23 March 2013
The role of joints in controlling rockfalls
Posted by Dave Petley
In Iceland we flew over the spectacular Gullfoss waterfall. This is a wonderful site, although it was pretty cold, as the frozen spray around the waterfall demonstrates:
Just downstream is an amazing outcrop of columnar basalt in the gorge wall . This cannot be easily viewed from the ground (it is on the wrong side), but from the air one gets a pretty good view:
As well as being a genuinely increadible rock outcrop, this is a very nice illustration of the control that joints play in controlling rockfalls. Let’s zoom into a part of the image above:
To the right of the columns is a section of heavily-jointed rock. Here (relatively) small pieces can detach – and indeed below this section of cliff there are some blocks sitting on a ledge. The result is a face that is comparatively planar. The columnar basalt.itself yields rockfalls much less easily. To detach, a block needs to topple, or to fracture, both of which are quite hard to achieve. The result is a rockface that, at least along a horizontal line, has a great deal more relief. Below the basalt is an almost unjointed section – this probably erodes primarily through abrasion of debris carried by the river when it is in flood. Leaving a smooth but undulating, surface.
It is for this reason that we often consider that the discontinuities (joints) are more important than the rock itself when trying to understand potential rockfalls.
It is nice to know that it is possible to get rockfall control for dangerous areas. This is a big help for those who may live by these types of areas or commute through them. My brother would find this fascinating since he was researching this type of construction. http://www.geotechsolutions.com/Rockfall_Control_Waipahu_HI.html