## 6 Conclusions  
Recycling of tunnel spoil is becoming increasingly important. This is not only because of the large amounts of rock mass that are excavated that otherwise must be disposed of, but also because of rising shortage of mineral raw materials like sand and gravel in the Alpine valleys. In times of sustainability, resource efficiency and minimisation of emissions it is a logic decision to recycle and reuse excavated tunnel spoil within the framework of the possibilities depending on rock material characteristics, processing effort and local demand for rock aggregate. By the given example of the Brenner Base Tunnel, it could be shown that even despite a restricted forecast concerning the possible recycling extent due to moderate rock properties, Bundner Schist rocks could be recycled at the construction site to produce value grain for drainage gravel and concrete aggregate manufacture.  
In this paper, it is shown that for a successful recycling realization, extensive preliminary research concerning rock quality and concrete mix design, as well as an excellent technical implementation of the processing and concrete mixing plant are necessary. In an initial step, comprehensive rock characterisation is needed to identify the range of the existing rocks and their mechanical and chemical properties, whereby rock properties are dictated by the existing rock types. These experiments are followed by aggregate test production from excavated raw material recording data of attained aggregate characteristics and processing procedure. Here, the aim is to test different crusher and mill types, finding the optimal machines as well as instrument combinations for crushing and sieving to optimize the geometric properties of the produced aggregates. The implementation of an efficient and powerful processing facility plays a key role with regard to the recycling implementation success. In the case of the current example, a three-stage crushing system using a jaw crusher, impact mill and vertical mill as well as an efficient wet processing including washing and sieving was applied to improve grain shape to a preferably cubic shape as well as to significantly reduce the amount of fines. After optimizing the aggregate quality, concrete testing--also in accordance with the chemical composition of the source rock--is needed to find concrete mix designs for the different concrete types and applications ensuring high stability and durability of the concrete structures.  
In the current example of the Brenner Base Tunnel, the extracted value aggregate could be processed and optimized for the application as aggregate for shotcrete, inner lining and structural concrete as well as filter gravel. Thus, the purchase and transport of aggregate from local quarries was minimized, and therefore resources were saved, accompanied by a financial benefit. In summary, through this example, it can be noted that reuse of tunnel spoil will also play an important role for future tunnel projects because of ecological and economic reasons.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Particle Size Class** & **Flakiness Index FI [\%] a)** & **Category of Fines a)** \\ \hline
4/8 mm & 7 & f1,5 \\
8/16 mm & 12 & f1,5 \\
16/32 mm & 17 & f1,5 \\ \hline \hline \end{tabular} a) in accordance with EN 12620.  
\end{table}
Table 5: Flakiness index and fines for processed excavation material at the access tunnel Wolf.  
**Author Contributions:** Conceptualization, investigation and writing, K.V., investigation and software, E.K. Both authors have read and agree to the published version of the manuscript.  
**Funding:** The open access funding was provided by BOKU Vienna Open Access Publishing Fund.  
**Acknowledgments:** This research was carried out with the support of the Brenner Base Tunnel company BBT SE. The authors would like to acknowledge the technical and financial support.  
**Conflicts of Interest:** The authors declare no conflict of interest.