A major tunnelling contractor required an improved methodology for filling the void behind tunnel segment rings with a cementitious grout and setting accelerator. Existing equipment employed two eccentric screw pumps, one supplying grout and the other accelerator, operating on a fixed speed ratio between the two. However, this open-loop control resulted in a large variability of the filling material quality, as the proportionate flows were affected by back-pressure level and pump wear, which, in turn, caused quality control delays to the operation. 

An improved, closed-loop control system was proposed, utilising highly specialised electro-magnetic flow meters in combination with inverter controlled electric pumps. In addition, a three-level control fall-back methodology was employed, since the system was critical to the tunnelling operation. 3D Solid Modelling was used to ensure that the equipment would fit successfully into a very constrained space envelope, whilst still allowing the pumps to be easily maintained. The whole project was completed in 6 weeks and ensured the 3m diameter 5Km long tunnel drive was completed ahead of schedule, since there were no delays due to quality issues or breakdowns.