Continuous Compaction and Permeability Evolution in Longwall Gob Materials

Abstract

Understanding the evolution of gob compaction and related gas transport behavior is necessary for the planning and optimization of gas ventilation and control in longwall coal mines. In particular, the detachment of the undermined roof into the gob leaves a loosely compacted perimeter that skirts the longwall panel. This permeable gob perimeter in plan view forms as a result of shear separation from support provided by the solid ribs. This detachment and the resulting rotated and reduced stresses limit compaction, elevate permeability and exert significant control on gas flow during active longwall mining operations. We report gob compaction experiments on in-mine-collected fragmented rock and conduct mechanical compaction on stacked samples that are either uniformly coarsening upwards (case A) or are coarsening upwards, but capped by a segregated upper layer of coarse rock (case B). Observed compaction is linked to a capillary model representing porosity reduction and permeability evolution. As applied uniaxial stress increases from 0 to up to ~ 2000 kPa, the porosity decreases from 0.64 to 0.41(~ 36%) for the uniform stacked material (A) and but only from 0.66 to 0.51 (~ 23%) where the gob is topped with a layer of coarse “roof” rock simulants (B). Particle–particle self-adjustment dominates the compactive behavior at initial low stress and results in significant strain—followed by a linearly elastic region through the remainder of loading. The elastic regime is used to predict the permeability of the loosely compacted gob, considering the redistribution of stresses induced by shear collapse at the rib. Permeability evolution is scaled through the evolving compactive strains and particle size distribution of the fragmented rock, enabling results to be up-scaled to mine scale. These results provide a first rational method for analyzing the interactions between caved gob and the ventilation system towards mitigating gas concentrations and minimizing the hazard.

Appendix 1

In this study, the raw experiment data for validating the proposed model was cited from Pappas and Mark (1993). In addition, the raw data was also organized by Karacan (2010). Pappas and Mark (1993) conducted compaction tests on gob materials collected from one site in a Virginia mine in the Pocahontas Coalbed, and two sites in an eastern Kentucky mine in the Harlan Coalbed. The test results of each simulated gob material were summarized in Table 2. The particle size distribution before and after corresponding loads were shown in Fig. 16 and the fragmentation fractal dimension $D_{\text{F}}$ was calculated based on Eq. (18) and the results were summarized in Table 3. As shown in Fig. 10, the stress–strain curves were monitored and the corresponding displacements were obtained through the compaction experiments. Based on the stress–strain curves and the initial porosities (Table 2), the stress-dependent porosities can be calculated by referring the methods illustrated in Sect. 3.2.

Table 2 Summary of the four tests from Pappas and Mark (1993) and Karacan (2010)

Fig. 16

Particle size distributions of four tests after maximum stress loads (Pappas and Mark 1993; Karacan 2010)

Table 3 Particle size distributions and the associated the fragmentation fractal dimensions of four tests (Pappas and Mark 1993; Karacan 2010)