Formation of benches:

Formation of benches:

The overburden and mineral deposit can be extracted by formation of benches or by keeping the surface sloped so that the angle of slope does not exceed 45° from the horizontal. A bench has two elements, the floor and the face (high wall). The width of the floor should not be less than the height of the face (high wall) and the heights of benches are as follows:—

1.  Loose material: In alluvium, morrum, loose earth, etc. which is likely to slide the bench height should not exceed 1.5 m. During the rainy season, there is possibility of land slide of the loose debris and it is desirable to keep the width of the floor much larger than the height of the bench.

2.  Coal: The height of coal bench, i.e. the coal face, should not exceed 3 metres.

3.  Sand stone and hard rock: Benches in sand stone and hard rocks are rarely vertical but generally sloping at a small angle with the vertical. The mining regulations do not stipulate definite bench heights in hard rocks except that the bench width has to be more than the bench height. In manual quarry the bench height is usually 3 metres to 4.5. metres and in mechanised quarries, more than 5.5 metres and depends upon the height of the boom of shovel above the bench floor. Suitable bench height for a 2 m³ shovel is 6 to 8 m and for a 3.5 m³ shovel, about 12m. The slope of the high wall is usually 20° off vertical and depends upon the travel of the bucket during loading. The width of bench floors in mechanised quarries is usually 15 metres and preferably more for movement of dumpers, tractor loaders and other equipment.

Gradients of roads in quarries for tyred vehicular traffic should not exceed 1 in 10.