Developing in a mine subsidence district

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Developing in a district

Subsidence Advisory NSW regulates development within mine subsidence districts (districts) to help protect homes and other structures from potential mine subsidence damage.

If you are planning to build or subdivide within a district, you must lodge a development application and obtain approval from Subsidence Advisory NSW prior to commencing work.

Subsidence Advisory NSW may place conditions on any development within a district to reduce the risk of potential mine subsidence damage. Conditions may include requirements related to the nature and class of any development, the size, height and location of new structures, and the use of certain building materials and construction methods. In most cases, Subsidence Advisory NSW's requirements for standard residential development are consistent with the Australian Building Code and do not result in increased construction costs.

Development applications can be lodged directly with Subsidence Advisory NSW or as an Integrated Development through your local Council.

Development guidelines

Subsidence Advisory NSW has set development guidelines to assist landowners that want to build or subdivide within a district.  

The guidelines set out the requirements for building on or subdividing a property based on potential mine subsidence risks. Different guidelines may apply to different areas within a district.

Subsidence Advisory NSW’s guidelines may include requirements related to the nature and class of any development on a property, the size, height and location of new structures, and the use of certain building materials and construction methods.

Subsidence Advisory NSW aims to process applications for proposed development that complies with the guidelines within five working days. Non-compliant applications will be considered on merit.

Assessment on merit

Development applications that do not comply with the mine subsidence development guidelines for the property are considered on merit.

Subsidence Advisory NSW may approve a development application with conditions to better protect the proposed structure from potential subsidence damage.

In some situations, you may be required to provide more information to support your application, for example, the results of desktop geotechnical studies and borehole investigations.

Large and complex development

If you are planning a large or significant development, you should contact Subsidence Advisory NSW for advice in the preliminary planning stages of the project.

Buildings and other structures can be designed to better respond to ground movement caused by mine subsidence. Our specialist engineers can provide information and advice on mine subsidence design requirements.

Some large and complex developments over old inactive underground mine workings have significant risks. Applicants may be required to eliminate the risk of subsidence by grouting the abandoned mine workings.

Designing for mine subsidence

The general advice on designing for different subsidence parameters, provided below, is aimed at developers and technical experts. It is not comprehensive but gives guidance on the issues involved in development on subsidence-affected land.

Design for vertical subsidence

Vertical subsidence can result in buildings sinking to a lower level. This is generally only an issue where they are located in areas prone to flooding. If you’re designing for vertical subsidence, contact the relevant local Council during the preliminary planning stages to find out whether a building is in a flood zone. Services such as water, sewerage and drainage may require additional care in design and detailing.

Design for horizontal strain

Mine subsidence can cause movement known as horizontal strain where the length of the ground surface between two points is changed. Increases are known as tensile strains and reductions are called compressive strains. The movements between the two points are usually small except where there are unusual topographical features such as steep slopes or gorges.

Both tensile and compressive strains can cause damage to structures, however, different design considerations can reduce the impacts.

Ground strains are generally transferred into footing systems by friction beneath and beside the footing elements. This friction can be reduced by making footings as shallow as possible and placing them on slip layers, which separate the footing structure from the soil and allow the ground to move without damaging the structure.

Where deep foundations are unavoidable, the transfer of ground strains can be reduced by excavating trenches around the structure. These trenches are placed as close as practical to, and extend to just below the underside of, the foundation. They can be backfilled with compressible material which is strong enough to support the sides of the excavation but more compressible than the natural soil. Material, such as coke, slabs of expanded polystyrene foam, vermiculite, cork and void formers, will be crushed and absorb the strain, rather than transfer all of the forces to the foundation.

Various techniques can be used to allow footings to slip relative to surrounding foundation material. The sides and bottoms of footings and slabs are kept as smooth as practical and are often poured on slip layers that incorporate plastic or bituminous membranes over layers of granular materials (sands). Exaggerated slopes are used on transition zones between stiffening beams and slabs to facilitate shearing actions.

The use of concrete slab on ground footing systems is now close to 90% in NSW; in recent years, the waffle raft system has emerged as the preferred reinforced concrete slab footing system. This system is ideal as a mechanism to isolate the superstructure from horizontal ground strains.

Design for curvature

Curvature results from differential settlement across the ground surface. It is considered as having the most damaging impacts of mine subsidence parameters on buildings. Curvature is normally defined by the deflection ratio or the radius of curvature. In practice, damage from mine subsidence will often be a result of the combination of curvature and ground strains.

The effects of ground curvature on a building can be reduced by panelling and articulating walls to move without developing strains or cracks or causing doors and windows to jam. Vertical articulation joints are provided at appropriate intervals and at sections where the wall stiffness changes. Damage due to curvature can also be reduced by eliminating brickwork above windows, doorways and arches. If such details are included, special attention must be paid to the provision of bond seams and strengthening panels that incorporate arches.

Design for tilt

A tilt caused by active underground mining refers to a change in the slope of the land surface between two points. It is generally the result of different levels of subsidence between locations and can be temporary or permanent.

Structures are generally only adversely affected by tilt if they remain in a significant permanent tilt once subsidence is complete. This normally occurs when a structure is located on or near the edge of a subsidence trough. Tilt can cause doors and windows to jam.

Small tilts generally don’t affect the usage of a building and can be allowed for in-building design by providing generous falls for services. Tilts over 7 mm/m can affect the serviceability of buildings. In this circumstance, Subsidence Advisory NSW may place conditions on the type of construction to ensure structures are safe, serviceable and repairable.

Suspended flooring systems can be relevelled economically where access is available to the supporting bearers and joists.

If sufficient ceiling height has been provided in building designs, and if appropriate detailing has been adopted, it may be possible to relevel floor slabs by adding a topping layer to recover original grades.

Domestic floor slabs are not normally strong enough to withstand relevelling by jacking. Other types of slabs may be designed with jacking points and sufficient strength to be relevelled after subsidence.

Designing for pothole subsidence

Potholes are holes in the ground surface resulting from subsidence. They usually occur in areas where old inactive mine workings are very shallow. Most potholes are relatively small. However, Subsidence Advisory NSW’s design parameters allow for potholes of up to five metres in diameter.

Subsidence Advisory NSW’s standard surface development guidelines for building in areas with pothole subsidence risks are single storey timber or steel frame clad with weatherboard or other similar materials.

Single storey brick veneer or two storey lightweight buildings can be constructed subject to certain design conditions.

Integrated development

Section 91 of the Environmental Planning and Assessment Act 1979, specifies that all development within a district (excluding exempt development) is considered ‘integrated development’.

Under Clause 66 of the Environmental Planning and Assessment Act 1979, Councils are required to refer all integrated development to Subsidence Advisory NSW for approval.

Deemed approvals

Subsidence Advisory NSW offers deemed approval for some minor construction works. Click here for a list of the items that do not require Subsidence Advisory NSW approval.