ABSTRACT

The paper deals with geotechnical study for optimum slope design of Tirap Colliery, Assam, India, with special reference to steep southern highwall slopes. Geotechnical mapping was done on the exposed benches of the quarry. The different geo-mechanical properties of lithological units were determined. Initially kinematic analysis was done to determine the types of failure in the slopes with respect to the measured discontinuities in the field. The failure analysis was done by GALENA software. An assessment of the engineering and structural geology, strength properties and the related geotechnical controls indicated that the 216m high ultimate southern slopes of the colliery may have an overall slope angle of 31°.

INTRODUCTION

A geotechnical study for optimum slope design of existing steep southern highwall slopes at Tirap Colliery was conducted. The colliery is part of North Eastern Coalfields Ltd. and situated in the Assam state of India. It was also aimed to know the influence of Water on the safety factor by sensitivity analysis, which tells the importance of the parameter in the slope. A more justified and suitable remedial measure Can be planned for any critical slope after sensitivity analysis.

The rock discontinuities were mapped at the exposed benches of the pit as per the norms of International Society of Rock Mechanics (ISRM 1978). Geotechnical mapping was undertaken to determine critical orientation of structural discontinuities.

After identifying kinematically possible failure Modes, detailed slope stability analysis is carried out by limit equilibrium method. Sensitivity analysis was done to determine the most effective remedial measure for any critical slope.

The area experiences a very high annual precipitation ranging from 300 to 425 cm. Although monsoon extends from April to September, the area practically experiences rainfall for about 8 to 9 months.

GEOLOGY

The area in general is covered by a thin cover of colluviums material/soil of varying thickness from less than 1m to 6 m. The general sequence of strata overlying ‘60-ft’ seam in the area from top to bottom is as follows:

  • Soil

  • Fine grained sandstone

  • Intercalated shale and sandstone

  • Intercalated shale, mudstone, grey to black carbonaceous shale with coal bands

  • Coal (‘60-ft’ seam)

  • Clay (floor)

The immediate roof and floor of the ‘60-ft’ seam are characterised by argillaceous formations. The coal core recovery was generally less than 55% because of the inherent soft and powdery nature of the coal.

GEO-MECHANICAL PROPERTIES

The different geo-mechanical properties of lithological units were conducted at the Rock and Soil Mechanics Laboratories of CMRI. The cohesion and friction angle of the topsoil was found to be 160 kPa and 200 respectively. The average density of slope materials is listed in Table 1.

(Table in full paper)

GEOTECHNICAL MAPPING

Geotechnical mapping was done on the exposed benches of the quarry. The mapping was done as per the norms of ISRM (1978). The general structural pattern in the rock mass is presented in Table 2.

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