Performance of pipeline suspension system for in-situ protecting large-diameter pressurized pipelines straddling deep braced excavation in clays

This paper investigates the in-situ protection technique for buried pipelines straddling deep braced excavation and the relevant structural performance. A deep braced excavation case history for constructing the West Yuanhetang tunnel in Suzhou, China is reported. The excavation is straddled by two shallowly-buried large-diameter pressurized pipelines, with a straddling distance of about 31 m. Considering the significance of the pipelines for the normal life of local citizens, in-situ protective measures is thus necessary for ensuring the pipeline serviceability and stability during excavation. In an attempt to overcome the shortcomings of traditional pipeline protection technique, a new pipeline protection technique based on the suspension method is proposed. This technique takes advantage of the excavation support system. It eliminates the need for installing an exclusive structure for suspending pipelines, which is frequently practiced in conventional methods. Moreover, the original use of several kinds of materials for restraining pipeline displacement and transferring pipeline weight also contributes to a better protection of the pipelines. Furthermore, the performance of the proposed pipeline suspension system during excavation is investigated by using three-dimensional finite element method. The method is validated via reproducing the observed typical behavior of the deep excavation. Results has shown that the maximum axial forces, shear forces, and bending moments in the suspension supporting beams, as well as the maximum axial forces in the finish-rolling screw-thread (FRST) steel bars and the maximum bending moments and shear forces in the composite steel beams, all exhibit a stepwise growth pattern as the construction stage progresses. This research provides practical reference for in-situ protection of buried pipelines within deep braced excavation.