Protecting Roads and Infrastructure from Falling Rocks

By Anna Bloom, Rockfall Specialist for Maccaferri

How is Infrastructure Impacted by Rockfall? When rocks above highways and infrastructure sites dislodge and slide, roll, or free-fall, the impact of these rocks and/or the resultant obstacle in a roadway may result in significant damage, road closures, mortalities, and injuries.

Developed areas commonly affected by rockfall include roadways, railways, construction sites, commercial and residential buildings, utility infrastructure, and mines. These rockfall hazards most commonly occur in mountainous terrain and are receiving greater recognition around the world due primarily to injury and mortalities. For example, according to Washington State Department of Transportation (WSDOT), Transportation Data Office, Collision Data and Analysis Branch, between 1970 and September, 2005, eight fatal collisions with a total of 10 fatalities were determined to most likely have involved falling or fallen rocks along highways in Washington state. Also, Hungr and Evans (1989) note that, in Canada, there have been 13 rockfall deaths in the past 87 years. Almost all of these deaths have been on the mountain highways of British Columbia.

Accident due to a rockfall incident on a highway.

Additional implications are also driving the increasing acknowledgment of rockfall hazards and implementation of rockfall mitigation systems. With increasing development and presence of roads and traffic, there is a need to stabilize rock slopes for this increased exposure to rockfall. Also, the implementation of rockfall mitigation systems is increasingly encouraged by government agencies as a safety measure. Additionally, legal recourse for injuries and loss of life resulting from rockfall events is becoming more common. As a result, new rockfall mitigation systems are being developed and rockfall mitigation systems are more commonly implemented.

How to Address Rockfall Hazards
When addressing rockfall hazards, it is usually not feasible to fully remove a rockfall hazard. As a result, the focus is placed on reducing the risk of the hazard. The first step in reducing the risk is to identify the hazard. The next step is to understand the level of risk from this hazard. For example, most of the state transportation departments in the United States implement a rockfall hazard rating system that is utilized to assess rockfall hazards along state highways and ultimately to prioritize the sites by the level of risk to infrastructure and humans. Factors that are commonly assessed to estimate these rockfall hazards are the following:  slope character, climatic conditions, geologic conditions, and discontinuity conditions. Subsequently, the traffic conditions and site history are also assessed in order to estimate the risk.

When addressing a risk, it must be decided what level of risk is acceptable. The rockfall mitigation system should then be able to be designed to achieve this level of risk. For example, most rockfall systems, particularly rockfall barriers (fences and embankments) have been tested to determine the energy level of impact that they can withstand and the length of elongation resulting from an impact. For this situation, the trajectory of the rock (bounce height and speed) and the potential energy of the impact can be estimated through rockfall design software. Ultimately, safety factors are applied to address the estimated bounce height of the rock and energy level of the impact. With this information, a rockfall barrier can be selected based on necessary height and the achieved energy level that has been certified during product testing. Other types of rockfall systems, such as drapery systems, may not have been tested as a system, but rather the primary components have been tested (anchors and panels) relevant to this application. Almost all rockfall sites have unique geometric configurations and needs; therefore, the exact configuration of components does not get tested.

Commonly Used Methods to Mitigate Rockfall
Rockfall protection systems are a key element in the design and maintenance of infrastructure networks and have a direct impact on safety and the preservation of infrastructure as discussed above. The following types of rockfall mitigation systems are the most common applications that are currently used around the world: rockfall fences, rockfall embankments, rockfall drapery systems, rock bolting, and rock shelters.

Rockfall fence barrier by Maccaferri, Inc. placed upslope of residential buildings.

Rockfall fences are systems composed of steel posts and steel cable net or ring net panels and mesh to create a fence. The fence is usually anchored by additional steel cables that run to anchors. Energy dissipaters are integrated into the system to absorb energy, and systems are designed to ultimately transfer the load to the anchors. Rockfall fences work as a passive system by absorbing the impact energy of the rocks before they reach the protected area. Rockfall embankments are made out of other construction materials, such as gabions or earth stabilization structures. The embankments serve as a catchment to detain rocks that have fallen. Rockfall drapery systems are composed of wire mesh, cable net, or ring net panels that are suspended by steel support cables. These systems are also designed to ultimately transfer the load to the anchors. Drapery systems may be draped loosely over the rock slope or they may be secured to the rock slope by rock bolting. Rock bolting may also be implemented without the use of a full-scale drapery system. Rock shelters are not commonly utilized in the United States due to their relatively high cost per risk reduction. Additionally, at times it is only necessary to scale the potentially problematic rocks by removing them from the rock slope.

