Most of the previous study focussed on the effects of cover depth, pillar depth and construction sequence to the interaction of multiple crossing tunnels. The study on the effects of interaction of perpendicularly undercrossing twin-tunnel on the existing horseshoe-shaped tunnel with different geometric sizes is not taken into account. The main objectives of this research are to understand the response of existing tunnel with different geometric sizes due to multiple crossing tunnel interaction at various intersection angles in sand and clay. Five series of numerical parametric studies with adoption of hypoplasticity constitutive soil model were carried out.

In dry sand, the induced invert settlement at the existing tunnel centre is insensitive to the change of size of the exist...[ Read more ]

Most of the previous study focussed on the effects of cover depth, pillar depth and construction sequence to the interaction of multiple crossing tunnels. The study on the effects of interaction of perpendicularly undercrossing twin-tunnel on the existing horseshoe-shaped tunnel with different geometric sizes is not taken into account. The main objectives of this research are to understand the response of existing tunnel with different geometric sizes due to multiple crossing tunnel interaction at various intersection angles in sand and clay. Five series of numerical parametric studies with adoption of hypoplasticity constitutive soil model were carried out.

In dry sand, the induced invert settlement at the existing tunnel centre is insensitive to the change of size of the existing tunnel (area ratio) because of similar magnitude reduction in vertical stress at below invert. In saturated clay, the induced settlement at the centre of the tunnel invert increases with increasing size of the existing tunnel at the end of twin-tunnel excavation. Compared to the excavation in dry sand, the reduction of vertical stress induced at below centre of the tunnel invert is lesser. At 10 years after completion twin-tunnel excavation in saturated clay, the induced invert settlement is furthered increased with increasing size of the existing tunnel which has exceeded the allowable limit of 20 mm (BD, 2009). In both dry sand and saturated clay, the computed settlement at the centre of the existing tunnel crown decreases with increasing size of the existing tunnel. This is due to the lesser transfer of stress to the crown for a larger size of the existing tunnel. Twin-tunnel excavation in clay induced excess pore water pressure which provides additional stress acting on the existing tunnel but not in the case of excavation in dry sand. Negative excess pore water pressure is induced at centreline below existing tunnel invert while zone of positive excess pore water pressure is induced at vicinity to the new twin-tunnel. The increase of invert and crown settlement after 10 years is attributed to the dissipation of excess pore water pressure below existing tunnel.

For different intersection angles with existing tunnel of having same geometric size (AR = 5), the computed results showed that the first tunnel excavation induced larger invert settlement when the intersection angle of new and existing tunnels is at 30°. This is attributed to the larger unsupported excavation length, resulting larger reduction of vertical stress. However, larger invert settlement is induced for the case of intersection angle at 60° at the end of twin-tunnel excavation. This is because first tunnel excavation has mobilized the soil stiffness at the centre of the existing tunnel invert.