Ground mechanics in longwall mining should consider surface and subsurface deformations as well as stresses and displacements in the vicinity of the longwall face as they impact face and mine stability. In the past, most studies [1] [2] [3] emphasized either surface deformations or in-mine stability studies. In this research, both surface deformations and in-mine stability studies are conducted, and its objectives are to study: 1) subsidence characteristics, including time effects; 2) stress and deformation changes in chain pillars as a function of time and face location; 3) roof, pillar and floor deformations in entries as a function of time and face location; and 4) relationships between the surface subsidence and underground strata behavior.


The mine instrumented extracts the Herrin (No. 6) coal seam in southern Illinois at an average depth of 198 m (650 ft) from the surface. The thickness of the coal in this area varies from 2.3 m (7.5 ft) to 3 m (10 ft). A detailed lithologic log in this area was given by Chugh et al. [4]. The chain pillars are designed on 36.6 m (120 ft) x 18.3 m (60) ft centers, using a three- entry system with 4.7 m (15.5 ft) wide entries. The longwall face is 293 m (960 ft) wide and 2,043 m (6,700 ft) long in the east-west direction (Figure 1).

The instrumentation chain pillars were selected about 220 m (720 ft) away from the panel's termination point and about 305 m (1000 ft) away from the retreating longwall. The surface and underground instrumentation sites are shown in Figures 1. Surface instrumentation consisted of subsidence monitoring for vertical and horizontal displacements. The underground instrumentation consisted of measuring roof-floor convergence, lateral pillar deformations, pillar loads and roof sag.

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