Annual Report,2009 - Directorate General of Mines Safety

DGMS Annual Report, 2009
as substitute and is invariably used in the industry including domestic uses. Cement is
equally important material to be used along with the steel in industries and
infrastructure fields.
Roof Bolts are now being used as common support in underground mines all over the
world along with the cement or resin grout to reinforce the strata and provide adequate
support in the excavation. Since past ten to fifteen years, cement grouted roof bolts
have almost replaced the timber supports in coal mines in India.
It is highly desired that the roof bolts once installed should immediately start taking
load of the roof strata and prevent dilation so as to eliminate chances of roof fall.
Quick Setting Cement Grout was the only option to provide required support
resistance. Various composition were tried by adding quick setting chemicals and
other re-agents in the cement composition. It was then possible to gain pull out load of
about 3 Tonnes within 30 minuters and 5 Tonnes within 2 hours of installation. The
Resin Grouts were also available for use in mines but it was cost prohibitive and
uneconomical for Indian conditions during nineties. With the advancement of
technologies and faster rates of extraction, resin capsule is now being invariably used
and are now economically viable in case of mass production.
However, the use of cement capsules would continue for medium and small scale
productions. A number of technical circulars have been issued from this Directorate
regarding load requirements, installation, testing and use of roof bolts from time to
time.
Technical Circular No.6 of 1996, Circular No. 3 and 6 of 1993 are highly elaborative
and exclusive on the subject. In spite of the best effort put in both from the mine
operators as well as from this Directorate, the results are not upto the expected level.
2.0 NECESSITY
It would be pertinent to mention here the trends of fatal accidents in coal mines
especially due to fall of roof and sides. The accidents data analyzed for the past fifteen
to twenty years during which the full column grouted roof bolts along with cement
capsules were being used, have revealed that :-
(i)
(ii)
(iii)
38 to 40% of the fatal in coal mines are caused due to fall of roof and sides;
57 to 60% of the total below ground accidents are caused due to fall of roof
and sides; and
58 to 60% of the roof fall accidents occur within 10 m of the face i.e. within
the Freshly Exposed Roof Area.
A number of accidents due to fall of roof have occurred within 2 to 4 hours of
exposure of the roof where cement grouted roof bolts were used that failed and fell
along with the failing roof of thickness varying from 0.25 to 1.2m. In many cases
during the course of inspection and enquiry, it has been found that the roof bolts
which were expected to take 3 Tonnes of anchorage load after 30 minutes of
installation were pulled out within 1.6 to 2.4 Tonnes. The roof Bolts which were
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