Recent exploration of Indonesian government coal concessions in South Sumatra by Shell Mijnbouw N.V. has revealed small-scale surface instability of recent origin, which could be important with respect to the construction of large spoil dumps.
The area of interest lies 145 km south-west of Palembang where Mio-Pliocene coals of the Muara Enim formation outcrop in country around Tanjung Enim. These coal seams and the related coal-bearing strata of claystones, siltstones, and sandstones are gently folded and faulted having dips in the range of 5°-30°. In places, the sediments are intruded by andesites which have locally upgraded the coal to anthracite or semi-anthracite, although away from the intrusions the coals have a semi-lignitic or sub-bituminous aspect.
Irrespective of the degree of alteration, the coals are frequently more competent than the associated strata and outcrop as the cap rocks to escarpments and waterfalls. Preliminary sampling of three coal seams in one such setting took place along the valley of the Air Muara Tiga. A series of trenches up to 90 m long were dug down the valley slopes to depths of 3–6 m. The location of the coal seams, a generalized geological sequence, and the positions of the trenches referred to in this note (A and B) are shown in Fig. 1. The side slopes to this strike valley are convex with gradients increasing downwards from 3° to 45°, although much of the slope is inclined at 12° –16°. The local vegetation consists of secondary jungle, giving way to tropical grasses (alang