Mapei’s UTT Team on Line 3 of the Sofia Underground Railway Network

Since 1974 Sofia, the capital city of Bulgaria, has been planning to construct a lightweight railway system. The first stretch of Line 1 was inaugurated in 1988 and, up until 2019, the rail network consisted of two separate lines with 35 stations, for a combined total length of 40 kilometres. Thanks to funds received from the European Union, construction of Line 3 was finally completed in 2019. The line is 16 km long and has 19 stations (11 of which are underground) and carries around 170,000 passengers every day.

The Mapei UTT Team was asked for their support during the complex construction work on the underground tunnels along Line 3, especially as for the products that need to be applied when tunnelling with a TBM (tunnel boring machine).

After carefully analysing the ground to be excavated, Mapei experts recommended injecting POLYFOAMER FP/L, a liquid foaming agent based on biodegradable, anionic surfactants in combination with a polymer, able to generate a stable foam, with great lubrication properties, and is compatible with every type of soil excavated with an EPB TBM. The foam generated by POLYFOAMER FP/L reduces the friction between the particles of soil, thus minimizing the wear on the cutting wheel.

To seal the cutterhead and the tail brushes, preventing the ingress of water, soil and other materials inside the TBM, it was recommended to use MAPEBLOX T tail sealant.

When building the tunnel segments, special concrete was produced by using DYNAMON SX super-plasticizer that gave the concrete enough workability, excellent final surface finish, and at the same time very high early strength, that allowed the contractor to increase the efficiency of the production process. DYNAMON SX is also particularly recommended for applications where there is the need for greater water reduction, along with very high compressive strength at the early and final stages.

Mapei Technical Services also recommended using the super-plasticizing liquid admixture for concrete MAPEFLUID N200. This product was used for production on site of backfill injection mortar with excellent fluidity and excellent initial compressive strength after injection. Because MAPEFLUID N200 also acts as a slight retardant for the hydration process of cement, it gave the injection mortar enough open time to be transported long distance in the tunnel.