Widely regarded as one of the biggest contributions to underground mine safety, roof bolting provided the most rapid technology change in the whole history of the mechanisation of mining in the United States, Mark said.
"By the late 1950's possibly three quarters of production were done on roof bolts but when you look at the statistics, the highest rate of roof fall fatalities on an exposure basis was in 1960," Mark said.
"In fact, during the 1950s, during the time roof bolts were introduced and gaining widespread acceptance, risks to the individual miner increased."
Mark believes this was a result of complacency and industry perceptions of what constituted an acceptable risk of roof falls to coal miners.
The crucial turning point occurred in 1968 after the Farmington mine disaster which lead to the introduction of the Federal Mine Health and Safety Act. Thereafter it became a requirement for all mines to have a roof control plan which in practice has meant a maximum of 5 foot bolt spacings. Interestingly, going back to the 1920s safety experts had been advocating using patterned support, Mark said.
"It was realised you need a roof control plan that requires a minimal amount of support everywhere and you can put more in if you need it but you don't leave it to the judgement of the individual at the face."
Mark said he had found it surprising that fatality rates dropped dramatically even before the law was fully implemented after 1970 and the reason for this appears to be because the industry self-regulated.
"In other words the industry had the capability, it had the technology and it had the knowledge to dramatically bring down roof bolt fatality rates before it actually did. But it only did so when the goalposts were moved, when the culture about what was acceptable risk changed."
The risk of roof fall injury today comes mainly from small chunks of rock falling between bolts. A bolt spacing of 1.5m is generally seen as just tight enough to prevent fatal roof falls. Mark believes 2m spacings are too wide while 1.2m spacings are most commonly used in US mines. Fatalities caused by roof falls are uncommon, but the US industry experiences hundreds of injuries every year related to roof falls .
"The conclusion you can reach is that you could use more mesh in certain conditions," Mark said.
Australian coal mines are characterised by higher levels of support, Mark said, for three main reasons.
Firstly, there is overall more weak roof in Australia than there is in the US. Secondly, almost all the mines are longwall which means more can be spent to support gateroads because the cost is dispersed over a larger tonnage than is the case with room and pillar operations.
"The third thing is cultural. There's a lower tolerance for less stable roof in Australia. Current regulatory environment too - duty of care - has changed the emphasis. In the US if you're following your Roof Control Plan you're not entirely off the hook if something goes wrong, but mine managers may perceive they are more off the hook than in Australia.
"You put all those factors together and you have a higher level of support," Mark said.