Longitudinal settlements during tunneling in soft Clay, using transparent soil models

The purpose of this study was to measure internal longitudinal displacements taking place during tunnel excavation and compare them to the displacements predicted using available analytical and empirical formulations. A physical model made with transparent soil permitted exploring the internal soil deformations, within the supporting soils. A soil surrogate made of Magnesium Lithium Phyllosilicate (MLPS) was used to represent soft clay. Tunneling was simulated using a moveable trap door arrangement for different face losses ranging from 5 to 30% D. Internal soil movements in the longitudinal direction were captured for three cover-to-diameter (C/D) ratios ranging from 1.5 to 3.5. Shear as well as volumetric strains were computed using the captured spatial displacements. The results demonstrate that the shape of the longitudinal profile is more dependent on the overburden depth of the tunnel in comparison to face losses, in particular: (i) subsurface settlement is approximately 2-4 times the observed surface settlement for the same C/D ratio; (ii) a longitudinal soil arch is likely to form with the increase of C/D ratio which limits the observed settlement; and (iii) shear strain fields showed that a cascading failure may occur with the increase in face movements, especially for shallow tunnels. Finally, the computed and measured surface and subsurface settlement profiles were generally in agreement especially as C/ D increases, thus validating methods used to predict subsurface settlements where measurements are sparse.