Drivers on winding West Virginia roads often encounter road damage caused by rockfalls, even if they are not present when the rocks tumble down the hillside.
Now, a pair of West Virginia University researchers hope to cut down on these events by examining 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 university's Wadsworth Department of Civil and Environmental Engineering, offer a framework to evaluate the cost-effectiveness of design options for rockfall damage prevention that focus on the state's rural roads.
Recently, the duo's research was published by the American Society of Civil Engineers (ASCE).
Their study is composed of three main steps: rockfall hazard assessment, prioritization and sensitivity analysis.
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 utilizing soldier piles and precast concrete lagging, the top-ranked alternative.
By way of definition, soldier piles are made of steel and vertically drilled into the earth prior to excavation; precast concrete lagging are panels 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 [known as thermal fatigue]," GangaRao said in a news release from WVU. "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."
Correctives Include Active and Passive Designs
Road designers and contractors working in mountainous areas are, as might be expected, concerned about the risk of rockfalls, Yoon and GangaRao said. That is why road sections located within a rockfall hazard zone require either active or passive countermeasures.
An active design is intended to mitigate the risk of rockfalls through prevention work at rockfall sources, while a passive design focuses on protecting the at-risk road structures from slides.
The use of steel rock bolts is one example of an active design. In this case, the bolts must be changed or maintained periodically.
One effective passive design measure utilizes shotcrete, where maintenance is not needed if the material does not deteriorate with aging.
Best Prevention for the Lowest Cost is the Aim
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 corrective among the various design alternatives is the one that offers optimal prevention and control of the rockfall at the lowest cost.
In the WVU press release, Yoon noted that sensitivity analysis tries to study the uncertainties in the decision-making process and consider all possible circumstances. Additionally, the framework's analysis provides engineers the confidential insight into their final decision on the best countermeasure.
"What rockfall countermeasures are the most important depends on the evaluation criteria you consider, to prioritize feasible design options," Yoon said. "An optimal design option will be determined for a rockfall countermeasure that best addresses a site condition."
The WVU researchers noted that they specifically chose to address the use of rockfall correctives on West Virginia's roadways.
"The reason we focused on rural roads is because local agencies are often lacking resources and risk management processes to adequately handle rockfall hazards compared to high-volume national highway systems," Yoon explained in the news statement.
"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 [here] to choose the top-ranked countermeasure option to implement."
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