Study on the protective effect of ground concrete bedding layer on buried pipelines under the rockfall impact

Concrete pipelines are widely used in major water transmission network projects in China, such as the South-to-North Water Diversion Project. However, buried pipelines in mountainous areas are prone to damage and leakage due to rockfall impacts. To investigate the protective effect of ground concrete cushion layers on buried pipelines, field rockfall impact tests were conducted by pre-burying multi-section bell-and-spigot concrete pipelines and casting in-situ concrete cushions on the ground. Combined with the DH8302 dynamic strain testing system, the spatial distribution characteristics of dynamic strain in the pipeline body and the variation law of earth pressure at the bell-and-spigot joints were analyzed. The LS-DYNA numerical simulation software was used to establish a detailed model of the rockfall impact test, and the reliability of the numerical model was verified by comparing simulation results with test results. By increasing the impact energy of rockfalls, the failure characteristics of buried bell-and-spigot concrete pipelines were studied. The influence mechanism of concrete cushion parameters (thickness and strength) on the protective effect was further analyzed by varying these parameters. The results show that: (1) Under the condition of a burial depth of 2 m, unstable crack propagation in the pipeline body is more likely to cause leakage of bell-and-spigot concrete pipelines under rockfall impact; (2) The peak tensile strain in the pipeline body decreases nonlinearly with the increase of cushion thickness and strength. The cushion thickness must exceed a critical value (15 cm) to significantly dissipate energy, and there is an optimal strength range (C30-C35) – excessive strength enhancement will reduce protective efficiency; (3) Cushion thickness accounts for 74% of the protective effect contribution, indicating that the design principle of "geometry prior to material" should be followed. It is recommended to use a concrete cushion with a strength of C30-C35 and a thickness of ≥0.2 m, which can significantly reduce the risk of pipeline impact damage and provide a quantitative design basis for pipeline protection in mountainous areas.