Mine subsidence

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What is mine subsidence

Mine subsidence is the movement of the ground that can occur after underground coal mining. After coal is extracted from beneath the ground, the land above can sink and fill the hollow mine workings. This can cause tilts and strains on the ground surface.

Subsidence caused by old inactive mines in New South Wales is generally localised and only affects a small area. In rare instances, collapses in the old mine workings result in holes in the ground surface known as ‘potholes’.

Most underground coal mines operating in NSW today use longwall mining methods. Subsidence from longwall mining occurs within two to five years following the extraction of coal. This means there are no long-term risks of damage from subsidence. Due to the depth required for longwall mining in NSW there is no risk of potholes forming in these areas.

Subsidence from active underground mining

Most active underground mines in New South Wales use longwall mining methods. Longwall mining involves the extraction of entire sections of coal using high powered machinery. As the machinery moves ahead, the mined-out area falls in, causing the ground above to subside.

Subsidence from longwall mining generally occurs over a large area. Subsidence occurs a short time after longwall mining has taken place and, in most cases, the ground completely settles within three years of mining meaning there is minimal risk of further subsidence after this time.

Generally, longwall mining in New South Wales occurs at depths of more than 200 metres beneath the ground. As a result, there is no risk of pothole subsidence in these areas.

Subsidence from inactive underground mining

Historically, underground mines throughout New South Wales extracted coal through bord and pillar mining methods. Bord and pillar mining occurred all over New South Wales but was predominant in the Hunter and Newcastle regions.

Bord and pillar mine workings were often shallow but left supporting structures after mining. Coal extraction may have occurred over 150 years ago. There are many homes built over old inactive mines that may never be impacted by subsidence as the old mine workings are considered stable.

Occasionally, parts of the old mine workings collapse resulting in subsidence. A range of factors, including water inundation, may cause these workings to deteriorate over a long time. Subsidence caused by collapses of these old mine workings is generally localised and typically only affects a single property.

Signs of mine subsidence damage

The signs of mine subsidence damage to buildings and other structures can range from cracking to walls and jammed doors to more significant structural issues. Generally, buildings damaged by mine subsidence remain safe and can be used until they are repaired.

If you are concerned about the safety or serviceability of your property, you should report it to Subsidence Advisory NSW’s 24 Hour Emergency Hotline on 1800 248 083.

The extent of mine subsidence damage can vary depending on the location of a building in proximity to the mine workings. It is possible for the ground directly above collapsed mine workings to subside without causing damage to above buildings; this may occur when the whole building subsides at the same rate.

Similarly, it is possible for a building that is not directly mined beneath to experience mine subsidence damage. Buildings on or near the edge of subsided ground may experience damage as a result of tilts and strains in the ground surface.

It’s important to be aware that damage to homes and structures could be caused by factors other than mine subsidence such as reactive soils, the roots of nearby trees and poor building practice.