Construction risks of large-section pipe jacking:The 9.1 m × 4.4 m large-section pipe jacking is prone to machine pitching and torsion, and in mucky soft-soil strata the construction risks increase sharply. Through end reinforcement, as well as measures such as strengthened attitude monitoring during jacking and auxiliary lifting by thick mud pressure via correction jacks, the pitching and attitude torsion of the pipe jacking machine were effectively controlled.

    Pressure of traffic and pipeline protection:The pipe jacking construction not only passes beneath the busy main road of Century Avenue but also beneath multiple pipelines such as rainwater and communications. The crossing range consists mainly of mucky silty clay and silty sand strata, and the highly disturbance-sensitive geological conditions increased the difficulty of controlling ground deformation and protecting pipelines.

    During pipe jacking construction, through reasonable muck conditioning, strict control of the balance between excavation volume and muck discharge at the face, and reasonable control of the faceearth pressure, stable pressure-retaining muck discharge was achieved. At the same time, patented technology was used to inject composite thick colloidal mud in front of the machine head, around the machine body and around the pipe segments to squeeze and fill ground loss, reducing stratum deformation and construction disturbance. Throughout the process, monitoring of the road surface and pipelines was strengthened and intensified at a frequency of 3 times per day, and monitoring data were promptly fed back to the operators to guide the jacking construction. Through the above measures, the safety of road traffic and pipelines was ultimately effectively ensured.

    Construction risks in unfavorable strata:The pipe jacking crosses mucky soil and silty sand layers, typical soft-soil geology. These strata are particularly sensitive to construction disturbance, especially the silty sand layer, which has a loose structure and poor self-stability and is prone to quicksand under hydrodynamic action, aggravating soil and water loss and causing ground settlement or heave, and is also prone to screw conveyor gushing. During construction, by strengthening muck conditioning measures and optimizing the ratio of conditioning agents, the workability of the muck was well improved and screw conveyor gushing was effectively avoided. By closely monitoring groundwater levels, quicksand and seepage failure were prevented; by optimizing jacking construction parameters, strengthening grouting technology, and real-time monitoring and other measures, the impact of stratum disturbance sensitivity was minimized to the greatest extent, making ground deformation controllable.

    The launching shaft structure for pipe jacking is complex, and construction faced three main difficulties:First, the existing reaction wall was only a 300 mm thick partition wall, and the distance from the main structure of the shaft wall reached 2.2 meters, so the jacking reaction force could not be effectively transmitted to the main structure; second, there was a large height difference between the shaft bottom slab and the outer bottom surface of the pipe jacking passage, so the pipe jacking machine could not directly jack along the guide rails on the launching shaft bottom; third, the already constructed shear walls and floor slabs were numerous, resulting in narrow working space and difficult equipment lifting and installation. In response to the above difficulties, measures such as adding a temporary reaction structure between the partition wall and the main structure, erecting a 5-meter-high temporary launching steel platform, and optimizing lifting equipment effectively solved the problems.

    Efficiency constraints of the three-cell utility tunnel:The pipe-jacked utility tunnel was designed in a three-cell form, and no partition wall was provided in the middle during precasting of the pipe segments. Because of the large span of the pipe segments and the lack of mid-span support, insufficient load-bearing capacity at the pipe crown was found during pipe jacking construction, so two rows of steel supports were added inside the pipe. However, this operation compressed the pipe jacking working space by 40% and reduced muck discharge efficiency. During construction, the problem was effectively solved by optimizing the size of the muck cars and increasing their number.