ABSTRACT
Dahor Formation in Bunati area is Mollase like deposit comprising of rockmass and sediments which deposited during/since Miocene Meratus Mountain uplift. Preliminary geotechnical investigation hasbeen conducted in Bunati Area, South Kalimantan which gave insight at depth more than 20 m its formation comprise of various rockmass such as thin intercalation of siltstone-claystone, claystone, coal and sandstone with basic GSI value range 27 from 43 with its intact rock strength range from 3 to 16 MPa. Its rock mass found have systematic isotrop oriented discontinuity orientation controlled by strike-slip deformation during or post Meratus Uplift. Furthermore, there’re also discontinuities which its orientation tend to be anisotropic controlled by sedimentation, diagenetic and mechanical compaction of overlying rocks and sediments. The interaction between these two types of discontinuities with its respective orientations are very likely to trigger failure mechanisms in the rock mass which results in two types of failure in the form of step-path failure and wedge failure reponse to mechanical excavation of undercutting low wall and side wall pit slope. Finite element method and wedge block failure was utilised in order to recognized instability mechanism involving discontinuities interaction and to asses rock mass slope factor of safety and strength reduction factor utilizing rock mass strength parameters quantification.
