It has been well known that the stability of "permanent" slopes in the open pit coal mines in Indonesia does not remain constant over long periods of time. This has been mainly attributed to the weathering process that intensively occurs in the tropical regions. It is therefore appropriate to carry out an investigation of shear strength characteristics on rock samples of the predominant coal bearing strata such as claystone and mudstone which are obtained from a number of open pit coal mines in Indonesia. The investigation includes the behaviour of long term strength. The research reveals that the long term strength of those two kinds of rock is significantly lower than their peak strengths. It is also found that the Burger reology model does fit to the rehology of these rock samples.
Aseries of studies on time dependent shear strength behaviour at the Department of Mining Engineering (DME) Institute Technology Bandung (ITB) Indonesia was initiated by the occurrences of considerable slope failures at MuaraTiga Besar open pit coal mine ofTanjung Enim PT. BukitAsam (PTBA) in South Sumatra and at Melawan west open pit coal mine of PT. Kaltim Prima Coal (PT. KPC) in Sangatta, East Kalimantan. The interesting part of the former one was that despite the bedding planes were obviously oriented against the slope face direction, significant failure of slopes still occurred.The cause of the failure was much involved with intact material characteristics of claystone rather than developing along a plane of weakness, and this type of failure being common in highly weathered geo-materials. It was recorded that the failure area was poured with heavy rains for a period of about a week prior to the slope failures. In addition, subsequent to heavy rain for more than 4 consecutive days a huge low wall slope failure took place at night shift at the Melawan pit. The failed materials were mudstone sitting on top of a coal seam. The failure was previously believed attributed to geological structure. On the contrary, a thorough research carried out not long after the failure indicated that this slope failurewas convinced primarly due to a deterioration process of the geomaterials constituting the slope and geometry rather than that of weak geological structure. Laboratroy test confirmed that the rock materials in this particular area was classified as weak materials (UCS less than 7.5MPa).
In general, there are three interesting geology phenomena of Indonesia as indicated by the fact that about 80–90% of Indonesian land is covered by quarter sediment resulted from volcanic activities, as well as trass alluvial, and soils resulted from rock weathering. Second, the rock formation in Indonesia is very young, and third there are many tectonic activities occurring in Indonesia. These types of geo-materials do not seem quite dense and consequently the porosity is relatively high.
