I was wondering about the trucks myself -- if they were caught in the slide I bet the cabs were pretty stinky afterwards!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse
Well, I don't think that big of slides are easy (or maybe even possible) to predict -so, I would say rock bolts probably were never even considered. That kind of metal fence/mesh is made to stop small rocks -the kind that break windshields and dent roofs, not the kind that squash semi trucks.
I would ask for hazard pay if I were driving.