Going underground: an integrated system for reuse of excavated tunnelling materials
What happens to material excavated from tunnels? A three year EU project, completed in September 2015 has investigated how valuable mineral resources from underground construction projects can be used rather than ending up in landfill. DRAGON (Development of Resource efficient and Advanced Underground Technologies) has prototyped a system to do this. ‘Future tunnelling projects in Europe are expected to generate around 800 million tonnes of excavated material,’ explains Professor Robert Galler, Chair of Subsurface Engineering at Montanuniversität in Leoben, Austria. ‘Recycling of this material would substantially reduce the demand for primary mineral resources, and reduce environmental impacts.’ Montanuniversität was the co-ordinating partner of the project, which also has industrial and academic participants from Germany, Switzerland, France and the UK. Many partners are involved in large ongoing underground projects, where the use of excavated material has already been realised. One such project is the Les Farettes Hydropower Project in Switzerland, where excavated rock provides the job site with usable aggregates. ‘ Underground analysis The challenge for the DRAGON consortium was how to create an economic and efficient recycling process. ‘The goal was to clearly separate materials which can be used in industry from those which can only be used for landfill.’ says Prof. Galler. ‘Automated online sampling and characterisation of physical, chemical and mineralogical properties provide the basis for assessing the excavated material.’ The DRAGON project solved the challenge by developing a materials characterisation system, directly integrated into the tunnel boring machine. The entire process, from analysis to sorting takes place underground. The project’s five prototype units each automate a different process step. The prototype use automated sample analysis including X-ray elemental analysis, high-precision microwave moisture measurement, and photo-optical grain-size analysis. ’The equipment for the elemental analysis has to be protected by a special housing. An underground separation plant then handles the material, based on the test results and online materials classification. The excavated material can be sorted in-stream, and used directly on site or transported for another industrial sector,’ says Prof. Galler. Environmental and economic benefits The DRAGON consortium has also demonstrated environmental benefit, through a Life Cycle Assessment. ‘Generally the results for most indicators show that environmental performance improves as the proportion of excavated material that is diverted from landfill increases. The benefits accrue from avoiding impacts associated with landfill and from avoiding primary production,’ explains Prof Galler. One important condition for both machine manufacturers and construction companies has been that the tunnelling rate should not be adversely affected by DRAGON’s new process. The team is confident this condition can be met, but whether recycling is economically viable also depends on material demand at the job-site as well as from the surrounding industry. The DRAGON consortium calculated there is potential to reuse around 80 % of excavation material and that transport of excavated material to external receiving industries is economical within a 150 km radius. Future impacts The team now proposes that in any tunnelling project, a material management strategy should take into account excavated material in the planning stage, with the view to making projects resource-efficient, aiming at zero waste. Prof. Galler concludes, ‘Our newly developed technologies will have a strategic impact on sustainable management of limited mineral resources and could decrease EU dependency on imports, improving competitiveness for companies associated with underground construction and new resource-efficient technologies.’ The DRAGON consortium now intends to use their results as an example for other companies in the construction industry, as well as their own ongoing projects. ‘If the results of DRAGON could be applied to these projects, an immense amount of material could potentially be used now and during the next decades,’ concludes Prof. Galler.
Keywords
DRAGON, underground construction, recycling, excavation material, tunnelling, automated analysis, life cycle analysis, waste, resource efficiency, sustainability
