Mine Subsidence
There is no mine subsidence map in Newcastle Environment Plan (2012) since it is not administered by City of Newcastle. Mine Subsidence Districts are administered by Subsidence Advisory NSW under the Coal Mine Subsidence Compensation Act 2017 to help protect homes and other structures from potential mine subsidence damage.
Newcastle Mine Subsidence District
Much of the inhabited parts of Newcastle are in the Newcastle Mine Subsidence District.
“Mine Subsidence Districts are proclaimed in areas where there are potential subsidence risks from active or non-active underground coal mining.
A district is a land zoning tool administered by Subsidence Advisory NSW under the Coal Mine Subsidence Compensation Act 2017 to help protect homes and other structures from potential mine subsidence damage.
Building or subdividing within a district
Subsidence Advisory regulates building and subdivision works within districts to ensure new homes and structures are built to an appropriate standard that reduces the risk of damage should subsidence occur.
All development within a district is required to meet Subsidence Advisory’s approval requirements. Building and subdivision applications must be submitted and approved prior to commencing work.”
The following map shows a closeup of the Newcastle Mine Subsidence District around Wickham. See History/Ferndale Colliery for information about the Ferndale Colliery north of Throsby Street, and examples of subsidence occurring 30 years after its closure.
Work done by Subsidence Advisory NSW has “determined that the risk of subsidence in parts of Wickham is much more extensive than previously anticipated” (Wickham Masterplan 2021 Update (2022), p.3). This meant Wickham Master Plan 2017 had to be updated and hence Wickham Masterplan 2021 Update(2022) was issued. The red area in the following map is where localised mine grouting is considered unviable and the entire mine area will most likely need to be bulk grouted, which is outside the financial viability for a single development to fund.
The impact of this is best described in the Wickham Masterplan 2021 Update (2022). It reads:
“Mine Subsidence
The anticipated yields in the WMP were based on the understanding that old mine workings could feasibly be remediated comparable to other parts of the Newcastle City Centre, due to the cost of remediation beyond the site area being reimbursed at successful completion through the Newcastle Mine Grouting Fund (NMGF), administered by Hunter Central Coast Development Corporation.
However, Subsidence Advisory NSW (SA NSW) have since advised City of Newcastle of the outcomes of investigative works carried out as part of preparing the Newcastle Central Business District (CBD) Mine Subsidence Risk Model, to identify the level of remediation works required across the Newcastle city centre to enable redevelopment. SA NSW have determined that the risk of subsidence in parts of Wickham is much more extensive than previously anticipated.
Furthermore, it identified that a bulk grouting solution is needed to remedy the undermined area of Wickham to ensure adequate stability. Bulk grouting refers to filling the underground voids with a material (usually consisting of fly ash and cement) but where this flows well beyond a site area before a stable mass is formed. Hence, bulk grouting is more expensive than where targeted ‘strategic grouting’ is possible.
Despite the NMGF, the upfront cost, the burden of risk, and the long timeframes before funds are reimbursed, means redevelopment in undermined areas is prohibitive for individual proposals. Without an alternate means of funding or facilitating remediation to allow redevelopment over the current two storey limit set by SA NSW, it is estimated that redevelopment yields will equate to a shortfall of 340 dwellings out of the total 1200 extra dwellings envisaged by the adopted WMP.
While this loss is significant at a local scale, the capacity for redevelopment across the city centre is sufficient to ensure housing supply forecast and targets are still met without the need for additional density being provided on land not impacted by subsidence from the adopted WMP.
City of Newcastle will continue to liaise with State agencies on alternate funding arrangements or recoupment mechanisms that better facilitate the bulk grouting identified by SANSW”
(Wickham Master Plan 2021 Update 2022, pp. 3-4)
Subsidence Advisory NSW has set development guidelines to help property owners building within a mine subsidence district. The guidelines provide a streamlined approval process that allows complying development options, that are approved within seven days. For example, the map below shows in red the sites in Wickham that are assigned Surface Development Guideline 2. The guidelines range from 1 to 8, with 1 being the most restrictive and 8 having virtually no restrictions. On the map below, areas in the yellow mine subsidence district that are not shown red are either Surface Development Guideline 7 or 8. Subsidence Advisory NSW provides a step-by-step guide for development applications within Mine Subsidence Districts; see Development Approval Process. This page also shows how to find the Surface Development Guideline that is assigned to a property.
Surface Development Guideline 2 defines allowable residential construction as:
“Allowable residential construction …
Guideline 2 allows for up to two residential buildings that conform with the following description:
Single or two storey brick veneer residential developments erected on reinforced concrete footings/slab to comply with AS 2870-2011. These improvements are limited to a maximum length of 24 metres and maximum footprint size of 400m².
- The buildings are to be designed and constructed in accordance with the current editions of AS1684, AS 2870, AS3600, AS3700, AS4773, the National Construction Code, and applicable Australian Standards. Site classification “P” for Mine Subsidence Site is to be ignored for these structures.
- Masonry is to be articulated, in accordance with the current editions of Australian Standards AS3700 and AS4773.
- Both slab on ground and lightweight bearers and joists are permitted.
Refer to Surface Development Guideline 2 for prohibited works, other allowable additions and improvement, and exempt developments.
Applications for proposed improvements that do not comply with the Guideline assigned to the property may be assessed under Subsidence Advisory’s Merit Assessment Policy. For example, the 45 m high building at 73 Railway Lane behind the Lass O’gowrie Hotel requires a merit assessment. Approval by Subsidence Advisory NSW under section 22 of the Coal Mine Subsidence Compensation Act 2017 was obtained in July 2019. This approval is subject to a number of general terms of approval. For example, prior to construction geotechnical investigations are required to confirm the ground conditions, and an engineering impact statement is required. On-going monitoring is also required during and post-construction.