What is top-down construction (walls-and-roof method)?
Top-down construction, known in Dutch as the “wanden-dak-methode” (walls-and-roof method), is a way of building tunnels, deep stations, car parks and other underground structures in cities in which the walls and roof are built first, and the ground beneath the roof is excavated afterwards. It is a variant of cut-and-cover.
Sequence
- Walls: diaphragm walls are formed from ground level, sometimes combined with combi-walls.
- Roof: a reinforced concrete roof slab is cast onto the walls, often a few metres below ground level.
- Reinstate the surface: the street above the roof can largely be restored.
- Excavation: the soil under the roof is removed in stages. Temporary props between the walls keep the structure stable.
- Base slab and fit-out: the floor and interior follow last.
Grout injection below the walls can create a low-permeability layer, so less or no groundwater lowering is needed.
Pros and cons
Advantages
– The street is back in use relatively quickly.
– The roof acts as a working platform and as propping.
– No pile driving, so less vibration and noise for neighbours.
Disadvantages
– Diaphragm walls are not fully watertight. Leaks between panels can carry soil and water and affect surroundings, although leaks can be traced and repaired.
– Excavating beneath a roof is logistically harder than in an open pit.
Applications
In the Netherlands it was used for the tram tunnel in The Hague and the deep stations of the North/South metro line in Amsterdam. Where a bored tunnel is possible, it usually causes even less disruption at street level.
Related terms
- Diaphragm wall
- Dewatering
- Formwork
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