The paper discusses preliminary study on the effect of underground mining on large open pit slope stability in Mae Moh coalfield in Thailand. Mae Moh mine is currently operated as open pit mine and it is the largest open pit coal mine in Thailand. In the near future, however, the final pit limit of the mine will be reached and underground mining will be developed from the final highwall in the depth of 400–600 from the surface. However, due to the adverse geological and geotechnical conditions, such as weak mechanical properties of rock, extra-thickness (20–30 m) of coal seams, and huge final pit slope, various studies for underground mine are being carried out at the present. In this study, it was investigated for appropriate distance between open pit wall and underground mine, response of pit slope in different extent of rib pillars, which is measured horizontally minimum distance from the open pit wall to underground mine, and panels, and different mining height, and the interaction between open pit slope and underground working. The investigation was conducted by means of numerical techniques using three dimensional finite difference code FLAC3D.

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