New Sustainable Technologies for Application in the TBM Tunnelling Industry

The TBM tunnelling industry has lost the technical reasons why backfill grouts are pumped behind the tunnel lining during TBM advance. Primary objective is the avoidance of ground settlements that can be automatically generated by the creation of the annulus void due to the passage of the TBM in the ground. Secondary objectives, not less important, are the avoidance of segment floating, the prevention of water ingress into the tunnel, and the mild transfer of longitudinal and radial stresses from ground to lining. In most of the cases, conventional two- component grouting methods do not fulfil the above basic requirements, thus jeopardizing the quality of the final tunnel and putting at risk the life of people using the final tunnel on daily basis. All backfill grout objectives are strictly linked to raw material quality, TBM operations and the quality of the final tunnel as a product per se. In this context, the backfill grout durability concept is still, anachronistically speaking, highly dominated by the concrete industry mentality where the unconfined compressive strength (UCS) is measured at 28 days; however, accountability on curing of laboratory samples and detailed procedures of testing, quality control are not yet properly addressed by the industry due to the lack of a proper tailored-made standard to regulate TBM backfill grouts. The Global Warming Potential (GWP), defined as the impact of carbon footprint emissions (CO2 eq.) per cubic metre of grout, considering how long it remains active in the atmosphere, is however still rarely calculated. In an average of 300 GWP/m3, cement as binder of component A and sodium silicate as activator of component B are the major contributors of approx. 60% of the total CO2 eq./m3. This paper illustrates a new two-components backfill grout solution with reduced carbon footprint emissions that hopefully will be accepted by the industry.