ICGCM Papers:
Pillar Design and Global Stability
 
 
Roof-Pillar Interface Affecting the Pillar Behavior
35th International Conference on Ground Control in Mining
Roof-Pillar Interface Affecting the Pillar Behavior
by
Andre Cezar Zingano, Universidade Federal Do Rio Grande Do Sul, Porto Alegre, Brazil
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Pillar Design and Global Stability
[Author] Andre Cezar Zingano, Universidade Federal Do Rio Grande Do Sul, Porto Alegre, Brazil
[Abstract] 
Key Conclusions:
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this work demonstrated that the cause of pillar yield was the low strength interface represented by the thin clay layer, and the inclination of the coal seam. Both causes acted together, because no pillar fail where the seam dip is lower. The higher number of fracture and faults could facilitate the rib displacement, but it was the cause of pillar failure. This thin clay layer was not observed during drilling and core description, because it clay is washed by the drilling water. It turns important alternative investigation methods, like tele viewer system of the borehole wall and geophysics log. The numerical models represented the effect of the interface on the final pillar strength and behavior, which could count on the empirical methods.
Key Findings:
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this study started with intense monitoring of the roof-floor convergence, the measure of the position of the centers of the intersections, and the real pillar size. The objective was to determine where the shear movement and its orientation. Also direct shear tests were conducted to determine the thin clay layer mechanical properties, and the geomechanics characterization to determine the rock mass properties of the other layers and coal seam. The coal seam strength is 3.6MPa, based on the rock mechanics characterization. The safety factor calculated to the 11x11m pillar was 1.8 or higher, based on the ARMPS approach. For pillars fail due to low strength, the coal seam strength should be less than 2.0MPa, based on the back analysis of pillar design. However, the inclination of coal seam could change the pillar load. The vertical pressure splits in two loads: one normal to the coal pillar, and the other parallel to the seam orientation. Numerical models in two and three dimension had been built to simulate the low strength interface and coal seam inclination. The simulations demonstrated that both the, the low strength interface layer and the coal seam strength were the causes of the pillar yielding. The load on the pillars is very small to cause pillar failure. In others section at the same mine where it was found the clay layer no pillar yield was observed, because the seam dip is low (less than 5°). To reduce the pillars yielding and avoid pillar failure the ribs of the pillars were bolted and meshed. After rib reinforcement, the entire section will be backfilled with pumped material from the preparation plant. The objective of the backfill is to increase the confinement on the pillar ribs and reduce the void space that the pillars could deform.
Objective of the Paper:
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the objective of this paper is to determine the cause of the pillars yielding, and study the effect of the clay layer at the interface between the roof and pillar, including the inclination of the coal seam.
Problem Statement:
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: the pillar strength and behavior will depend on the coal seam strength and the vertical pressure caused by the overburden thickness on the coal seam, and this vertical pressure is based on the tributary area. The effect of the interface shear strength on the final coal pillar strength had been studied since 60’s. However, few case studies had been reported, and special in shallow underground coal mining (less than 100m). This interface could be represented by a thin clay layer or an abrupt contact between the coal seam and a rigid sandstone layer of the immediate roof. These two interface conditions are found in the coal mines in Brazil. The effect of the interface shear strength could increase its effect when the inclination of the coal seam is higher. The effect of the interface strength has been observed in a production section of a coal mining, which mines the Barro Branco (White Mud coal seam). The depth of the coal seam is 60m in average, and 2.3m seam high. This particular section is 12-14% (7-8°) of seam inclination. The coal pillars started to yield at the rib, including rib spalling. These rib yielding and spalling were noted mainly on the pillar ribs at the direction of the inclination. The first idea was the pillars are yielding because they have low safety factor due to the low coal seam strength. The cause of this low seam strength is the high amount of fractures and faults that cut the coal seam and roof. However, the pillars had been designed using the criteria of width to high ratio larger than 4 and the safety factor higher than 1.5 (based on the ARMPS methodology). The pillars are the dimension of 11x11m, and more. At this panel, it was noted a thin clay layer (picture) exactly at the interface between the siltstone layer of the immediate roof and the coal pillar. Also, it was observed a relative movement between the immediate roof and pillars (picture).