Report: APP CMHS Project 1

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3.1.9. Water Control

Current Technologies

Two main inrush sources exist in Australian underground coal mines: inrush from mining under water bodies such as lakes, creeks, sea, etc. and inrush from mining adjacent flooded mine workings. Both situations have caused incidents resulting in loss of life or assets. The management of both hazards is typically by a detailed site hydrogeological assessment and strict adherence to surveying standards and strata measurement with a range of advance drilling options for final verification. The Wardell Guidelines (
) detail the requirements for mining under surface water bodies.

In seam, mines are required to leave a defined barrier of coal between possible or known water bearing strata and current mine workings to reduce the potential for inrush. This must be supplemented with flanking drill holes to ensure adequate separation and to detect any unsurveyed workings encroaching.

Typically monitoring entails measuring water inflows and discharge from the mine – increasing trends may indicate a possible water source. The final monitoring step is that of physical monitoring by personnel to detect any increase in water inflow that may indicate elevated water levels in close proximity.

In the recent years, increased groundwater inflows have been experienced due to mining induced changes in the surrounding aquifers. This has involved a complex coupled process between strata and groundwater during mining. The strata in a coal mine environment are generally well bedded, having a significant impact on load-deformation characteristics. Mining may induce shearing and separation along the bedding planes, which may result in bending and subsequent fracturing of the rock layer. This may substantially change the in-situ fluid flow properties of the rock mass, such as permeability and porosity. A proper coupling of mine induced deformation, fluid flow properties and the process of fluid flow itself is a prerequisite for reliable prediction of rock mass deformation, water and gas flow into a coal mine.

Currently a number of commercial numerical codes are available independently for stress-deformation analysis and water flow simulation. Commercial numerical codes such as FLAC/UDEC have combined stress changes and water flow, but need excessive run times and therefore limit their usage.

To address these shortcomings, a fully-coupled code that can accurately capture the coupled process of 3D strata-deformation and fluid flow called COSFLOW was developed at CSIRO in collaboration with New Energy and Industrial Technology Development Organisation (NEDO) and Japan Coal Energy Centre (JCOAL) in 2003 (Guo et al, 2002). The code provides an effective approach to predicting and assessing the complex interaction between fracture, deformation and water flow during mining.

A project undertaken by CSIRO in 2008 aimed at investigating the impact of underground longwall mining on hydrogeology using COSFLOW. In that study, detailed water inflow simulations were carried out to predict the mine water inflow at Springvale Colliery located in NSW. The COSFLOW results obtained in this study matched well with the mine inflow measurements to date. COSFLOW has been applied to study the mine water inflow issues in a number of mines in Australia. In those studies COSFLOW has been found to accurately predict water inflow into mines.

Application Sites

An example of successful site hydrogeological assessment and mining induced groundwater impact prediction and management can be found at the Springvale mine in New South Wales.

Technology Gaps/Needs

The remote identification of underground water bodies (water filled mines and old workings) will be beneficial to Australian mines as they continue to encroach on previously mined areas. This is particularly relevant in the Newcastle and Southern coalfields, but will eventually extend to other currently less developed areas.

Ideally there is a need for remote mapping technology to further verify the existence or otherwise of unmapped water bodies or mine workings. Technologies such as seismic and Radio Imaging Method (RIM) surveying may provide some assistance in this regard.

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