It’s quite a sight when you’re stuck in traffic on the N1. The Gautrain viaduct curves gracefully on top of its giant pillars and sometimes, just sometimes, you spot one of the trains on their runs, flying by overhead.
But while all we see is curving bridge and giant pillars, there’s a whole lot going on underneath the surface, hidden from view.
And it all comes down to dolomite, a mineral composed of magnesium and calcium carbonate. Rocks made up of dolomite are called dolomitic rock, and it’s this rock that runs below a chunk of the Gautrain route.
Simple enough, right? Wrong.
“Dolomite is soluble – it dissolves in water,” explains Gautrain geotechnical and tunnels manager Ron Tluczek. “As water seeps in between the cracks in the rock, it erodes them away.”
The cracks become wider and wider, blocks of rock fall in, creating cavities, and then – without warning – the bridge above the cavity collapses, forming a sinkhole.
And it’s a risk that runs for a 15km stretch along the line, says Tluczek: from the Mint just after the Midrand station, over Centurion, and up to the military area, just before Eeufees Road.
So how do you deal with it?
“If you built the line like a road, a sinkhole occurring anywhere along the line would wipe the whole line out,” says Tluczek.
Instead, the engineers decided to take the train off the ground and raise it up a notch.
Tluczek points to the “footings”, the foundations underneath each of the pillars holding up the viaduct.
Each footing is designed to suit the geography at that spot. For every level of sinkhole risk, there’s footing designed to eliminate it.
So a sinkhole can occur anywhere along the line with no effect. - The Star