Newcastle is the second largest city in NSW, located on the east coast of Australia, and situated on lands of the Awabakal and Worimi people. Surrounded by golden beaches, and a beautiful working harbour, it is a city undergoing a transformation with a rich history in mining and trade but embracing the future of a vibrant and growing population.
For its city authorities, planners and commercial developers, it is a challenge navigating the mining past due to the vast area of the Newcastle CBD having been undermined by a century of mining activity. Associate Geotechnical Engineer Simon Baker is driving insights and solutions to address these problems to ensure the city has a viable solution for future viability.
Growing the city
Outlined in the Greater Newcastle Metropolitan Plan 2036, the city will need to prepare for an expected 20% population growth. Under the plan, the former industrial city is expected to expand its workforce in health, education, defence, tourism and the creative sectors.
According to Simon, one of the main challenges faced by planners and developers is the impact of mine workings on the stability of the city. Although some areas are considered stable, some areas have a history of collapse with marginal factors of safety. Over time mine workings can become unstable if localised ground conditions change – this is particularly risky with respect to high rise property developments. Changes may include increase or decrease in water head, roof fall, degradation (loss of strength) of units within the pillars, loss of support above the coal pillars.
For some areas of Newcastle, the coal seam mine workings underneath Newcastle are affected by water recharge, which can result in subsidence and damage to buildings and infrastructure. Tetra Tech is helping our clients to investigate and grout the mine workings, and to monitor the water levels and pressures.
“So normally you expect hydrostatic pressures to be giving a buoy effect, but it’s actually doing the reverse. It’s actually causing these pillars to destabilise, so obviously there is a large frictional component to these coal pillars that’s losing this capacity, just because they are balancing and there’s not enough cohesion to actually counteract that,” explains Simon.
For larger buildings and higher utilised buildings (schools, hospitals etc) it becomes necessary to remediate the mine workings. One remediation strategy is to fully grout mine workings within the zone of influence which can exponentially increase the costs of development projects. For these development projects to remain viable finding ways to reduce remediation costs is essential.
Tetra Tech Coffey has been engaged on numerous projects to develop a mine void grouting strategies. We work closely with clients, developers and most importantly the Subsidence Advisory NSW (formally the Mine Subsidence Board) to carefully design mine void grouting strategies modelling the effect of the overlying strata and then model the failure potential under various loading conditions,” adds Simon.
“Working with the geotechnical and heritage challenges, Newcastle is transforming into a city designed to support a vibrant and growing population with significant redevelopment occurring right across the Local Government area.”
Mapping and surveying – enabling sustainable growth at scale
With a long-established reputation for mine subsidence expertise, Tetra Tech’s Hunter team has been providing geotechnical and environmental services for projects in the area for the past 50 years.
To future proof a city it makes sense for planners to future plan to enable the diverse use of larger retail centres including housing, offices and recreation areas, with jobs closer to homes by boosting city centres in regional areas. The former CBD at the eastern end of Newcastle has several challenges of respecting the heritage of Newcastle with many buildings more than 100 years old, steep tertian causing slope stability issues and shallow unmapped workings from approximately 200 years ago (1800 -1850).
Simon explains that the main opportunity is in the growth of the CBD with the commercial and residential developments. “The planners are pushing to move the CBD to the Newcastle West area, where the Wickham interchange is located, with the demand for high-rise buildings and mixed-use developments”.
Recent examples of this commitment to build include the NSW State government with a 14-story commercial building adjacent to the Wickham interchange.
This area is not without its own challenges with foundations regularly needing to be extended to 40m depth for buildings more than 8 storeys.
Geotechnical challenges to Urban development
The City of Newcastle’s population forecast for 2024 is 174,529 and is estimated to grow to 205,445 by 2046. Simon explains that the demand for high-rise buildings and mixed-use developments in and around Newcastle will only continue to grow, but the geotechnical and environmental challenges faced by these projects, especially mine subsidence, is a big factor which is limiting development in parts of the city.
“Until recently there was funding for grouting of the mine workings for new developments, however with the change in State government this funding has closed to new developments. As such it will be important as ever to ensure that grout is placed efficiently,” adds Simon.
Helping build the future of Newcastle
For Simon, it is the diversity and complexity of the geology and geography of Newcastle.
“As a geotechnical engineer I am constantly challenged by the different coal measures, anticlines, alluvial floodplains, and rock formations that shape the landscape of Newcastle and the surrounding areas,” says Simon.
“Every day is different and at any one time I can be looking at old mining maps to performing different modelling or in the field completing site assessments. Working with our clients at Tetra Tech enables me to utilise all types of technologies and methods to deal with the challenges I face, such as vibrating wire wells, historic research, and numerical modelling of the mine workings.”
“Most of the time I find it rewarding. It’s certainly a challenge, and I face different challenges on different days but that’s what’s interesting about geology and geotechnics in the very unique city of Newcastle” says Simon.
Connect with Simon | [email protected]
