What is mine subsidence?
Learn about mine subsidence and how it occurs.
To ensure community safety, it is important that people living in active or non-active mining areas are aware of mine subsidence and immediately report any subsidence safety issues to Subsidence Advisory NSW.
About 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.
Most active underground mines in NSW 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 gradually, over a large area. Subsidence occurs a short time after longwall mining has taken place. In most cases, the ground completely settles within 2 to 5 years following the extraction of coal, meaning there is minimal risk of further subsidence.
Generally, longwall mining in NSW occurs at more than 200 metres beneath the ground. As a result, there is no risk of pothole subsidence in these areas.
Historically, underground mines throughout NSW extracted coal through bord and pillar mining methods. Bord and pillar mining occurred all over NSW but was mainly used 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. Many homes are built over old non-active 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 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.
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.
If a property is damaged as a result of mine subsidence in NSW, the property owner can lodge a claim for compensation with Subsidence Advisory.
Types of damage
Mine subsidence cracks can range from hairline to significant splits in building materials. Cracks on the inside of structures are often located near cornices and door and window frames. Externally, subsidence can cause brickwork to crack in a diagonal or step pattern.
A subsidence hole (or pothole) is a hole in the surface of the ground that can appear because of subsidence in non-active mining areas. These holes are generally localised and only affect a single property. Subsidence holes vary in size but they can all be dangerous. It is important to keep a safe distance as the ground around subsidence holes may be unstable and can easily collapse.
A strain due to subsidence is a change in the length of the ground surface between 2 points. An increase in the length of the land between 2 points is a tensile strain and a reduction is a compressive strain. Tensile strains can cause cracks in brickwork and internal walls and separation of joints in paving. Compressive strains can cause brickwork to break apart, door and window openings to shrink, and pipes and paving to buckle.
A tilt caused by active underground mining refers to a change in the slope of the land surface between 2 points. It is generally the result of different levels of subsidence between locations. Tilts can be temporary or permanent depending on their location in relation to the workings. Structures are generally only affected by tilt if they remain at 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 affect the operation of doors and windows, causing jamming.