ConclusionsDifferential ground sett

ConclusionsDifferential ground settlements caused by T1 and T2 are observed in cases of closely spaced twin tunnels from both practical projects and model tests. This study aims to investigate the inherent mechanism behind the phenomenon. On the basis of closed-form plasticity solution for an unloading cylindrical cavity, subsurface ground settlement troughs at various elevations are employed for determining the shear stiffness. By radial displacements toward the twin tunnels, the influence of T1 construction on the shear modulus of soils around T2 is evaluated. Observations of 1g model tests for closely spaced twin-tunnelling in overconsolidated kaolin clay from literature are interpreted. The main conclusions can be drawn as follows:1. The subsurface ground settlement troughs induced by T1 construction can be expressed well by typical Gaussian curves, and those caused by T2 are fitted by Gaussian curves with horizontal eccentricity. The subsurface settlement induced by T2 construction at the side close to T1 is larger than that caused by T1, while smaller at other side.2. The graphs of r1/r plotted against r/dr around T1 (= 0°~45°) prior to T2construction are almost straight lines. The gradient m of the graph increases as the angle from vertical centerline of T1 increases, implying that the shear modulus G decreases as increasesDuring T2 construction, due to the influence of T1, r2/r