Enhancing face stability in soft ground conditions: A study of cross passages construction in the Kolkata east west metro project

The Kolkata East West Metro Project, an ambitious endeavor to address urban transit needs, involves the construction of underground rail tunnels connecting Howrah Maidan Station to the West End of Esplanade Station. Among its key-components are cross passages facilitating maintenance and emergency access between twin tunnels. This study focuses on the design and construction challenges of cross passages, particularly in soft cohesive ground conditions, with a special emphasis on Cross Passages 01 and 02 situated beneath existing infrastructure. Employing the New Austrian Tunneling Method (NATM), the construction sequence involves temporary shotcrete linings reinforced by lattice-girders and splices, followed by permanent reinforced concrete linings. However, face stability and surface deformations, especially in soft ground, pose significant challenges. To mitigate risks associated with excavation in such conditions, an unconventional ground improvement approach utilizing chemical grouting is explored. This method, involving partial permeation and soil replacement grouting, aims to create an interconnected grout matrix, enhancing ground strength and reducing water pressure. Physical results demonstrate substantial improvements in ground shear strength and minimized-surface settlements, indicating the efficacy of the ground improvement technique. This study not only evaluates the impact-of ground improvement on face stability but also revisits parameters crucial for assessing stability in cohesive ground-conditions, offering valuable insights for future metro tunnel projects in similar geotechnical settings.