Design of grouting procedures to prevent ground subsidence over shallow tunnels

Tunnels 3 m (10 feet) in diameter used for road and railroad crossings of water pipelines in Hillsborough County, Florida suffered ground surface subsidence problems due to their shallow depth and sand soil cover. The soil depth above the top of the tunnels was typically no more than 3 m (10 feet). A controlled low strength cementitious grout was designed to fill the void between the tunnel and the liner plates, and to maintain pressure around the tunnel boring machine (TBM) equal to the overburden pressure. Pre-grouting could not be done due to the low permeability of the fine sand soils and the lack of surface access. Therefore, the grout had to be designed for injection from ports inside the TBM without causing damage to the machine. This paper describes the design of the grout and the testing program to document the engineering properties of the grout, as well as the field grouting procedures developed for the project. The results of field measurements and laboratory test results are also presented.