Subsurface settlement profiles above tunnels in clays

Designers assessing the possible effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop, and how these relate to surface settlement profiles. The effect on piled foundations or existing tunnels depends on the width of the subsurface settlement profile and on the magnitude of the settlement. This note uses available field measurements and centrifuge model test data to determine how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays. The results reveal a fairly consistent pattern of behaviour: the width of the subsurface settlement troughs at depth is significantly greater than would be predicted by assuming a constant trough width parameter K of 0.5. Smaller more realistic subsurface settlements and strains will be predicted if account is taken of the proportionally wider trough widths at depth.