Pre-grouting reinforcement of underwater karst area for shield tunneling passing through Xiangjiang River in Changsha, China

•A new controllable grouting method and two controllable grouting materials were proposed.•The feasibility of the pre-grouting scheme were verified through a series of assessment in field.•The pre-grouting activities is environmental and ecological friendly. Underwater shield tunnel construction in karst ground is a huge challenge for engineers and technicians since such engineering case is rarely seen and there are no reported experiences in the literature. The shield tunnel of Changsha metro line 3 had to pass through underwater karst regions in a water conservation district of Xiangjiang River. Therefore, guaranteeing the construction safety and protecting the river water quality during the shield tunneling process are the main difficulties need to be solved. A new controllable grouting method and two corresponding grouting materials were proposed in this paper to pre-reinforce the underwater karst region before shield tunnel passing. The karst caves in the vicinity of the tunnel body were pre-grouted by a so called vertical layered controllable grouting, which can ensured the grouted are to be easy cutting by the shield machine but possess sufficient strength to keep the stability of the tunnel. This specific field pre-grouting scheme mainly includes a waterborne construction platform and a series of anti-pollution measures. The effectiveness of field application of the proposed scheme were evaluated by a series laboratory tests taking the samples from the grouted area. The result indicates that the grouting activities not only meet the engineering requirements but they are also environmental and ecological friendly. The presented study could provide a detailed case experience and reference technology for tunnelling in underwater karst area.