Study on the Effect and Mechanism of a New Capsule Technology on Tunnels Under Multi-Step Excavation

With the intensive development of urban underground space, excavations adjacent to existing tunnels have become increasingly common. This study investigates the response of adjacent tunnels and surrounding soil to multi-step foundation pit excavation and the effect and mechanism of a new capsule technology. Centrifuge model tests and finite element analysis were conducted for models both with and without a capsule. The results show that the soil deformation caused by each excavation step is confined to an influence zone. As the excavation deepens, this influence zone progressively expands. Excavation causes the tunnel to move outward and the retaining wall to rotate clockwise. The results demonstrate that capsule pressurization can effectively reduce the maximum horizontal displacement of the adjacent tunnel by approximately 30–40% compared to the case without reinforcement. Capsule pressurization alters the earth pressure distribution on the retaining wall and reduces tunnel displacement. The effect of the capsule decays with increasing distance from the capsule to the tunnel. The excavation impact propagates to the tunnel via wall–soil and soil–tunnel interactions. Capsule pressurization mitigates the tunnel response by ensuring that the surrounding soil experiences a smaller reduction in horizontal stress and exhibits a higher modulus during subsequent excavation. This enhanced state of the soil produces smaller deformation, which ultimately transfers less of the excavation effect to the tunnel and controls its displacement. The study concludes that the active pressure control offered by the capsule technology is a promising method for protecting existing tunnels during adjacent deep excavations.