Analysis of Roof Caving Characteristics at a Coal Mine by Using Full Scale 3D Numerical Modeling

Abstract

Regular and efficient caving of roof strata behind is essential in maintaining a trouble free operation in underground longwall mining when especially mined area is left for caving. As the face advances, roof strata should be regularly caved forming a goaf as homogenous as possible. In case of having an uncaved roof behind the coal face, load on the face increases dramatically leading to serious fall of roof conditions. Therefore it is of paramout importance to have the roof regularly caved behind a longwall face. This paper presents the problems encountered at a coal mine in Turkey due to high face pressures and subsequent flow of roof at the face roof junction. The height of the fully mechanised longwall face is 4.5 m. Sliding of face coal and later the fall of roof strata in front of the shields created serious stability and safety problems in the mine. Stopes opened in the roof had to be filled by usign forepoling, foam and concrete. This rescue operation had to be completed securely before starting of the cutting operation at the longwall face. Obviously rate of production of the longwall face has been severly declined during this period. There were a couple of reasons for having such a difficult condition in the mine. The longwall panel was located near to a syncline axis leading to high tectonic stresses. There were lots of small faults through the working face. Longwall face was extremely loaded by a very strong limestone layer having a thickness of up to 80 m located at 120 m above the coal seam. Although the strata between the limestone layer and the coal seam has a readily caving characteristics, the limestone caved at long intervals causing high face pressures due to its cantilever beam effect. Moreover during caving of the limestone, severe dynamic loads are experienced in the vicinity of longwall face deteriorating stability conditions. Therefore it was decided to model the effect of limestone layer’s behaviour by means of numerical modelling. A full scale model was created in accordance with all geometrical conditions and operational parameters by using FLAC3D software. The face advance is also simulated on the model. Stress and deformation state of the coal face, surrounding rock and especially the problematic limestone layer are analysed. To solve the problem, a blasting pattern is selected to weaken the limestone layer by using drill holes opened from the surface. This paper presents the numerical modelling results in relation to selection of the best blasthole geometry to decrease loading on the face and hence maintain a safe, efficient and stable longwall operation.