Secured drapery system by Maccaferri, Inc.

Rockfall hazard sites are almost always unique in their many characteristics. Therefore, there are often situations which present a need for a combination of rockfall mitigation systems. For this reason, industry leaders, such as Maccaferri, have developed their product lines to include all of the necessary components for the application of a combination of rockfall systems to create unique solutions to rockfall problems.

Rockfall Product Testing
Rockfall fences have been tested in both the United States and Europe by a variety of methods. The most common methods have been by inclined field tests and (click to view vertical drop tests video). The inclined field test, which is more commonly used in the United States, implements the rolling of real boulders into in-situ, installed rockfall fences. This test is very useful because it considers the rotational component of the rockfall impact because the boulder is likely to be rolling upon impact. The vertical drop test is implemented by instead dropping a pre-fabricated boulder of strict specifications onto a rockfall fence. This method is relatively more repeatable and is therefore allows a more accurate comparison between rockfall fence systems. The biggest limitation of this method is that it does not test the effects of the rotational component of the falling boulder, because the boulder impacts after a free fall.

In the United States, rockfall fence testing was first implemented in the early 1960's by the Washington State Department of Transportation. And then in the early 80's, the California Department of Transportation rolled hundreds of rocks into catchment ditches and catchment fences on numerous road cuts around California. In 1986, Canadian Engineers and geologist performed rock-rolling test where rocks were rolled into a maintenance designed cable net fence. Beginning in the late 80's, the California Department of Transportation, the Colorado Department of Transportation, and various manufacturers began a series of test, extending over a period of several years up to the present, on a variety of rockfall fences. Within recent times, rockfall fence testing has become an increasingly integral part of the selection and design of rockfall fences. As a result of all of these tests, many states have begun to use a performance specification. By the mid 1990's, many types of barriers have been tested by various methodologies within the United States and/or Europe; including cable nets, ring nets, double twisted wire fences, single twisted wire fences (chain link), timber walls, mechanically stabilized earth embankments and rubber tires.

The current trend in the United States is to move towards more standardized testing methodologies, and perhaps future product certifications. Currently, companies such as Maccaferri, an industry leader in the United States, comply with established standards and certifications. Implementation of this product testing has resulted in Maccaferri producing more effective designs. As a result, their systems have become more cost efficient and have ultimately increased safety and saved lives.

Liability Issues Regarding Rockfall
Liability is an increasing concern in the United States and worldwide. In respect to rockfall, liabilities regarding injury, loss of life, and damage to infrastructure, vehicles, and residences are a realistic concern in today’s society. There have been numerous successful lawsuits regarding negligence due to unmitigated rockfall hazards. For example, in some cases the courts may see that the “responsible party” could readily foresee the risk of harm to highway users if the highway was not maintained within reason. This maintenance may include prevention of rockfall hazards. Liability can be placed in situations to protect property and/or persons from rockfall activity above a roadway, parking lot, residential building, school yard or building, commercial building, infrastructure property, active construction sites, and in many other situations. Ultimately, the highest concern is safety; however, rockfall mitigation is aimed at reducing risk, because completely removing the hazard is often unrealistic.

References
Hungr, O., and Evans, S. G., 1989, Engineering Aspects of Rockfall Hazard in Canada: Geological Survey of Canada Open File Report 2061, 102 p.
Washington State Department of Transportation (WSDOT), Transportation Data Office, Collision Data and Analysis Branch, http://www.wsdot.wa.gov/NR/rdonlyres/722330E4-E25D-4F69-86D8-F67F348233EB/0/Sections4_5_6_RockfallHist_HwyClos_AvalHaz.pdf.

Author details:
Anna Bloom, Rockfall Specialist for Maccaferri
T: (301) 223-6910
E: abloom@maccaferri-usa.com
www.maccaferri-northamerica.com