Rozwój osuwiska 22S w najgłębszej części kopalni Bełchatów

The open-cast mine at Bełchatów is one of the deepest in the Central Europe. Landslides are there the main geotechnical hazard. The principal factors causing landslides in the mine are: complicated tectonic setting in the Kleszczów Graben, high lithological variability and the considerable depth of the excavation (over 200m). Landslide 22S is one of the biggest in the mine. It is situated at the SE corner of the so-called "second-order graben", where seam exploitation reaches 280m in depth. In this corner there are a set of faults forming a step-like pattern and alluvial sandy cone splitting the brown coal seam. The result is that layers are inclined to the open-cast central axis. There is a clay layer near the bottom of the brown coal seam inclined in the same direction Analyses of these factors enabled the landslide hazard zone to be established. In September 2003 slope movement monitoring began. During the first year slow horizontal displacement was recorded (1.4-5.0mm/day). In July 2004 the tempo of the displacement doubled and one of the inclinometers was truncated at the depths ca 20m. A simulating calculation showed that in order to prevent a slide a widening of a bench ca 400m is necessary, and in consequence restrict brown coal exploitation. It was decided not to widen a bench, but to intensify the monitoring and control process. On 17.02.2005 the speed of displacement rate exceeded 30mm/day. From the experiences with earlier landslides such a value was regarded as critical. The landslide hazard zone was closed and equipment was evacuated. On 2.03.2005 the tempo displacement attained 66mm/day (the alarm value of 50mm/day was crossed). On 15.03.2005 rapid mass movement occurred and the landslide 22S was formed; more exactly it was end of first stage of its development. The volume of the landslide was estimated to be 650 000m3. Protection works were undertaken to restrain landslide progress. However an increase in the depth of an excavation may result in the possibility of a landslide, and an intensive monitoring of a displacement in the hazard zone was ordered. During the first half-year after the first stage of the landslide 22S, no displacement was recorded. In July 2005 initiation of movement and fissures formation were observed. Per analogiam to the investigations of the development of the earlier landslides in the mine, a reactivation of the landslide 22S was expected at the end of 2005. On 4.12.2005 the second stage of the landslide occurred. After two stages the landslide volume reached ca 1 million m3. Site-specific geological conditions and the great depth of the Bełchatów mine excavation result in the landslide development having some peculiar features, and an investigation of them provided important pointers for prognosis hazard of the slope instability.