Question

Why do shale, slate, and schist pose engineering hazards?

Answer 1

For the engineering point of view, we require massive rocks (devoid of any fracture or cleavage plane) for good construction, but rocks like shale, slate and schist possess fissility property and foliation planes. So construction over those rocks is not safe than the construction over massive rocks like granite, granodiorite etc.

Shale shows fissility property, slate has slaty cleavage and schist has foliation planes. So if any construction will be done along the foliation plane direction then it is very unsafe, whereas perpendicular to the foliation plane/slaty cleavage direction is a little bit safer then the constitutions along that plane. But overall these regions are not that much stable/rigid for any massive construction. That's why they can make severe engineering hazards.