Subsidence Management Plans to provide greater certainty

ONCE fully implemented, the proposal for mine plans in NSW to include Subsidence Management Plans (SMP) before any mining approvals are given, will offer greater certainty to coal miners, writes Ross Seedsman of Seedsman Geotechnics. Seedsman led a subsidence management workshop in Wollongong in February.

Staff Reporter

Until recently, the Mine Subsidence Board provided a de-facto management system for subsidence-related issues in NSW. The board could declare a mine subsidence district, control the nature of surface improvements so that they would be compatible with high extraction mining, and then repair any subsequent damage.

The board’s ability to optimise coal extraction was reduced to some degree by their inability to compensate for business interruption. Over time, the community’s demand for low-cost slab-on-ground dwellings led to the need to adopt mine layouts that kept subsidence to less than 150 mm.

The board provided mine planners with tolerance limits in terms of tilts, curvatures, and strains to which it could design. With some notable exceptions, the measured mine subsidence was within the predicted limits.

In the 1990’s, new and highly productive longwall mines were opened in non-urban areas in both NSW and Queensland. At the same time, the general community became more concerned with environmental issues and in particular with the health of water systems – surface and groundwater.

There have been a number of successful longwall operations conducted under major water supply infrastructure (Cataract Reservoir, Cataract Tunnel). However, a number of unplanned impacts to river systems highlighted the coal industry’s need to better identify and consider the tolerance of natural features to subsidence impacts.

The Commissions of Inquiry into proposed longwalls at Mandalong and Dendrobium changed the way in which subsidence was to be managed.

In 2003, the SMP is seen as the way of improving the way in which subsidence impacts are incorporated into mine planning and also how the impacts are communicated to the community prior to mining.

The new mine subsidence plans require a risk assessment approach to be used for assessing the impact on natural features.

For the foreseeable future, it is unlikely that a formal risk assessment by way of likelihood and consequence will be possible – we simply do not have the case histories that are required for this approach.

The focus will need to be on the acceptability of the consequences but given that we are dealing with natural systems, this raises some important questions:

- How is the potential damage to be rated? What is meant by ‘loss of amenity’

- How can tolerance levels, in particular tilts and strains, be set for natural features?

- How can the normal variations of natural systems be addressed - for examples droughts, or even different vintages in Hunter Valley?

- How can appropriate baselines be defined in acceptable timeframes?

Attendees at the workshop concluded that the consultation requirements in an SMP are possible. It was estimated that one full time employee would be needed in planning stages and up to three full-time employees for operating mine.

Community consultation needs to start as early as possible so that a level of trust is built up. This is particularly important because, in recognising that we are dealing with limited knowledge of natural systems, it is almost inevitable that some of the subsidence impacts will be underestimated.

A recurrent theme was that there is inadequate knowledge of how the geology of the overburden can significantly change the surface subsidence impacts. In particular, the absence of spanning units that are assumed to be present can cause a serious under-prediction.

Currently, there is a large amount of research into shallow horizontal stresses, but this will need to be complemented with studies of bedding and joint structures in the near surface rocks. Likewise, better knowledge of the role of different soil types and soil thicknesses in modifying subsidence impacts is urgently required.

Being able to predict subsidence parameters is only part of subsidence management – the next step is being able to assess the impacts.

This will be challenging in the absence of objective criteria of damage. A cooperative approach with regulators and bodies such as the Sydney Catchment Authority will be required if such criteria are to be set, especially during the transitional phase over the next few years.

How to separate mining impacts from natural cycles such as drought, fire, or even poor seasons leading to poor vintages was a recurrent question from the workshop.

If subsidence impacts are to be reduced by different mine layouts it will be necessary to narrow extraction panels or increase pillar sizes. There is a need for regulators to recognise that the timeframe to change a mine plan can be in the order of 3-5 years if longwall continuity is to be maintained.

A better understanding and utilisation of pillar dimensions may be able to address some of the problems on the South Coast and other deep mining areas.

It would appear that in the middle of a longwall district, ground strains (=cracking) are a function of panel width only and not pillar width. However, wider pillars could reduce the strains that develop at the leading and trailing sides of the district.

When fully implemented SMPs should offer greater certainty to coal miners. The transitional phase will be difficult for the mining operations and for all the government agencies involved, with the major concern being the availability of qualified people on both sides.

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