886429 Some considerations regarding the stability of underground workings which are to be mined by opencast methods : Morris, R; Fourie, G A Proc International Conference of the South African Institute of Mining and Metallurgy: Planning and Operation of Openpit and Strip Mines, Pretoria, 9–13 April 1984P71–77. Publ Johannesburg: South African Institute of Mining and Metallurgy, 1986