Slide 10 of 35
Notes:
The most common rockslide trigger is water.
Water in soil pores and rock fractures creates powerful pressures capable of triggering both large and small rockslides and rockfalls.
Water from intense storms (like hurricanes), snow melt, as well as manmade sources are known to have caused innumerable large and catastrophic landslides.
When a location that has been relatively free of rockslide activity for hundreds of years becomes active, with multiple events over a short period of several years, it indicates that there has been a change in the environment that has altered the equilibrium state.
In the absence of vibration and freeze-thaw, and in an environment where water is visibly seeping from fractures in the rockfall source area, then changes in groundwater flow patterns must be considered as a likely trigger.
The drawing above is a generic water pressure diagram from Hoek and Bray’s text entitled Rock Slope Engineering. It illustrates the effects of water pressure forces on a rock block bounded by fractures partially filled with water. Standard rock slope stability analyses indicate that as little as 400 to 800 gallons of water would be sufficient to trigger the Glacier Point types of rockslides if the water finds its way into critical areas behind rock blocks.
