Geotechnical Appraisal of the Thar Open Cut Mining Project

This paper is concerned with a slope stability appraisal of the proposed open cut mining
operations in the Thar lignite field in Sindh, Pakistan. The Thar coalfield covers an area of approximately
9 000 km2 and is estimated to contain 193 billion tonnes of lignite resources. The design of safe high wall
slopes is necessary to ensure mine safety and overall economical viability of the mining operations. In the
Thar lignite field, the presence of three main aquifers induces pore pressure in the rock mass surrounding
the lignite seams and makes high wall slopes potentially unsafe. It is, therefore, necessary to dewater the
rock mass before commencing mining excavations. A proposed mine dewatering scheme to facilitate
rock mass dewatering surrounding the mining excavations and a description of the slope stability analysis
of the high wall using the software “SLIDE” version 5 is outlined. Three computer models with slope
angles of 28o, 29o and 30o, incorporating a plane failure mode, were analyzed to investigate the stability of
pit slopes. The generalized stratigraphy of borehole RE-25 has been used for the development of the
computer models. The main conclusions of this study are that the slope angle of 28o is quite acceptable
for a Stability Factor (SF) ≤ 1.3 whereas the excavated slopes with slope angles ≥ 29o are not safe
against the plane failure for SF>1.3. This assessment was followed by a slope stability analysis
incorporating circular failure modes. Five models incorporating various slope angles ranging from 23o to
27o and one model incorporating combined slope angles of 23o in dune sand and 26o in the rest of the
strata were developed and analysed. The main conclusions from this study are that the dune sand layer
(having a thickness of 48 m) is acceptable for a SF of 1.3 at slope angle ≤ 23o, while the rest of the strata
is acceptable for SF=1.3 at slope angles ≤ 26o. The overburden to lignite extraction ratio for this slope
design has been calculated as 3:1 or 3 m3 of overburden over 1 t of lignite.