INVESTIGATIONS INTO the funicular railway tunnel fire in Kaprun, Austria that killed 156 people last month have been hampered by the risk of rock falls.
The Austrian army has removed 3t of loose rock to allow investigators access to the scene.
Temperatures are thought to have reached 1000degreesC in the blaze.
'The mica schist has been burnt badly, [up to] 50mm deep and at faults and joints up to 350mm, ' said Salzburg regional government geologist Gerald Valentin.
Fire broke out on the morning of the 11 November, 600m into the 3.29km tunnel at the ski resort, as a train began its ascent to the slopes at Kitsteinhorn.
The blaze intensified within minutes, fuelled by air sucked into the lower end of the steeply inclined 3.6m diameter single bore tunnel, despite the train being made of fire-resistant materials.
Its cause was still unknown as Ground Engineering went to press.
Lubricants used on the track and the cable used to pull the train are being examined, as is mechanical friction.
Although smoking is not allowed on the train, it was not being ruled out as a cause.
Flammable equipment such as gas canisters and fuel is only transported at night, when no passengers are on board.
When the funicular railway opened in 1974 it was one of the first of its kind to serve a ski resort. The tunnel, with an average 42.8% gradient, was excavated in 'good rock' by tunnel boring machine and has no structural lining. Rock anchors and bolts were used where needed.
Trains approach the tunnel on a 600m long steeply inclined viaduct before being pulled up by two electric motors at the top of the tunnel. They run on a single track through most of the tunnel with a passing loop at the midpoint, where the 638m horizontal escape tunnel is located.
Each train carried only two fire extinguishers and the tunnel has no fire doors at either end nor a sprinkler system.
All but 11 passengers perished in the disaster. Survivors fought their way downhill through the fire at the back of the train to the bottom entrance of the tunnel. About 60 people were overcome by smoke and poisonous fumes within 60m of the train as they tried to reach the escape shaft 900m up the tunnel.
The driver of the down train also died, trapped on the passing loop. Three people near the tunnel entrance at the top of the mountain died through smoke inhalation.