Stabilizing Undercut Slopes Using Counterweight Balance - The Case Study of Mae Moh Coal mine of Thailand

The purpose of this research is to investigate the applicability of counterweight balance on stabilizing undercut slopes with potential plane failure in open pit mines. For this purpose، the phenomenon of arch action in geomaterial and undercut slopes was reviewed briefly. It was confirmed that by excavation at the toe of an undercut slope، the load transfers from the central undercut part to sides to produce some areas of load concentration. Therefore، making stronger the load concentrated areas beside the undercut part، by counterweight balance، leads to more stable undercut slopes. Because the main focus of this research is on open mines where always a huge amount of waste rock is available which can be used as counterweight balance، the technique of counterweight balance is suggested as an appropriate technique for stabilizing undercut walls. Some advantages of using counterweight balance are reducing the risk of slope failure during operation as well as reducing the transportation cost of waste rock. The proposed technique was applied to the Mae-Moh lignite mine of Thailand as a case study. The existence of a shearing zone at the interface between under-burden layers at the northeast area made some slope stability problems from 1990 to 1996. In 1998 occurrence of a huge slope failure in this wall leads to a mining operation suspension in the area for about 10 years. Because of the huge size of the unstable area none of the common stabilizing methods such as anchoring or grouting were economically applicable. Finally in 2010 the technique of counterweight balance was accepted by authorities and applied to the unstable area through 2011 to 2013 successfully.