Structural And Geotechnical Engineering In The Sutherland Shire

Hawkesbury Sandstone sits close to the surface across most of the Sutherland Shire, so a basement there is usually a rock job and the footings often reach rock within a short dig. The engineering questions move to how the rock gets cut, what that does to the neighbours, and what bushland and sand bring at the edges of the Shire.

We have designed apartment buildings over three basement levels in Engadine, Miranda and Sutherland, and commercial work in Caringbah. Each one sat in the same sandstone, and each still needed its own answer.

The ground across the Shire

The Shire sits on the sandstone plateau south of the Georges River, cut by steep gullies that run down to the Woronora River, Port Hacking and the bays. Over the rock there is usually only a thin cover of sandy soil and clay.

Lower ground around the bays and creeks carries deeper weathered profiles and softer soils. Near the beaches at Cronulla the ground turns to sand, and a basement there has more in common with a sand site in the Eastern Suburbs than with a sandstone site in Sutherland.

Shallow foundations

With sound sandstone close by, footings often go straight onto rock as pads or short piers, about the most economical foundation a building can have. Our commercial job in Caringbah went that way, as described in the [Taren Point Road, Caringbah](/blog/taren-point-road-caringbah) project.

The rock still needs classifying. Sandstone quality changes over short distances, and a footing on a weathered band or above a clay seam won't carry what the class on the drawing says. The founding approach is in [founding in Sydney sandstone and rock sockets](/blog/founding-in-sydney-sandstone-and-rock-sockets).

Basements in sandstone

A three-level basement in the Shire is mostly a hole cut in sandstone. Above the rock, the soil is held with soldier piles socketed into the sandstone. Below it, the rock face often stands close to vertical, held with rock bolts and shotcrete where the joints need it, and with anchors where the face has more to hold back.

Joints in the rock decide the detail. A joint dipping towards the excavation can let a wedge slide out of a face that looks perfect, so an engineer inspects each stage of the face as it is exposed.

Rock excavation and the neighbours

Breaking sandstone takes rock hammers, rock saws or ripping, and all of them send vibration into the ground. Older houses and units next door feel it first, and the complaints usually start with cracked render or a door that suddenly sticks.

So vibration limits get set before the dig starts, monitors go on the closest buildings, and the method is chosen to suit them. Sawing the perimeter of the cut before breaking the rock inside it reduces the vibration that reaches the boundary. Dilapidation reports before and after record the neighbours' buildings, so any claim can be checked against fact. The monitoring side is in [vibration monitoring on construction sites](/blog/vibration-monitoring-on-construction-sites).

Seepage in basement faces

Sandstone carries water along its joints and bedding rather than through the whole mass. A basement face often weeps along those lines after rain, and water perched on top of the rock runs in along the boundary between soil and rock.

The basement design has to collect that seepage behind the walls and get it out, or keep it out altogether. The choice depends on how much water arrives and what the council will accept.

Bushland edges

Engadine and Sutherland sit close to the Royal National Park, and plenty of Shire streets back onto bushland. Lots on that edge are mapped as bushfire prone land, and the Bushfire Attack Level set for the site changes the outside of the building, from the glazing to the decks, as covered in [Bushfire Attack Level and what it changes](/blog/bushfire-attack-level-and-what-it-changes).

Steep blocks above the gullies bring their own checks too, for slope stability and for loose boulders sitting on the rock.

Authorities

Two of our apartment jobs in the Shire needed Specialist Engineering Assessments for building near Sydney Water assets, and one of them also needed an engineering assessment report for Transport for NSW. Both kinds of report come from the same engineers who designed the shoring they describe.

How we approach it

We plan a Shire basement around the rock. The boreholes core the sandstone, the shoring is designed for the joints we find, and the excavation method is chosen with the neighbours' buildings in mind. Structure, civil and geotechnics are designed together, so the footings, the seepage drainage and the stormwater all work off one set of levels.

Sandstone is generous ground to build on, as long as nobody mistakes it for one solid block.

ACSES provides geotechnical engineering in Sutherland and structural engineering in Engadine. Talk to us about a project.

Building Relationships Beyond Structures