Falling rocks are a special category of the large family of gravitationally-driven phenomena called landslides. What are commonly called rockfall events generally fall into four technical definitions: rockfall, rock topple, rock avalanche, and rock slide. Obviously nature doesn’t always follow our pigeon-hole classifications, so rockfalls commonly grade into one another. Generally an individual rockfall includes one to only a few rocks, with sizes that vary from gravel to boulders (~2 inches to 5 feet dia.), although they may be of any dimension. When a large mass of rock fails and the resultant fall spreads out into a fan of utter devastation, it is referred to as a rockslide or even a debris avalanche.
Rockfall is the fastest type of landslide and is common in mountainous areas near cliffs of broken, faulted, or jointed bedrock, on steep slopes of rocky soils, or where cliffy bedrock ledges are undercut by erosion or human activity. The loss of support from underneath, or detachment from a larger rock mass destabilizes the rocks and gravity does the rest. The criteria for rockfall is simply an exposure of broken rock, gravity, and a slope steep enough that when a rock detaches or dislodges from the ground surface, it will move down the slope rapidly. Complex interactions between physical parameters of both the rock and the slope cause the falling rocks to move down the slope in a high-velocity, seemingly random and erratic manner. When people, buildings, vehicles, or highways are in the path, these rockfall events can lead to tragedy — property loss, personal injury, or even loss of life.
It is important to note that rockfall is a natural catastrophic erosional process that has been occurring in steep terrain for as long as the Earth has existed. Young mountain ranges such as those in Colorado do not weather away grain by grain, rather, they tumble down in a punctuated but perpetual sequence of rockfalls, rockslides, landslides, and debris-laden floods over millions of years. Many regions and towns on the Western Slope of Colorado are exposed to the rockfall hazards of nearby mountains and cliff-rimmed mesas. More information about rockfall and other case histories in Colorado can be found in CGS RockTalk, Vol. 11, No. 2
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 free-fall 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.
Rockfalls can demolish structures and kill people. Rocks falling on highways may strike vehicles, block traffic, cause accidents, and sometimes damage the road. Minor but costly consequences is the work of clearing highways and borrow ditches in rockfall areas. Any structure in the path of a large rockfall is subject to damage or destruction.
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 block of sandstone, for example, rolling downhill into a typical unprotected house, would likely 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 are likely 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 may be used safely, but the cost of protection from the potential hazard may, in many cases, exceed the economic gain from the change in land use.
Booth Creek, East Vail, Colorado
An example of a rockfall hazard and high risk area affecting a neighborhood is in East Vail at Booth Creek. The north valley wall of Gore Creek is benched with two high rock cliffs. Above the cliffs, the 1,100-foot high valley rim is composed of an eroding slope of glacial till, which is also composed of very large rocky material. After several repeated, potentially lethal, rockfall events that damaged several homes in the early to mid 1980s, CGS was asked to provide assistance to the Town. The neighborhood created a special Geologic Hazards Abatement District (GHAD) affiliated with the Town of Vail. The GHAD funded a rockfall hazard study that included a mitigation design. The construction of a rockfall catchment ditch and berm above the homes on the valley slope was completed in 1990 (Figure 1).
Owners of adjacent condominiums elected to not participate in the GHAD, and that poor judgment was brought into sharp focus in March 1997. Another large rockfall event fanned down the slope toward the residential areas at the property line between the homes and condominiums. The existing rockfall ditch and berm was 100% effective in catching the rocks, but several rocks impacted the unprotected condos (Figure 2).
After that incident, which luckily resulted in no fatalities, the condominium homeowners association petitioned the town for their own mitigation. In 2001, specially designed impact barriers (Mechanically Stabilized Earth wall) were constructed on the slope behind the condos to provide a similar level of protection (Figure 3).
Glenwood Canyon Thanksgiving Day Rockslide
On Thanksgiving Day in 2005, a very large rockfall event occurred in Glenwood Canyon affecting a portion of Interstate 70. A segment of rock over 1,200 feet high on the canyon wall and 2,000 cubic yards in volume, detached from the cliff face, broke into many large blocks that rolled down a rockfall chute, and slammed into the highway at the canyon bottom (Figure 1). Thankfully, the westbound lanes were temporarily closed at the time. No vehicles were hit, but there was severe damage done to Interstate 70 highway structures, requiring the westbound lanes to be closed for almost three months for repairs.
Figure 1. Thanksgiving 2005 rockslide area in Glenwood Canyon. Detachment location, shown by black arrow, is 1,200 vertical feet above Interstate 70. Note the prominent nonconformity where this 600-foot thick cliff of Sawatch Quartzite lies over Precambrian basement rock. The rockfall path is well defined by the show-filled chute in the underlying Precambrian rocks.
The rockslide occurred near the Shoshone Interchange, which is a tightly constrained section of highway structures in one of the narrowest sections of the canyon. A series of bridges and retaining walls enable the highway to cross the Colorado River to the Hanging Lake Tunnel portal while still providing road and bicycle access to the Hanging Lake Rest Area. The rockfall was caught on the closed circuit video cameras used to monitor Interstate 70 traffic in the canyon. The video showed many rocks, up to 12 feet in diameter, impacting the on-ramp retaining wall of the rest area, as well as the bridges to the tunnel portal. A dust cloud generated by the rockslide filled the canyon afterwards.
When the dust cleared, the highway was littered with boulders of all sizes (Figure 2). Upon closer inspection, the true nature of the damage became apparent as large holes were punched though the concrete deck and the westbound retaining wall (Figure 3), demolishing a section of the bicycle path below (Figure 4), as well as damage to the bridge girder of the adjacent eastbound bridge.
Figure 2. Huge blocks of Sawatch Quartzite litter both the eastbound and westbound lanes of I-70. Photo by Ty Ortiz, CDOT.
Figure 3. Westbound deck and retaining wall with extensive damage. Photo by Ty Ortiz, CDOT.
Figure 4. Damage to retaining wall and bike path. View is below bridge at hole location shown in Figure 3. Photo by Ty Ortiz, CDOT.
The town of Glenwood Springs in west-central Colorado lies at the confluence of the Roaring Fork and Colorado Rivers. The town is tightly constrained by the steep river valleys so land-development pressure is causing more residential growth to push into rockfall hazard areas. In West Glenwood, on the west side of the Roaring Fork River, the valley is rimmed with dipping sandstone outcrops of reddish Maroon Formation. The sandstone layers are being undercut by the erosion of underlying softer siltstone and shale so that large sandstone blocks are being actively undermined and destabilized. In this area, there have been several large rockfall events from the valley rim; some that have severely damaged homes on the valley floor, 1,100 vertical feet below. Fortunately, there have been no injuries or fatalities. While there has been rockfall mitigation in some locations, the threat remains in other areas.
Figure 1. Valley rim west of the Roaring Fork River in Glenwood Springs looking north towards the confluence with the Colorado River. Note slumped (tilted) sandstone blocks in the exposed rock layer. Some of the rock blocks shown in this picture from 1994 have now fallen/rolled to the valley floor. Photo by Jon White.
Figure 2. Hole in side of house from impact of two large boulders in 2004. Note smaller rock embedded in roof. Photo taken from Midland Avenue embankment above home. River shown is the Roaring Fork. Photo by Jon White.
Figure 3. The main rock that slammed into the house shown in Figure 2 rolled through the home to come to rest against an easy chair in the living room. This rock came from the source area shown in Figure 1. The homeowner built a rockfall protection fence afterwards. Photo by Jon White.