Strood and Higham tunnel lining project, Kent

This article describes the process for installing a concrete lining to two railway tunnels in Kent, south-east England, to prevent chalk falls from the tunnel sides. The concrete, which was sprayed onto a reinforcement mesh, needed a low water/concrete ratio and high cementitious content, and incorporated a superplasticiser and plastic fibres. The concrete was delivered by a rail-mounted agitator to a pump connected to the formwork. The contractor required the concrete to have up to twelve hours placing time: varying temperatures in the tunnel made it necessary to monitor and adjust the dosage rate of the retarder.