Engineering Apartment Buildings In Burwood And Strathfield

An apartment building in Burwood or Strathfield starts in residual clay over Ashfield Shale, usually close to the railway and almost always with an older building on the boundary. Those three facts shape the basement before anyone designs a floor, and they shape it differently on every site.

Our office is on Swan Avenue in Strathfield, and Burwood is the suburb next door. Our work in Burwood alone runs from a nine-storey building over four basements to a 50-level tower over six.

The ground under the Inner West

Ashfield Shale is the lowest layer of the Wianamatta Group, the shales that cap much of Sydney west of the city. Near the surface it has weathered to clay, which is often reactive. Below that sits weathered shale that gets harder with depth, and under the shale is Hawkesbury Sandstone.

How deep that change to sandstone sits varies from site to site, so the boreholes decide whether a basement stays in shale or reaches rock. Sydney's ground types are compared in [deep excavation solutions for Sydney's geology](/blog/deep-excavation-solutions-sydney-geology).

Shale in a basement face

Fresh shale looks solid when the excavator first opens it up. Left exposed, it softens and breaks down as it wets and dries, and a face that stood well on the day it was cut can start shedding flakes once a few rain events have been through it.

So shale faces get sprayed with shotcrete as each stage of the dig opens up. The bedding and the joints matter too. A smooth, clay-filled seam dipping towards the excavation can let a block slide, so the faces are inspected as they open and bolted where the joints call for it.

Groundwater

Shale lets little water through the rock itself. The water that does reach a basement tends to come along the joints and bedding, or sits perched on top of the rock after rain.

Whether the basement is drained or tanked comes down to how much water the site delivers and what the council will accept. That is why the standpipes get read over time before the choice is made, because a single wet joint can deliver more than the boreholes suggested.

Salt and reactive clay

Wianamatta shales are a known source of salt, so the soil and groundwater get tested for it during the investigation. The result feeds into the concrete grade and the cover for everything in contact with the ground.

The reactive clay above the shale affects anything sitting shallow. Ground slabs outside the basement footprint, pavements, driveways and the neighbours' footings can all move as the excavation changes the moisture around them.

The railway and the authorities

The railway runs through the middle of both suburbs, and much of the denser development sits close to the line. Jobs near the rail corridor or a state road need the shoring and excavation assessed for Transport for NSW, and our Burwood work has included those reports, a process covered in [Transport for NSW engineering assessment reports](/blog/transport-nsw-engineering-assessment-reports).

Building near or over a Sydney Water main brings a Specialist Engineering Assessment, and two of our Burwood jobs have needed one.

Neighbours and heritage

Older shops along Burwood Road and around Strathfield Square often sit hard on the boundary on shallow footings, and heritage controls apply around Strathfield Square. A basement beside them needs shoring or [underpinning](/blog/underpinning-adjoining-structures-explained) designed to protect them, dilapidation reports before and after, and monitoring while the excavator is close.

Where part of an existing building on the site is kept, it gets assessed before the new design starts. One of our Burwood projects started with a pre-construction assessment of the heritage building on the site, then assessment and strengthening of the existing structure.

Tight sites and tall buildings

Many sites here are narrow and boxed in, so the crane, the concrete pump and the trucks all compete for the same street frontage. On the taller towers, floor plates are small for the height, which puts real demand on the core and the lateral system, and the column grid has to suit apartments above a car park that fills the whole lot. Our [Bede Street, Strathfield](/blog/bede-street-strathfield) write-up shows how we coordinated the columns with the car park layout.

How we approach it

We design the structure, the civil works and the geotechnics for these sites together, from the boreholes up. The shoring is drawn with the neighbours' footings on the same sheet, and the reports for Sydney Water and Transport for NSW are prepared by the engineers who designed what those reports assess.

Every basement in this ground starts as a question about the shale, and the boreholes are the only honest answer to it.

ACSES provides structural engineering in Burwood and geotechnical engineering in Strathfield. Talk to us about a project.

Building Relationships Beyond Structures