Three-dimensional finite element analyses for optimal ground improvement for tunnel cross passage in Bangkok subsoils

Abstract

Jet grouting is a common technic for improving the stability during the construction of tunnel cross-passages in soft grounds. In this study, the optimal soil-cement mass for a cross-passage from a tunnel to an intervention shaft in Bangkok subsoils was determined using three-dimensional finite element analyses. The soil-cement mass was assumed to be octagonal prism along the route of the cross-passage which has the radius of 2.25m. The result showed that the factor of safety increases linearly with the improvement radius. The improvement radius of 3.60m was selected since it is satisfied with the FS more than 1.4 and the thickness is more than 0.1m to prevent from the short-term seepage problem. In addition, the relationship between the improved groundmass and ground surface settlement curve was made. It was found that the predicted settlement profile can be captured by the formula proposed by Peck and O'Reilly providing that the empirical coefficient of 0.6 agrees well with the results from the 3D simulation and this value was obtained from the average of K = 0.4 and K = 0.7 which is the empirical coefficient value of stiff clay and soft clay respectively based on the overburden thickness.