Traditional methods the B.C. government uses to prevent rock slides wouldn't have stopped the rock tumble on Tuesday night that blocked the Sea to Sky Highway, says Simon Fraser University geologist John Clague.
The bolts and netting that engineers use to hold unstable rock in place or minimize the effect of a slide wouldn't have prevented the 16,000 cubic metres of boulders from falling.
"The failure occurred deep enough in the slope that I don't think a bolting program would have prevented it - it was just too big," he said.
The stretch of highway near the Porteau Bluffs where Tuesday's disaster occurred is the perfect storm of rock-slide-prone areas: The narrow road winds along steep-walled cliffs that lean heavily toward the highway, and in many cases overhang it where they have been undercut to widen the road. The rock is solid but riddled with faults that deepen over time until they give way and send huge chunks plunging toward the sea - with the highway directly in their path.
Rock slides are common in that area, but only four have been big enough to block the highway in the past half-century, Prof. Clague said. In the 1960s, a falling rock killed a motorist within a kilometre of this week's slide.
Regular inspection for any kind of movement can help authorities predict when a piece of cliff is likely to come free. Drilling holes to allow water to drain can also help prevent rocks from weakening.
In recent years, highway authorities have used everything from wide nets designed to keep rocks from tumbling onto the road, to metal bolts anchoring unstable chunks of rock to the cliff.
But even those measures couldn't have prevented a slide of this size, Prof. Clague said.
Although slides that big are rare, the government should prepare for them because, as the highway is more heavily used, it's inevitable that at some point, people will be injured or killed in such a slide.
The only sure way to prevent rock slides from wreaking havoc on the highway is to tunnel through the hill rather than building a road around it, so that even the most major rock slide won't affect the road. Prof. Clague said costs have ruled out this option before, but added that to make the highway safe, there isn't any other choice.
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Planning ahead
-Engineers have many ways at their disposal to secure rock faces and prevent rock slides from occurring.
-Unstable material is removed from upper part of slope
-Concrete dowel to stop slab sliding off
-Tensioned rock anchors secure planes of weakness
-Overhangs blasted away
-Dense concrete used to stop fractured rock disintegrating
-Concrete buttress supports rock above cavity
-Trees removed to stop roots breaking up rock
-Drain holes drilled to reduce water pressure within slope
-Fault zones reinforced
-Ditch to catch smaller debris
-Road
NINIAN CARTER/THE GLOBE AND MAIL
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