Ground movements generated by sequential twin-tunnelling in over-consolidated clay

The expansion of urban populations comes with an associated demand for increased public transport. An often utilised solution is to construct a rapid transit system within tunnels. Generally, a pair of tunnels are constructed within relative close proximity. The construction of these tunnels will generate ground movements which have the potential to cause damage to existing surface and subsurface structures. Modern tunnelling practice aims to reduce these movements to a minimum; however there is still a requirement for accurate assessment of settlements. For tunnels driven in clay, superposition of settlement predictions made by considering a single tunnel is an accepted method used to estimate movements around pairs of tunnels. This presumes that the movements generated from the construction of the second tunnel are not influenced in any way by the presence of the first tunnel. A series of plane strain centrifuge model tests have been conducted to explore the validity of superposition as a prediction method. The tests consisted of a sequential twin-tunnel construction with varied centre-to-centre spacing in over-consolidated clay. Relatively complex apparatus facilitated a predefined volume loss whilst monitoring surface settlement, tunnel support pressures and pore-water pressures. The measured data were assessed against superposition for surface vertical settlements in the plane perpendicular to an advancing tunnel face. The results highlight some inconsistencies with the superposition method.