how wide the concourse has to be, how many escalators and lifts connect the levels, and where the entrances sit. Those decisions set the length, the width and the depth of the box, and once the walls are in the ground they are effectively permanent. On most projects the design is frozen at this point under a formal change process, because a change that would be trivial in a building becomes a change to a retained excavation that is already under construction.

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   Step 2: Clear the footprint at the surface

   Before anything can be sunk, the ground where the box will go has to be emptied of everything else. Utilities are traced, proved and diverted to new routes outside the footprint. Buried structures, old foundations and forgotten basements are located and dealt with. Traffic is taken through a staged series of layouts, pedestrians are rerouted, and the site hoarding is set at whatever line the highway authority and the local authority will accept. This phase is routinely the longest single item in the early programme and the one most often underestimated, because it depends on third parties who have their own outage seasons and their own approval processes.

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   Step 3: Install the perimeter walls

   The walls of the box are built into the ground before the ground is taken away. Depending on what the designer selects for the conditions, that may be a diaphragm wall, a secant or contiguous piled wall, or a sheeted solution, and it may be designed to serve as the permanent structure or to be lined afterwards. Verticality, joint quality and the continuity of the wall matter more here than almost anywhere else, because the wall is simultaneously the ground support, the water cut-off and, frequently, the finished structure. The specification and the wall type are the designer's, and the installation tolerances are proved as the panels go in rather than discovered later.

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   Step 4: Control the groundwater

   Most station boxes go below the water table, so the design has to decide whether to exclude the water, to lower it, or to build a structure that resists it. Exclusion relies on the perimeter wall reaching a low-permeability stratum and on a base treatment where it does not. Lowering relies on pumping, which draws water from beyond the box and can settle the ground and the buildings on it, so abstraction is licensed, monitored and limited. Discharge has to be consented and treated. Whichever route the designer takes, groundwater is designed and monitored continuously, and the response to a reading outside the expected range is written down before the reading happens.

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   Step 5: Excavate and prop in the designed sequence

   The box comes down in stages, and each stage is supported before the next is dug. In bottom-up that means temporary props or ground anchors installed at levels the temporary works designer has set. In top-down it means casting the permanent slabs progressively and excavating beneath them. Either way the sequence is not a suggestion - the wall is only stable in the configuration it was checked for, so the propping goes in at the level and the stage the design says, and removing a prop early is a design change, not a site decision. Muck removal, ventilation, lighting, water management and access all get harder with every level down, and on a top-down box they are constrained by the size of the openings left in the slabs above.

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   Step 6: Monitor the ground and the neighbours throughout

   Instrumentation is installed before the excavation starts and read continuously through it: wall movement, ground movement, groundwater levels, and settlement and tilt of the buildings, tunnels and utilities around the site. Trigger levels are set by the designer with agreed actions at each level, and a named person receives the alerts. Condition surveys of surrounding property are carried out and signed off before the works begin. On sensitive projects the mitigation is designed in from the start, and the instrumentation is what proves the assumptions were right rather than what discovers they were wrong.

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   Step 7: Build the permanent structure inside the shell

   With the box complete, the internal structure is formed: base slab, columns and walls, intermediate slabs, stairs, escalator pits and the openings for shafts and services. Waterproofing is a governing consideration, because a leak in a station box is very difficult to reach once the finishes are on. Tolerances matter more than they might elsewhere because escalators, lifts and platform equipment are manufactured items with limited adjustment, and their pockets and fixings have to be right. The concrete frame is also the mounting point for the systems that follow, so cast-in provisions are coordinated before the pour rather than drilled afterwards.

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   Step 8: Fit out and hand over to the systems teams

   Fitting out an underground station means moving a very large quantity of material through a small number of openings, in a fixed volume, with many trades on top of one another. Ventilation and smoke control, fire systems, power, communications, drainage, lifts and escalators, wayfinding, finishes and the station control room all have to be installed and then proved. Access, craneage, lay-down space and lift capacity become the real currency of the programme. The structure is handed over to the systems teams in agreed zones, and from that point the station is on the commissioning route to entry into service rather than the construction route to practical completion.