Bringing science to subsidence

MINE subsidence research in the Southern Coalfields of New South Wales has established that UK subsidence standards are inappropriate for Australian conditions.

Mine subsidence research in the Southern Coalfields of New South Wales has found that UK methods used to assess the potential impacts of mine subsidence on building structures are inappropriate for Australian conditions.

The testing is being conducted as part of an Australian Coal Association Research Program (ACARP) project, headed by Mine Subsidence Engineering Consultants (MSEC) director Arthur Waddington.

Mine subsidence requirements have changed dramatically for mining companies in the last few years, Waddington told Australian Mining.

“Regulatory authorities have increased the extent of assessments that are required prior to giving approval,” he said.

The Mine Subsidence Board polices the Mine Subsidence Compensation Act in coal mining areas of New South Wales.

The Government’s push to deliver on environmental issues, says Waddington, has made the mining approval process more complex.

Government departments have input into mining approvals for good reason, according to Mine Subsidence Board chief executive Greg Cole-Clark.

“It is important that a comprehensive assessment to determine any effects on environmental features and surface structures is undertaken before mining,” Cole-Clark told Australian Mining.

Approval

The approval process generally limits mining, says Waddington, so that the level of damage to buildings is not excessive.

“However, there is already adequate protection for building owners under the Mine Subsidence Compensation Act,” he said.

“Any person whose building is damaged by mining has the right to have that building repaired or be compensated for the damage that has occurred.”

Cole-Clark says the Board’s surface development guidelines require damage to be limited so structures remain safe, serviceable and economically repairable, and impact on the property owner is limited.

“These definitions are relatively broad, and the Board uses the relevant codes and standards to assess the classification of damage,” he said.

The definition as to what constitutes an acceptable level of subsidence is too subjective, according to Waddington.

“To apply definitions on such a rigid basis is wrong, and there ought to be more science used to determine whether mining should be allowed to proceed beneath buildings,” he said.

The Board supports R&D to broaden knowledge of mine subsidence and review guidelines.

“While research may lead to improvements based on scientific analysis, it is important that the risk of damage to structures is maintained at acceptable levels so any damage can be readily repaired,” Cole-Clark said.

“The Board undertakes R&D to improve knowledge of mine subsidence and is a partner with MSEC and Tahmoor Colliery in the Australian Coal Association Research Project (ACARP).”

Damage assessment

Buildings affected by subsidence are currently assessed by the level of tilt on the structure and the extent of cracking in the walls of the building.

“It has been generally established that for buildings to remain safe, serviceable and repairable, tilts should not exceed 7 mm/m and that damage due to cracking in walls should not exceed category two as defined in AS 2870,” Waddington said.

Waddington says tilt is not a factor that can be simply defined in any building.

“Most buildings comprise a number of different units, and some units of the structure may tilt in different directions to others,” he said.

To adopt a limiting maximum value of tilt of 7 mm/m on houses affected by undermining is not very scientific, according to Waddington.

Tilts are only one component considered during a mining application with strain and curvature responsible for damage, according to Cole-Clark.

“Localised areas of a dwelling may have higher levels of tilt that exceed 10 mm/m and the dwelling performs for the purpose of which was intended. However, where residual tilts of greater that 7mm/m exist across the whole structure the effect may be readily observable to residents and the amenity of the building may be affected,” he said.

Research

Waddington hopes his research will inform the approval process prior to mining, and clarify appropriate levels of damage from mine subsidence.

“Mines must look very carefully at the methods used to assess damage, and take into account the findings of this research,” Waddington said.

“The methods that have been used in the past are based on the type of building structures found in the UK. Most houses undermined in the UK were not of flexible construction, says Waddington, and were not very forgiving when subjected to mine subsidence movements,” he said.

Cole-Clark says there is a strong Australian database of information relating to mine subsidence.

“As with other international data, the UK information only provides a base for comparison,” he said.

Detailed measured data for the research was collected at Tahmoor Colliery, and the findings were supplemented with archived Mine Subsidence Board records from the Newcastle and Southern Coalfields. Mining was allowed to proceed beneath 1500 homes at Tahmoor.

Arthur Waddington

Mine Subsidence Engineering Consultants

enquiries@minesubsidence.com

www.minesubsidence.com

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