Abstarct: The line 7 of Tehran subway is one of the longest (26 km) of projects that are under implementation in Iran. Excavation operation is done in two sections, South-North (14 km) and East-west (13 km).The used TBM is an EPB with excavation diameter of 9.164m and the lining is with pre-cast concrete segment following the concept of the Universal Ring. According to the results of periodic inspection of water in boreholes along the tunnel rout from kilometerage 5600 to 11100, excaviation will be carried out below the groundwater level. In the Earth Pressure Balance (EPB) TBM tunneling methodology, the instantaneous filling of the annulus that is created behind the segment lining at the end of the tail during the TBM advance is an operation of paramount importance. Its main goal is to minimize the surface settlements due to any over-excavation generated by the passage of the TBM. To correctly achieve the goals, a simultaneous backfilling system and the injected material should satisfy the technical, operational and performance characteristics. In this project, two-component backfill system injection has been selected. To optimize the mix design of two-component grout against water and use of additives to enhance stability and consistency, tests have been performed. The optimal admixture plan is selected by testing more than 190 admixtures in laboratory and assigning effective parameters. The materials portion of optimal admixture grouted in tunnel rout that located below the groundwater level is 400 kg of cement, 40 kg of bentonite, 853 kg of water, 4 kg (1% of cement weight) of retarder, 70 kg of silicate sodium (as accelerator) and 0.6 % of cement weight anti wash-out type K was selected as optimal design for this project