INTRODUCTION

In India, about 50 per cent of the total accidents are attributed to roof and side a Is, the proportion of fatalities on account Of roof fall and that due to side fall being almost equal. Statistics reveal that nearly 50 per cent of the total accidents have taken place by fall of roof rocks of up to 30 cm thickness and in majority of eases the area of the fallen roof rock was less than 10 sq.m.(Prasad 1975). It is generally, therefore, the immediate 30–50 cm of roof which needs close attention to ensure safety against roof falls. In this paper characteristic of weak roofs associated with the mining of some coal seams in the Raniganj Coalfield of India are given and the new technique of securing Such roofs are described.

COAL MEASURE STRATA

The main coalfields of India are found in the Damodar Valley and the areas adjoining and lie roughly east and west along the 24°N parallel. They belong to the Lower Gondwana period, which is subdivided in ascending order in three series: Talchir, Damodar and Panchet (Table 1). The Talch-irs lie unconformably on Archaean rocks of Precambrian age.

Most of the coalfields of India have a number of coal seams. For example, Jharia coalfield has more than 25 workable coal seams, the Raniganj coalfield has seven main coal horizons in the Barakar coal measures and nine in the Raniganj coal measures and some 30 coal seams have been traced in Bokaro coalfield. These seams occur in quick succession, the parting at places being very thin; all are generally banded the bands being from a few milli-metres to several centimetres in thickness.

(Table in full paper)

The thicker seams have as a rule, more bands. Balls or nodules and rounded fragments of coal are not uncommon. Pyritic intrusions are also very common.

The roof and floor of the seams are sandstone or shale. Fireclay as a rule is absent; if present, it is overclay. The strata consist mainly of sandstones and shales, there being various types of gradation into one another, e.g. sandy shale, shaly sandstone, etc. The percentage of sandstone in the strata is usually high (especially in the Raniganj coal measures) and may reach 88% in certain localities. Table 2 shows the relative proportions of different rock types in the Raniganj coal measures. Sandstones form massive deposits and are at times difficult to cave but they form good roof. Shale roofs on the other hand are weak and invariably present roof failure risks.

SITES OF INVESTIGATIONS

The present study was done in three mines, Dhemo Main, Jamuria 7 & 8 pits and Ranipur Colliery, all in Raniganj coal measures. Roof associated with Raghunathbati seam at Dhemo Main, Taltore seam roof at Jamuria 7 & 8 pits and Dishergarh seam roof at Ranipur Colliery were the subjects of study. In the Raghunathbati seam the workings 194.15m deep lay 156.66m over the goaf of Dishergarh seam.

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