Aquifer Inflow Prediction above Longwall Panels

Water inflow into coal mines has been a design issue for many years. Guidelines as to the potential for water inflow have been developed in many countries based on local experience and the form of mining being undertaken. In most instances, the guidelines relate to inflows which would endanger underground personnel and operations.

In more recent times, water inflow criteria for mines has been widened to include lesser inflows which may not impact on mine safety or operations, but have the potential to reduce water flow within streams and surface aquifers. For the purpose of this report the larger inflows relating to mining safety are defined as mine inflow and the lesser inflow relating to aquifer water loss as environmental inflow.

The water losses referred to are those which enter the mine from the surface or from near surface aquifers.

The empirical relationships developed as mine design guidelines or regulations typically relate to high flow (mine inflow) situations and in this report these have been reviewed as to their application to environmental inflow. In many cases environmental inflows were unlikely to have been noted or recorded in many mining operations in the past.

The aim of this report was to assess inflows into a mine which occur through the overburden above and adjacent to longwall panels. This study does not address near surface effects such as valley fracturing which may locally redirect near surface flow laterally but not into the mine.

In order to study flow about mines, both flow within in situ strata and that within strata impacted by mining needs to be assessed.

The key finding is that for simple mine geometries the resultant conductivity above longwall panels can be summarised relative to rockhead depth and subsidence. The impact of more complex geometries and multi seam layouts is best done on the basis of specific geometry of the site by computer modelling.

The use of strain as a criteria in this report should be seen as a means of comparing the overseas empirical experience with the conductivity obtained from the modelling. The empirical strain values do not relate to actual measured strain but to the equivalent theoretical strain or systematic strain which is a function of subsidence and depth. Similar criteria can be created from tilt. The experience and that from the modelling indicates that measured strain can be variable over a site due to surface features and this value does not necessarily relate to the theoretical or systematic strain value associated with inflow potential.

The best application of these results is considered to be via the data contained in Figure S1/64 on the basis of rockhead and subsidence. Such data typically relates to single panels or multiple panels at shallow depth where chain pillar compression is not significant. In deeper mines where individual panels are subcritical and chain pillar compression or yield is significant, computer modelling of the overall panel geometry is recommended.