Geotechnical investigations are one of the first essential steps in all major infrastructure projects. For Westport, these are taking place from two jack-up barges across the proposed site of the new port in Kwinana. It’s a significant milestone in bringing the Westport vision to life.
What are geotechnical investigations?
Geotechnical investigations are a study of the subsurface conditions. The process involves drilling bore holes to extract samples which are then analysed in a laboratory to find out the properties of the ground layers, or the geological units. Geotechnical investigations are done before all major projects as these subsurface conditions impact the design and construction of the infrastructure, from where the pile foundations are laid to the types of materials used.
“Effectively we are identifying the type, depths and strength of soil and rock conditions” Erik Brouwer said, Principal Project Manager at engineering firm WSP who were awarded the contract to conduct the investigations for Westport.
“With the data obtained engineers can decide what foundations are most suitable for the proposed structures.”
A geophysical investigation of the site was done in late 2024 which used soundwaves to map the subsea area of the proposed new port, providing a broad picture of the topography and properties of the ground layers. In combination with the geotechnical data obtained, a clear geological profile can be created which will be used to optimise the foundation designs.
Westport Project Manager Roger Goga explained that so far the geotechnical investigations have confirmed some of the initial assumptions, but the process is also about collecting further details.
“We thought we would find limestone down there and we have, but it's really the details of the limestone that we’re trying to determine, how strong it is, at what depth its being encountered at,” he said.
Roger explained that parts of the Westport development would utilise the Osborne Formation that lies along the Western Australian coast.
“The Osborne Formation is a layer of interbedded sandstone, siltstone and shale millions of years old that runs through the area, right up to about Scarborough and further south as well. That's the ideal spot to found the bottom of the piles on and we’ve been finding that at about 15-20m depth,” he said.
Sealift 2. coming into harbour in Henderson to have 30m legs installed.
Types of investigations
There are three types of geotechnical investigations taking place across the Westport site.
Geotechnical boreholes
Holes are drilled between 5 - 70 metres into the ground to collect “cores” - cylindrical samples of sediment and rock. Standard Penetration Tests (SPT), Pressuremeter Tests and Downhole Geophysics are also conducted to gather additional data about the strength and composition of the seabed. For Westport, the majority of these bores are being drilled into the seabed from jack-up barges - floating platforms which can be raised out of the water to create a stable platform. Tugboats are used to move between locations.
Vibrocore sampling
A Vibrocorer is a vibrating machine that drives a cylindrical barrel between 1 and 7 metres into the seabed. The extracted cores are logged and tested in a laboratory, similar to borehole samples.
Cone Penetration Test (CPT)
A CPT involves pushing an instrumented cone into the ground. Sensors within the cone measure resistance and other parameters, helping to assess the strength, density, and layering of the subsurface materials.
Leg extensions for one of the jack-up barges to allow it to go into deeper water.
Core logging and testing
For the samples collected from the jack-up barges, testing begins while they’re still out on the water.
“There is a geotechnical engineer on board who starts logging the samples by analysing it, recording depths of samples, taking photographs and preparing the samples for transport to the laboratory,” Erik said.
Each borehole requires detailed logging of the properties and thickness of the geological units and what meterage each layer sits at. This cataloguing continues when the cores are brought to a dedicated shed onshore from where the samples are collected to be sent to the laboratory for detailed testing.
For the vibrocore samples, two types are collected for laboratory testing - geotechnical samples which are analysed for structural and material properties and contamination samples which are tested for potential pollutants.
Core sample being analysed offshore before samples are taken to send to the laboratory.
There are several other tests done onsite, such as Standard Penetration Tests (SPT) and Downhole Geophysics, to gather additional data about the strength and composition of the seabed.
“With the geotechnical boreholes, at regular intervals we do an SPT test which involves hammering a sample tube in the ground at the bottom of the borehole. How far the sample tube travels in each blow determines how strong the soil is. A high blow count represents dense sand, a low blow count represents loose sand,” Erik said.
