Some of the most demanding work disappears underground.

A finished development reveals very little of the work beneath it. At this project, construction included substantial underground STP and UGR structures extending approximately 10 metres below ground level. Once completed, much of this work becomes invisible — but its execution depends on excavation, structural sequencing, access, waterproofing and careful coordination long before the development above is finished.

Walk through a completed development and you see roads, landscaping and buildings. You do not see the sewage treatment plant or the underground reservoir sitting roughly ten metres below the finished level — and that is precisely the point of building them well.

Excavation and sequencing

Deep excavation is governed by the ground and the space around it. The sequence of cutting, supporting, casting and backfilling was planned together rather than as separate activities, because each one limits what the next can do. Access for excavation plant, and later for material and shuttering, had to be maintained down to the working level throughout.

Waterproofing and finishing below ground

Below-ground structures of this kind are only as good as their waterproofing and joint detailing, and both are effectively unrepairable once backfilling is complete. Surfaces were prepared, treated and checked before the structure was closed up, because there is no convenient second attempt.

• STP and UGR structures at approximately 10 m below ground level
• Staged excavation with support and dewatering as required
• Waterproofing and construction-joint treatment before backfilling
• Access and material movement maintained down to the working level

The teams below finished ground level

For several months, a part of the workforce on this project worked entirely below the level that anyone would later walk on. It is ordinary construction work, carried out carefully, and it becomes invisible the moment it is finished.