Construction of cross passages completely submerged under ground water

This paper is a case study of Construction of Cross Passages between South Extension and Lajpat Nagar Metro Stations of line-7 of Phase-III of Delhi Metro. The geological strata at the cross passages i.e. CP-18 and CP-19 mainly comprised of silty sand and were completely submerged under ground water. CP-19 was located-below a vacant field and CP-18 was located just below a road junction surrounded by residential houses. Initially Forepoling and TAM grouting was carried at both the cross passages. At the time of excavation, flowing ground-condition encountered at CP-19 and some settlement was also observed at surface due to ground loss. For-stabilising the ground above CP-19 and to fill any cavity formed because of ground loss, compensation grouting-was carried from the top surface. Also during the excavation TAM grouting was carried on the excavated face of cross passages to ensure stability of ground inside the cross passage. At CP-18, due to space constraint and the presence of various utilities, compensation grouting was not possible from top. Therefore, to encounter the problem faced at CP-19 and to mitigate any risk to the buildings above CP-18, in addition to Forepoling and TAM Grouting, chemical grouting was carried on the crown of cross passage to avoid any ground loss. For restraining ingress of water and to provide compressive strength to ground, Polyurethane injection grouting with very low viscosity was carried. By adopting this methodology, CP-18 was successfully constructed with minimal ground loss and with maximum settlement of -11mm observed at ground surface.