Subsidence impact on mine planning

THE New South Wales Government’s requirement for mines to submit a subsidence management plan (SMP) has the potential to impact mine planning as coal companies strive to meet tougher environmental regulations.
Subsidence impact on mine planning Subsidence impact on mine planning Subsidence impact on mine planning Subsidence impact on mine planning Subsidence impact on mine planning


Angie Tomlinson

In March 2004, the NSW Government introduced a revised process for approvals of underground coal mining where surface subsidence may result. Any leaseholder planning to embark on underground coal mining that is likely to result in surface subsidence is required to prepare and submit an SMP for approval.

Since March 2005 mines have been required to submit the plan for first and second workings – covering all types of secondary extraction, such as pillar extraction or quartering, longwall or miniwall mining; first workings that directly support any proposed secondary extraction by longwall or miniwall mining, such as gateroads, installation roadways or bleeder headings and the associated main headings; and any other case where the proposed underground coal mining will potentially lead to subsidence.

The limit, magnitude and nature of subsidence movements are dependent on numerous factors, including geological conditions, surface topography, the distance between the mine workings and the ground surface, and the mine design.

Mine design can be changed to meet the regulations by re-orienting the panels, reducing the width, length and/or height of seam extracted, or leaving larger pillars of unmined coal between extracted areas.

The mine plan can especially be affected when mines look to mine under water. Mining under watercourses on floodplains, or in other relatively flat-lying areas, may result in localised diversion of water flows and possible increases in the incidence of flooding, erosion and other impacts.

Mining under watercourses in steep valleys may result in small areas of ground in valley floors being forced upwards to relieve stress – called “upsidence”. Upsidence can result in a number of short-term impacts. Surface water may permeate into the shallow groundwater system through the resulting open fractures.

This water will usually return to the surface further downstream, but may be chemically altered by minerals in the rock strata. During extended periods of dry weather, the loss of surface water from rivers or creeks may become evident with a noticeable but localised reduction in surface flows.

Altered flows and stream chemistry may have an impact on the life cycle of aquatic species and riparian vegetation.

When operations submit an SMP they must include a proposed mine plan. This plan must take into account areas that may be impacted by mining activity, including areas of environmental, heritage or archaeological sensitivity.

According to the NSW Department of Primary Industries, approvals will be granted only after consideration of all potential impacts on landholders and the environment.

“Should the potential impacts be deemed significant and unacceptable, particularly with regard to adjacent rivers and water supplies, then the proposal will be either modified or refused,” the DPI said.

Once the mine is approved, the operation must continue to monitor subsidence and report to the department.

Monitoring requirements include the comparison of predicted subsidence impacts with actual impacts to assess whether the observed subsidence is consistent with predictions. All significant incidents or variations to predicted subsidence or subsidence impacts must be reported to the DPI within 48 hours of their identification.

In consultation with the SMP committee, the DPI may then require the mining company to increase monitoring, make changes to the mine plan, or immediately stop mining.

The department has considered 16 first-workings SMPs and two full SMPs since the regulations were brought in.

“The review committees were very complimentary about the way that Beltana approached the SMP process and in the thorough manner that they developed their application and addressed stakeholder issues,” DPI said.

“The agencies commended Beltana for the high standard of the application, stating that it will provide a benchmark for industry. Beltana’s efforts in compiling the application were rewarded with the ease in which the application was processed by the respective departmental and interagency committees.”

The department is still looking at finetuning its regulations under a review and in consultation with the Interagency Committee and an industry working group.

“It has been suggested that the requirement for baseline monitoring needs to be strengthened and an improved submission policy is necessary to streamline the requirements for SMP documentation,” DPI said.

“This is being undertaken through improved guidance on agency requirements for monitoring, a decrease in the number of hard copies to be submitted with an increase in electronic submissions on CD, an opportunity for industry to review the recommended approval conditions prior to final consideration, and the possibilities for improved communications between agencies and digital submission of SMP mine data.

“In this, the review will provide industry operational continuity following early identification and resolution of issues and ensuring certainty for future longwall extraction.”

The DPI said these changes to the guidelines will ensure that monitoring requirements are sufficiently detailed to provide the information required to ensure that management of subsidence impacts satisfies Government objectives and causes minimal detrimental impact to surface features.