Results from more than 3,100 underground observations by Deutsche Montan Technologie (DMT) have resulted in a base for making practical recommendations and for selecting the most effective design of shield support.

This was the topic of the paper “Longwall Roof Control Prediction and Shield Support Recommendations” by DMT’s Ulrich Langosch, Ulrich Ruppel and Holger Witthaus at the 22nd International Conference on Ground Control in Mining.

The paper stemmed from moves by the German mining industry in the 1990’s to introduce a new generation of shield supports. The new design of support had a maximum load capacity of 10,000 kN, making these units as strong as the shields used in Australia and in the USA.

DMT made numerous observations in order to verify the roof fall frequency by statistical analysis.

“The results of this work have led to practical recommendations for roof control and the required shield support system on longwall faces,” the paper said.

The researchers correlated the information made during observations into Rock Mass Classification, to stress calculations and to the angle between the direction of the fissures and the direction of longwall mining. The analysis work yielded two sets of results.

Firstly, they found there was a critical distance between the canopy tip and the coal face (TF) related directly to the thickness of the first roof layer and its uniaxial compressive strength.

“The face support should have a TF that is less than the TF in every underground longwall situation. Exceeding the TF can immediately result in a roof fall,” it said.

Secondly, using the obtained regression equation DMT they were able to calculate the probability of the roof fall frequency (FF), which describes the roof fall sensitivity. Roof fall frequency relates to the measured support resistance of the shield support, the calculated vertical stress, the fissure-direction index (DI = angle between main fissure direction and direction of mining) and the distance by which TF is exceeded.

“Armed with these results DMT is now able to predict the critical distance between the tip of the canopy and the coal face (TF), as well as the roof fall frequency, for all shield designs.”

By applying the new calculation method DMT are now able to compare alternative longwall layouts and different shield support types under pre-set geological conditions.