“We collect all this data to come up with a control profile across the whole site. This means when the engineers are working on the design, they can understand all the soil and rock formation parameters in each section. From that they can determine the pile length and thickness.”
“Any missing data here could result in design issues and higher costs when we build the port if there’s something unexpected that requires a different approach than what was planned. So, it’s really crucial we get as much information as we can through geotech investigations to inform the design process,” Roger said.
Once testing is complete, the samples for Westport will be archived and stored for up to 10 years, ensuring the raw data is available to check during the construction of the port.
Determining the dredging plan
The geotech data will be used alongside the data from the WAMSI-Westport Marine Science Program to manage the impacts of dredging for a new shipping channel and turning basin.
“The material we find will determine the dredging plant that is required. It also helps us understand what that dredged material will do in the water column - the particle size distribution - whether it floats up and then sinks down or whether it would stay suspended,” Roger said.
“This also feeds into the hydrodynamic modelling being done for the environmental approval, the seagrass reclamation project and desalination.”
Collaborating with Water Corporation
Hydrodynamic modelling is used to map water movement and flow. For Westport, it is being done to assess how changes to Cockburn Sound’s seabed during the port development could change flushing and circulation of the water.
“Water movement and the particle size distribution in the water column could also affect the existing desalination plant and any future upgrades,” Roger said. “This is because if the particles do become suspended in the water it can impact the desalination process. So, it’s been an important part of the project to work with the team at Water Corporation to share the findings from this modelling. We are also providing geotechnical investigations in the area where new pipes might be located if the plant were expanded in the future.”
The logistics of geotechnical
“On a project of this size it all comes down to planning,” Erik said. “We're trying to optimise the drilling schedule as much as we can and direct the work in the most efficient way. We have to be aware of the constraints of the area, both logistically and physically. Before starting the work, we did a constraint mapping exercise to pick up on all the things we need to be aware of while drilling in the water, to make sure we are not drilling on a pipelines, telecom cables, wrecks or on seagrass.”
There’s also the day-to-day logistics of navigating the vessels, the marine life of Cockburn Sound and even the weather.
“Weather has been a challenging with some of those big winter storms,” Nick Di Lallo from Soil Mechanics said, who as Operations Manager is part of the team overseeing the day-to-day logistics of the investigations for Westport.
The team monitor the weather when planning upcoming barge movements and works, while regular contact with Fremantle Port Authority to keep them up to date with movements of other vessels in the area so there’s no crossover.
Nick said they have been lucky enough to see marine life such as whales from the jack-up barges, as well as dolphins who were initially curious about the activity going on.
“They came and had a look at what we’re doing but they’re mostly just looking for fish now,” he said.
Erik continued, “We always keep an eye on any marine life and if there is an animal approaching, we will stop the work, in line with our environmental management procedures."
Heritage monitors
Engaging with Aboriginal Traditional Owners has been embedded in all steps of the Westport program and for geotechnical investigations this has included having heritage monitors visit the site.
“We have Gnaala Karla Booja Traditional Owners coming out three times a week to have a look at the cores that we've extracted. They ask a few questions of the core loggers or the geologists to understand what time period that particular core would have been from and they’ll talk about their stories and what was happening in that area at the time,” Roger said.
The skill of drilling
Soil Mechanics Managing Director Daniel Van Liefde said with the project containing multiple types of geotechnical drilling, it’s given his workers experience in a variety of skills.
“With geotech, the drilling has to be done slowly to get quality samples, so there’s a lot of skill and refinement in the drilling process,” he said. “We have several trainees gaining valuable experience on this project including our first two female trainees.”
Nick said it’s rare to have so many different types of material to be drilled through on one project, including on land and under the sea.
“It’s been a pretty interesting project, you don’t always get to do cool stuff like this,” he said.
What’s next?
The geotechnical investigations for Westport are scheduled to be completed by mid-2026. The jack-up barges will continue their journey across the proposed port footprint in Kwinana and make their way along the shipping channel, to create a full picture of subground conditions, providing the data to ensure an efficient and reliable design for future construction.
