ABSTRACT
Various forms of hard erratics may be present in the overburden of lignite mines, such as layers of sandstone, quartzite horizons, and concentrations of boulders. Their presence requires that a number of special measures are taken to maintain the efficiency of mining. These may include one or more of the following: changes to the digging tools; modifications to the existing mining equipment; selection of specialized mining equipment; or modifications of the chosen mining methods. This paper describes the measures implemented in two mines. The first one contains lenses of hard sandstone in the overburden, and the other one contains thick bedded quartzite horizons. Furthermore, a method is proposed to assess the influence that concentrations of boulders have on the efficiency of mining.
