Basements In Sand In Sydney's Eastern Suburbs

Saturated sand with water close to the surface makes a basement around Mascot, Botany, Pagewood and Kensington a groundwater job before it is a concrete job. The shoring has to hold back the water as well as the soil, and the basement usually ends up tanked because pumping it forever isn't an option you can count on.

This ground is called the Botany Sands, and it behaves nothing like the sandstone and clay across most of Sydney. Design a basement here the way you would on a sandstone site and the water will correct you.

The ground under the Eastern Suburbs

The Botany Sands are layers of marine and dune sand filling an old basin in the sandstone. Reports on the basin describe them running from under ten metres thick around the edges to more than 60 metres in the middle, with Hawkesbury sandstone below.

Water moves through them easily, flowing south and south-west to Botany Bay. The sand also holds bands of peat and peaty clay, and those soft layers matter a great deal when anyone starts pumping.

Why the sand won't stand up

A clay or sandstone face will stand for a while, long enough to get shoring in. Wet sand won't. Expose it below the water table and it runs into the hole, taking the ground behind it along.

So the wall goes in before any digging, and it needs to be close to watertight. That points to secant piles, soil mix walls or diaphragm walls, each taken down to a layer that slows the water. A wall of soldier piles and lagging, perfectly good on dry ground, doesn't do that job. Sheet piles can cut off water too, but driving them in loose sand can shake and settle the ground around them, and we weigh that against what stands next door.

The base of the excavation

The wall is only half the box. Water can still come up under the toe of the wall and into the floor of the excavation.

If the upward flow gets strong enough, the base loses its strength and the sand boils. To stop that, the wall has to go deep enough to lengthen the water's path, or the excavation needs to be pumped in a controlled way inside the cut-off. The checks are in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations).

What pumping does to the neighbours

Lowering the water table outside a site lets the ground compress, and peat compresses badly. Buildings on shallow footings can settle because of a pump running on a site they have nothing to do with.

That is the reason cut-off walls are the default here, and why the dilapidation reports and settlement monitoring start before the wall does. On low-lying ground, pumping can also expose [acid sulfate soils](/blog/acid-sulfate-soils-on-sydney-development-sites), and some groundwater around Botany carries contamination from past industry, which limits where the pumped water can go.

Approvals for groundwater

Botany Sands is a regulated groundwater source, and taking water from it for a build sits under the Water Management Act and the NSW Aquifer Interference Policy. A proposal has to account for the water taken during construction and for the life of the building, then show that it keeps the effects on the aquifer within the policy's limits.

The licensing rules for construction dewatering have changed in recent years, so we check the current position with the NSW water regulator on each job. Any DA with a basement in this ground should expect questions on it.

Why the basement ends up tanked

A drained basement keeps pumping for as long as the building stands, and this aquifer isn't a place where that gets an easy approval. A tanked basement seals the water out, so nothing needs pumping once the building is finished.

The price is uplift. The slab and the walls have to resist the full water pressure, and early in the job, the building isn't heavy enough to hold the box down. Piles in tension or a thicker slab do that work, as set out in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

Damming the aquifer

A wall that seals a whole site works like a small dam in the path of the groundwater. Water can mound up on the upstream side and drop on the downstream side, and a street of deep basements adds to that effect.

The hydrogeological assessment has to look at that, and on a larger site the answer can include passing the water around or under the structure. We would rather raise it at concept stage than have the regulator raise it.

Footings and piles

Under the basement, the sand can carry a raft well where it is dense, and the design then turns on settlement and uplift together. Where piles are needed, bored piles in running sand need casing or support fluid, and continuous flight augers are a common way to place them. The choices are compared in [piling types and when each is used](/blog/piling-types-and-when-each-is-used).

Salt in the groundwater near the coast also feeds into the concrete mix and the cover.

How we approach it

We have standpipes installed early and read them through wet weather, since the highest credible water level sets the wall, the slab and the uplift. Our geotechnical and structural engineers then work the wall, the waterproofing and the slab as one design, with the groundwater assessment running alongside it.

In this ground, the water was there first.

ACSES provides geotechnical engineering, including shoring and groundwater assessment, across Sydney. Talk to us about a project.

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