Tunnel excavation techniques have advanced phenomenally in the past decades, resulting in the construction of longer and wider tunnels even in very remote locations as well as in difficult geologies and topographies (e.g., sea bed). Because of this, tunnels have increasingly become essential parts of many road and railway systems. Tunnel construction requires the excavation of huge amounts of rocks, which are usually recycled as road or railway embankment material. However, our recent studies have shown that some of these tunnel-excavated rocks actually pose significant threats when exposed to the environment because they release substantial amounts of hazardous trace elements. Many of these elements (e.g., arsenic, selenium, lead and cadmium) have adverse human-health effects when ingested via contaminated drinking water and food crops. Unfortunately, the leaching behavior as well as the factors and physico-chemical processes controlling the release of these toxic elements from tunnelexcavated rocks are still poorly understood. In addition, management of these voluminous rocks using traditional waste disposal facilities (e.g., special landfills) is impractical and unsustainable. This paper presents a comprehensive summary of our laboratory and in situ studies, which evaluated the leaching behaviors and release mechanisms of the most commonly encountered toxic elements in tunnelexcavated rocks. We also described several alternative mitigation approaches that could minimize the negative impacts of these rocks on the environment.

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