A study of pile row barriers close to the retaining wall of a deep excavation to protect existing tunnels: physical testing and a case history

This paper investigates the use of a protective pile row barrier to mitigate the risk due to a deep excavation in soft soils adjacent to an existing tunnel. The study includes two reduced-scale 1 g model tests with and without the barrier, along with monitoring data from a case history in Wuhan, the Huazhong Science and Technology Industrial Park basement; physical testing and numerical sensitivity study results are compared to the case history to evaluate the barrier efficiency. The monitoring data show successful excavation and basement construction within allowable displacement thresholds in the presence of the barrier. The entire tunnel settled and translated towards the excavation, although different types of ovalization of the tunnel cross-sections were recorded depending on their alignment to the excavation (centre and corners). Experimental results indicate that the pile row barrier can withstand a portion of the soil pressure on the diaphragm wall, leading to a substantial reduction in tunnel bending moments and displacements; in particular, experimental results showed a greater percentage reduction in settlements than horizontal movements of the existing tunnel. When a pile row barrier is constructed in proximity to the retaining wall, it can help to minimise the detrimental effects of the deep excavation on the existing tunnel, similar to twin retaining wall scenarios.