Testing of roof

In many situations the roof of mine roadways and faces is required to be systematically supported at regular distances apart, whether or not the places have been tested and found to require supports. Where such systematic support is not considered essential, it is the duty of mine official to test regularly and systematically the places where workmen have to pass or work. This testing is usually done.

(1) Visually with the help of a strong light,

(2) By hearing the sound of the roof and side or a prop-set when tapped with a stick, and

(3) By feeling the vibrations when the roof is tapped with a stick.

Visual test aims at detecting any cracks in the roof or sides or any signs of weighting on supports such as bending of lids or burr of the tapered end of the existing prop, water percolation from roof, or increased flow of water percolation. These indicate the need for additional supports.

A good roof or side gives a ringing sound when tapped with a stick; dull or drummy sound indicates the need for supports. When testing the place, the official should stand to the rise side of the place being tested as loose chunks of rock of bad roof may give way when tapped. The roof above gallery height of 2.5 m may be tested with a bamboo with its testing end shod with iron. Caution is necessary when testing the shale roof, which emits a ringing sound even when the roof is bad, if the thickness of the shale bed is grater than 0.3 m. A prop, if already erected, may be tapped to get an indication of weight on it; a ringing sound indicates good condition but a dull and heavy sound gives a warning of bad roof condition, requiring additional supports.  Fig. 9.6 Vibration   test for   high roof,

Vibration test is done by experienced men by resting the palm of the hand against the roof when tapping it with stick and feeling the vibrations, which are different for bad and good roof conditions. This is possible for height not exceeding 2.2 m.

Visual test is carried out if the roof is upto 5 m high by a strong focussed beam of light from a torch or an electric lamp. For vibration test a bamboo end is kept pressed on the roof by one worker. A second worker taps the roof by another bamboo and vibrations are felt on the first. (Fig. 9.6)