ABSTRACT: Subsidence may be observed above partial extraction shallow room-and-pillar mines where the coal seam(s) is associated with weak and thick claystone in the floor. This paper presents results of subsidence studies at such a mine. During the 26-month study period, a time dependent subsidence causing a maximum vertical movement of 50 mm was observed. After a subsidence rate of 6.0-6.5 mm/month for the first two months after development of mine workings, the rate decreased to 0.8-0.9 mm/month after 17 months. However, presumably due to settlement of barrier pillars, it increased therafter to 1.7 mm/month for the next 9 months. Angle of draw, based on zero subsidence, was calculated to be about 29 degrees. Maximum tensile strain and compressive strain values observed were 0.040 pct and 0.039 pct respectively. A hypothesis was formulated to correlate surface subsidence movements with observed underground movements. The data collected to date fits reasonably well within the formulated hypothesis.
1 INTRODUCTION
Trough or sag type surface subsidence may be observed above partial extraction room-and-pillar mines, especially at shallow depths and where the coal seam/s is associated with relatively weak and thick claystone in the floor. An overall extraction ratio of about 50 pct is normally achieved in such a mining system; the remaining coal is left in the form of pillars to support overburden strata. Studies of the characteristics of such subsidence are sparse in the literature. A knowledge of such subsidence is important from a mining as well as an agricultural industry's point of view. It may change land slopes, soil characteristics, agricultural productivity and may cause damage to surface structures. This paper presents results to date of such a study at a mine in the midwestern United States. An attempt has also been made in the study to correlate surface subsidence movements and observed underground pillar settlements as a function of time.
2 MINE DESCRIPTION AND AREA GEOLOGY
The mine extracts a relatively flat 1.5 - 2.0 m thick coal seam at a depth varying from 80 - 100 m. Coal is extracted in panels approximately 1150 m x 250 m (Figure 1) with 5 m wide openings and pillars varying from 17-25 m in different parts of the mine. Barrier pillars of about 54 m width are left between two adjoining panels. The extraction in a panel generally varies 40-45 pct. Roof to floor closure, primarily due to floor heave, is commonly observed in the mine at a long-term rate of approximately 0.5 cm/month. Peak closure rates of 7-10 cm/month are observed immediately after mine openings are developed. The surface topography in the mine area is flat to gently rolling, with a relief of about 6 m. Total thickness of the glacial material over the panel is about 40-45 m while the thickness of the rock overburden over the coal seam is approximately 35 m. The rock overburden primarily consists of shales (65-70 pct), limestones and sandstones. The average overburden thickness over the study panel is 75-80 m. The immediate roof stratum consists of a weak thin band of shale varying in thickness from 0.3 - 2.5 m (average 1.0 m).
