ROCKFALL is the falling of a newly detached mass of rock from a cliff or down a very steep slope. Rocks in a rockfall can be of any dimension, from the size of baseballs to houses.
Rockfalls are the fastest type of landslide and occur most frequently in mountains or other steep areas during early spring when there is abundant moisture and repeated freezing and thawing. The rocks may freefall or carom down in an erratic sequence of tumbling, rolling and sliding. When a large number of rocks plummet downward at high velocity, it is called a rock avalanche.
Rockfalls are caused by the loss of support from underneath or detachment from a larger rock mass. Ice wedging, root growth, or ground shaking, as well as a loss of support through erosion or chemical weathering may start the fall.
Man’s activities often cause rocks to fall sooner than they would naturally. Excavations into hill and mountainsides for highways and building frequently aggravate rockfalls. Vibration from passion trains or blasting can trigger them, as can changes in surface and ground water conditions. Rockfalls have been attributed to earthquakes and sonic booms.
H.B. 1041, Part 1, 106-7-103(8) Rockfall is defined only as a kind of geologic hazard.
In a rockfall, relatively large fragments of rock become detached and by means of free-fall, rolling, bounding or rapid sliding, or a combination of these methods, moves rapidly down a very steep slope under the force of gravity. Rockfall can be a continuous process over a considerable period of time or a single or series of single, intermittent events. Simultaneous activation of a large mass of rock can result in a rockfall avalanche or very rapid down slope and spreading movement of a large quantity of rock material. Rockfall can be initiated by several means. Most commonly this includes exposure to multiple freeze-thaw cycles, precipitation wetting and weakening of material under blocks, seismic activity, or undercutting of cliffs by erosion or flow of weak rock material.
Rockfall is common where there are cliffs of massive broken, faulted, or jointed bedrock; or where steep bedrock ledges are undercut by natural processes or activities of man. A major cause of rockfall is the repeated freeze-thaw action of water. Because freezing water expands, it develops pressures capable of wedging apart contiguous blocks of massive rock. Water from rain or melting snow also plays an important role in producing rockfalls by erosion, air slaking, and weakening of soft rocks, and by percolation of rainwater through joints. These actions remove the support for the overlying blocks of rock and can eventually initiate down slope movement.
Some rock types (shales) that contain a high percentage of clay become weak and slippery when wet. The result is a reduction of static friction at the base of overlying metastable blocks. This can cause slippage, which leads to forward rotation and results in subsequent rolling, bounding, or falling of rock fragments. Equilibrium of unstable blocks in rock exposures can be upset by shock from natural earthquakes, blasting, or movement of heavy vehicles.
Undercutting of rock slopes by stream erosion or construction excavations such as road-cuts, that remove support for overlying or overhanging rock, can result in conditions conducive to rockfalls. Talus and talus slopes are the usual natural result of numerous small rockfalls, and their constituent rocks have come to rest in metastable equilibrium, especially those rocks on the surface of the talus slope. Thus, cuts into, and construction on, these slopes can interfere with the active natural rockfall process from the cliffs above, or cause increased movement or falling of the talus material below. Certain over-steepened road-cuts or other excavations are common and dangerous areas for rockfalls.
Severity of problem
The combination of conditions that produce rockfalls is common in the hilly, mountainous, and tableland areas of Colorado. Rockfalls can result in almost unpredictable, nearly instantaneous losses of life and property, when man chooses to live or build structures in their paths without due consideration for the danger. Fortunately, many rockfall areas can be identified (see Criteria), and with proper recognition and engineering, much of the potential danger can be alleviated, if economic costs and benefits are justified and proper actions taken.
Criteria for Recognition
Many areas where rockfall may occur are relatively easy to recognize. Other areas where rockfall is a potential hazard are difficult to identify and evaluation of the degree of hazard present may be virtually impossible. Potential rockfall areas are those where relatively steep or barren cliffs rise above less steep talus or colluvial slopes. The talus slope and areas adjacent to it, occupied by larger angular randomly oriented rocks, constitute the long-term potential rockfall danger zone even though the talus may be partially overgrown with vegetation. Active rockfall areas are those showing evidence of recent falling and rock movement. Rock displaced or damaged vegetation, fresh "tracks" of rocks rolling down-slope, fresh scars on cliffs, anomalous or disoriented lichen growth on rock blocks, eyewitness accounts, and damage to fences or man-made works are some common criteria for identifying active rockfall areas. The most common difficulty with ‘inactive" rockfall areas is unexpected reactivation due to activities of man or exceptional natural conditions. Questionable rockfall areas should be monitored if there is the possibility that reactivation of a rockfall may take place and present a hazard to man.
Consequence of Improper Utilization
Improper utilization of rockfall areas is any use for which occasional, unpredictable, rolling, bounding, or falling of rocks could constitute a threat to life or property. Unless completely protected (see mitigation), buildings, some roads, pipelines, railroads, and most other works of man are in potential jeopardy in rockfall areas. A 3-ton of sandstone, for example, rolling downhill into a typical unprotected house, probably would destroy it, whereas this same block crossing a concrete roadway probably would do relatively little damage. A major rock avalanche could, however, destroy a roadway or a whole subdivision. In the case of costly engineered structures, expenses for mitigation of rockfall danger would likely be warranted, especially if alternative locations are prohibitively expensive. Housing, on the other hand, might easily be planned elsewhere with less expense if other potential sites are available.
Areas of potential rockfall are subject to constraints similar to those of active rockfall areas. However, if activation can be prevented, such areas could be used safely, but the cost of protection from the potential hazard can in many cases exceed the economic gain from the change in land use.
Colorado Geological Survey