Mine blast: How the Pike River rescue will work

Workers at the entrance of the Pike River mine. File photo / NZ Herald

Rescuers will face a number of challenges as they work towards accessing the area where 29 coal miners are believed to be trapped.

The space where the men are trapped is not a vertical shaft like the one where the Chilean miners were trapped.

The Pike River mine is accessed by a tunnel which enters the side of the hill at a slight incline, about 2 to 2.5 kilometers in length. The rest of the mine then burrows up inside the hill.

From the top of the hill there is a ventilation shaft where fresh air is being sucked into the main mine, however the miners will come out the main access tunnel.

"The vertical distance [from the surface] is only 120 meters, but that is not the way [the miners] would look to get out of the mine," said Pike River CEO Peter Whittall.

Rescue procedure

Outlining the rescue procedure, Pike River chairman John Dow said tests still need to be done to ensure the air mixture in the mine is not explosive.

"The composition of air mixture needs to be evaluated to make sure there's no risk of a second explosion," Mr Dow said.

It was hoped these tests could be done by air, however helicopters cannot land at the entrance to the mine because of the thick cloud.

This means tests may have to be done by officials on the ground who have to traverse rough terrain to get to the top of the shaft.

Mr Dow said it will take at least two to three hours to get to the vent.

From there they will need to collect samples from deep down the ventilation shaft before officials can be certain it is safe for a team to move in, Mr Dow said.

Experts and equipment have been flown in from Australia to assist with the gathering and testing of air samples.
Mr Dow said the rescue team is waiting at the mine enterance to move in.

When they get the all clear, rescuers will enter through the main tunnel on foot, as there is already a vehicle in the shaft which could block traffic.

Once rescue staff enter the mine, Mr Dow says they will first need to establish a fresh air base for themselves, which is a safe point where they know they will be able to breathe safely.

"From there [the team] can foray into other parts of the mine to search and remove our employees and friends that are in there," Mr Whittall said.

Volatile methane
Methane goes hand in hand with coal as the two are formed together, the methane trapped in coal seams or the surrounding rock strata. How much methane exists depends to some extent on the geological pressures, but as mining activity takes place, the pressure is reduced and methane gas can be released, as this explanation of methane in the mining context shows:

"In underground mining, methane is released into the mine workings during mining. Mining regulations require methane to be diluted in the ventilation air, and then vented to the atmosphere. Mines can also remove methane before and during mining by using degasification systems. The gas can be vented, flared (not currently done in the U.S.), or recovered for its energy content. Emissions are reduced if recovered gas is flared or used."

How dangerous is methane? Well, that depends on what concentration it is present in. The US Occupational Safety and Health Administration explains further:

[Methane is] an odourless substance that is nontoxic and is harmless at some concentrations. Methane, however, can displace all or part of the atmosphere in a confined space(1); and the hazards presented by such displacement can vary greatly, depending on the degree of displacement. With only 10 percent displacement, methane produces an atmosphere which, while adequate for respiration, can explode violently.

A lot of science and research has gone into estimating how much methane is likely to exist in different kinds of mines, coming up with ways of extracting it and evolving technology to detect dangerous levels of methane in mine shafts.

Ventilation shafts are common features of underground mines and Pike River was employing ventilation as a safety measure at its mine. However, it is a tricky business estimating and managing methane levels in mine shafts, as faults in rock can become conduits for methane from other geographically removed sources. This paper outlines the potential for "unforeseen mine gas emissions in quantities sufficient to create hazardous conditions".

Coal dust and the "explosion pentagon"

The other potentially explosive hazard in coal mines is a buildup of coal dust. This hazard is as old as mining itself and has been responsible for countless deaths since the 19th century. Working at coal seams with industrial equipment throws up a large amount of dust, which if not properly extracted or appropriately dispersed can explode. OSHA again explains:

In addition to the familiar fire triangle of oxygen, heat, and fuel (the dust), dispersion of dust particles in sufficient quantity and concentration can cause rapid combustion known as a deflagration.

If the event is confined by an enclosure such as a building, room, vessel, or process equipment, the resulting pressure rise may cause an explosion. These five factors (oxygen, heat, fuel, dispersion, and confinement) are known as the "Dust Explosion Pentagon". If one element of the pentagon is missing, an explosion cannot occur.

In the confinement of a coal mine shaft, the conditions for coal dust combustion are obviously quite favourable.