CAUSE VERSUS TRIGGER IN ROCKFALLS AND ROCKSLIDES: IMPLICATIONS IN CASES OF PROPERTY DAMAGE AND PERSONAL INJURY

Both modern construction and leisure recreational activities can encroach upon areas containing obvious as well as hidden geologic challenges. Among the geologic challenges in some locations is the potential for rockslides and rockfalls. They result in significant incidents of property damage, personal injuries, and fatalities each year. Several case histories are presented here, including Sacred Heart Falls in Hawaii, Natural Bridge in Virginia, and Yosemite National Park in California. Often misunderstood are the relationships between the geologic causes and the immediate triggering mechanism for any particular event.

The causes of rockslides and rockfalls are tied to the geologic setting and structural conditions at the site. These include steep rock walls, zones of weak rock, adversely oriented joints, fractures, and faults. Over time, the normal processes of weathering and erosion weaken the geologic structures bringing those having unfavorable orientations closer and closer to failure. Eventually, the rock mass reaches a point near equilibrium where the driving forces (gravity) are just barely balanced by the resisting forces (rock friction and cohesion).

Once the barely stable condition near equilibrium is reached, changes in environmental conditions or major events can provide triggering mechanisms capable of pushing the rock mass beyond equilibrium leading to rockslides and rockfalls.

Well known triggering mechanisms around the world include: excessive vibration; freeze-thaw; and, changes in groundwater flow patterns that create excess water pressure. It is rare for rocks to fall without a triggering mechanism although the mechanisms are sometimes unrecognized at the time.

Case histories are explored to demonstrate the concept of cause versus trigger in different settings and to examine the potential legal aspects of various triggering mechanisms.