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Watching out for falling rocks

WVU engineers seek ways to prevent them on rural roads

WVU Today

Drivers on winding West Virginia roads often encounter road damage caused by rockfalls, even if they aren’t present when the rocks tumble down the hillside.

West Virginia University researchers hope to cut down on these events, as a pair of engineers have examined countermeasures considered useful in mitigating rockfalls, which can result in vehicle damage, traffic disruptions and injury or death to motorists.

Yoojung Yoon and Hota GangaRao, professors in the Wadsworth Department of Civil and Environmental Engineering, offer a framework to evaluate the cost-effectiveness of design options for rockfall countermeasures that focus on rural local road rockfall.

The framework is composed of three main steps: rockfall hazard assessment, prioritization and sensitivity analysis.

The duo’s research has been published by the American Society of Civil Engineers.

The two researchers’ prioritization framework was demonstrated with a case study in West Virginia, which confirmed that it was able to provide decisionmakers with a sense of certainty about the top-ranked alternative, which was soldier piles and precast concrete lagging.

Soldier piles are steel piles that are vertically drilled into the earth prior to excavation. Precast concrete lagging are concrete panels that are inserted behind the front pile flanges. The lagging efficiently resists the load of soil and transfers it to the piles.

“Rockfalls are caused by the freezing and thawing of rocks, resulting in expansion and contraction over a period of time (thermal fatigue),” GangaRao said. “Also, falls may happen due to increased overburden near steep cliffs due to building construction, change in direction of run-off water or other man-made disturbances at sites. Other potential reasons could be attributed to blasting of sites resulting in fissures in geological formation near the blast site.”

Yoon and GangaRao said  road stakeholders, such as designers and contractors, are concerned about the risk of rockfalls, which is why road sections  within a rockfall hazard zone require countermeasures that can be either active or passive.

Active design is intended to mitigate the risk of rockfalls through prevention work at rockfall sources. Passive design focuses on protecting the at-risk road structures from rockfall events.

An active design example is steel rock bolts, which must be changed or maintained periodically, and a passive design example is shotcrete, where maintenance is not needed, as long as the material does not deteriorate with aging.

According to Yoon and GangaRao, rockfall countermeasures are designed after analyzing rockfall hazards, including the susceptibility, magnitude, runout and exposure of the rockfall. The best rockfall countermeasure among the various design alternatives is the one that offers optimal prevention and control of the rockfall at the lowest cost.

 Rockfall design alternatives include soldier piles and precast concrete lagging, shotcrete, steel rock bolts and steel rock bolts and shotcrete.

Yoon said determining the best design option involves multiple evaluation criteria and that the sensitivity analysis of the framework provides decisionmakers with confidential insight into their final decision on the best countermeasure.

Sensitivity analysis tries to analyze the uncertainties in the decision-making process and consider all possible circumstances, according to Yoon.

Yoon and GangaRao specifically address the use of rockfall countermeasures for local roads because the need for rockfall countermeasures exists more often on low-volume roads in rural areas.

“The reason we focused on rural roads is because local road agencies are often lacking resources and risk management processes to adequately handle rockfall hazards compared to high-volume national highway systems,” Yoon said. “The state of West Virginia has local roads in rural, mountainous areas and we believe that the framework of this study can be useful for the local agencies in West Virginia to choose the top-ranked countermeasure option to implement.”

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