Basement tanking and hydrostatic uplift

Basement tanking and hydrostatic uplift

Two things go wrong with basements in Sydney, and they are opposite problems.

Water gets in, which is a waterproofing failure. Or the basement tries to float, which is a structural one. Solving the first without designing for the second is how you get the second.

Where the water pressure comes from

Below the water table, the ground is saturated and the water in it exerts pressure in every direction. That pressure increases with depth.

On the walls it pushes inward, and it is added to the earth pressure the wall already carries. On the base slab it pushes upward, across the entire footprint.

That upward force can be very large. A basement two or three levels down with a large plan area is being pushed up by a force measured in thousands of tonnes.

The building has to weigh more than the water pushes

The basic check is buoyancy. The weight of the structure has to exceed the uplift force, with a margin.

The trap is the timing. The uplift is at its maximum as soon as the excavation is complete and the tanking is in place. The weight of the building is at its minimum at exactly that moment, because the levels above have not been built yet.

So the critical case for uplift is usually during construction, not in service. A basement that is entirely stable once the tower is on top of it can float during the build.

How that gets solved

Dewatering during construction. Keep the water level down until enough weight is on. This works and it needs a licence to extract, monitoring, and a plan for what happens if the pumps stop. It also risks settling neighbouring buildings by drawing water out from under them.

Permanent tension piles or ground anchors. Physically tie the basement down to the ground beneath. Reliable, and it has to be designed into the foundations rather than added later.

Ballast. Thicken the base slab or add mass. Simple, expensive, and it uses up headroom.

A drained basement instead. Rather than resisting the water, let it in to a controlled drainage layer and pump it away. That removes the uplift problem and creates a permanent maintenance obligation, because the pumps have to run for the life of the building.

The choice between a tanked basement and a drained one is one of the earliest and most consequential decisions on the project.

The tanking itself

A tanked basement is only as good as its weakest detail, and the details are the construction joints, the penetrations and the junction between the wall and the base slab.

Water finds the joint. Waterstops, injection hoses and properly detailed junctions are what make it work, and none of them are recoverable afterwards. Once the slab is poured and the building is up, a leaking construction joint is repaired from inside at significant cost and with mixed results.

Penetrations for services are the other classic. Every pipe through a tanked wall is a potential path, and each one needs a proper puddle flange or sealing system rather than a sleeve and mastic.

Testing before it is buried

The point at which the waterproofing can be verified is before backfill. Once the ground is against the wall the membrane is inaccessible.

That means inspection and testing sit in the programme at a point when everyone wants to move on, and skipping it is the most common false economy on a basement.

The investigation that makes it all possible

Everything above depends on knowing the water level, and specifically the highest level it reaches rather than the level on the day of the borehole.

Seasonal variation, tidal influence near the harbour and rivers, and perched water in fill all matter. A geotechnical investigation with standpipes monitored over time gives the design level. A single reading does not.

We design basements and their waterproofing strategy together under structural engineering, because the uplift case and the tanking decision are the same decision.