Earth pressure balance shield tunneling in sandy gravel deposits: a case study of application of soil conditioning

Earth pressure balance shield (EPBS) tunneling techniques originally developed for fine-grained soil deposits have been adopted in a growing number of tunneling projects where challenging geological conditions, such as those with coarse-grained soils, are encountered. However, successful application of EPBS tunneling with soil conditioning could fully address technical challenges during tunneling in coarse-grained strata, such as tunnel face instability, blocking or clogging, severe wear of cutting tools, and low-effiency tunneling. In this paper, a comprehensive case study is presented that EPBS tunneling was successfully used in Beijing Metro Xijiao line with the aid of soil conditioning, where sandy gravel deposit with unfavorable particle size distribution, poor flowabilty, high permeability, and high abrasiveness was encountered. A series of both laboratory and in situ testing was carried to evaluate foaming agents, develop soil conditioning schemes, and verify their effectiveness of modifying the sandy gravel for EPBS tunneling purpose. The homogeneity, workability, permeability, and abrasivity of foamed soils were tested in the laboratory to develop soil conditioning schemes while the effectiveness of soil conditioning schemes during in situ tunneling was further evaluated with slump test and visual inspection. Parameters, such as foam expansion ratio (FER), half-life period, and foam injection ratio (FIR), which are used to evaluate the behavior of foaming agent and performance of conditioning schemes and tunneling schemes, were also discussed.