Initially, caps were installed on all grout ports within an isolated section of lining to ensure injected grout couldn't run from neighbouring grout ports. Grouting was carried out in a specific sequence ensuring the effective filling of voids behind the tunnel lining and commenced on the lowest grout ports of all the rings located within an isolated section. This process continued to higher grout ports in each ring until finally the highest grout ports were the last to be grouted in each isolated section effectively sealing the tunnel. Grout design & testing was completed prior to commencing the production works