No.DGMS(Tech.)(S&T) Circular 2

Dhanbad, dated 20/06/2001.

To
All Owner, Agent & Managers of Coal Mines.

Sub: Accident due to failure of slope in an opencast mine.

In one opencast mechanized coal mine, while 16 workers under supervision of two (2) mining sirdars and one (1) overman were working at the bottom of a 15m high coal bench, the overlying benches in sandstone and black cotton soil failed trapping the workers in debris, 10 of whom asphyxiated to death while the remaining six(6) persons escaped with minor injuries.

The immediate combined bench above coal was about 52m high with individual width of only 8-10m, this was overlain by benches in black cotton soil as high as 15m but with width of only 4 - 8m. The coal seam was earlier developed in two sections by underground workings and the coal was also on fire. The benches were formed against a fault plane along a barrier with the adjoining mine.

Inquiry into the accident revealed that if the workings were properly benched and special care taken while working near the fault plane and in coal benches affected by fire as required under statute, the disaster could have been averted.

The accident described above leaves no doubt about the fact that the design and working in opencast mines need to be reviewed seriously. In this particular case design of a safe slope and the method of working practiced left much of desire. It is emphasized here that proper design of benches and ultimate pits based on slope stability considerations is a must for safe working of an opencast mine.

A basic guideline for a scientific approach towards design of open pit slopes is given in the following paragraphs.

A systematic detailed work need to be undertaken for solving or understanding the problems of slope failure in open pit mines. The objective of such type of work could be, (1) increase the knowledge of the behaviour of mine slopes, (2) improve the ability to estimate rock mass strength for such slopes, and (3) develop an improved design methodology for forward design of rock slo0pes in the open pits. The fourth objective could be formulation of a detailed slope monitoring protocol for such slopes.

A geomechanical model needs to be developed based on extensive field work and review of previous studies at the sites. The model may comprise of a detailed description of the geology, joint sets and structures, mechanical properties of intact rock and joints, geohydrological conditions and virgin stress state. From this, a representative design cross-sections and parameter values need to be established which could later be used as input to stability analysis of the pit slopes.

Assessment of representative rock mass strength need to be addressed through the use of appropriate failure criterion in conjunction with rock mass classification. By comparing back-calculated strength available from old case studies a reliable estimate may be established. Failure mechanisms need to be studied preferably by means of numerical modeling. For this purpose the finite difference code FLAC and the distinct element code UDEC can be used. The work must be aimed at investigating failure mechanisms in detail, at the same time developing a reliable modeling technique for the pit slopes.

No discussion about slope stability in open pits can conclude without discussing the effect of groundwater. In the following para a general discussion about groundwater problems have been attempted. This most important parameter must be fully understood before any attempt is made to model or to study the slope stability problems in mines.

Rainfall and subsequent movement of groundwater greatly affect slope stability. The groundwater regime is considered the most changeable natural parameter which affects slope stability in several ways.

- by generating pore pressure, both positive and negative, which alter stress conditions,
- by changing the bulk density of the materials forming the slope,
- by both internal and external erosion, and
- by changing the mineral constituents of the materials forming the slope.

Groundwater in soil may be of two types, occurring above or below the water table (phreatic surface) respectively. Water above the water table may be transient percolation moving downwards to join the phreatic water below the water table or capillary water held above the water table by surface tension. Phreatic water below the water table is subject to gravitational forces and saturates the pore space in the material below the water table.

Usually three water zones below ground surface can be identified, (1) zone of permanent saturation - where pore spaces are always filled with water, (2) zone of intermittent saturation - where pore spaces are filled with water only after heavy rains and (3) zone of non-saturation - where pore spaces are never saturated, though water may pass through. Water table is the upper surface of the zone of permanent saturation. Its level migrates from season to season.

Water flows through soil or rock in different ways depending on the nature of ground material.

Aquifer refers to any water-transmitting soil or rock. Aquifers have various modes of ground water flow, such as homogeneous (inter-granular) flow and path-preferential (either fissure or conduit flow). The latter tends to be more rapid and can therefore significantly influence slope stability.

Flow in soil is more likely to be inter-granular, although fissure or conduit flow can also occur via fissures, pipes and drying cracks. Flow in rock can be inter-granular, but is more common through joints and other discontinuities. It is remarkable that joints in rock may get filled with water during intense rainfall, inducing high hydraulic pressure on the rock mass and therefore adversely impacting stability.

While designing an open pit mine slope, it is essential to consider all the issues discussed above. Moreover, it may be borne in mine that only design is not enough and a comprehensive design, implementation and monitoring protocol needs to be developed by every opencast mine operator in order to achieve safe working conditions in such mines.

Chief Inspector of Mines.