Effect of rubber-sand mixture backfill placement on responses of buried pipeline under static and cyclic loading

With the rapid expansion of buried pipeline networks, pipeline damage incidents have become increasingly common. Rubber-sand mixtures (RSM), as lightweight and sustainable backfill materials, have been proposed as an effective solution for pipeline protection. However, the influence of RSM placement and load amplitude on the longitudinal response of buried pipelines has not been systematically investigated. To address this gap, this study evaluates the protective effectiveness of RSM for buried pipelines, with particular focus on backfill placement and load level. Three model tests are conducted via a continuous loading scheme consisting of static stages ST1–ST3 (60, 120, and 180 kPa), followed by cyclic stages DT1–DT3 at the same load levels. The results show that pipe deformation (D), cyclic pipe deformation amplitude (ΔD), soil pressure (P), cyclic soil pressure amplitude (ΔP), and surface settlement (SS) all increase with loading amplitude. Compared with sand under loading at the pipe center, the top-layer RSM configuration reduces pipe deformation and surface settlement up to DT1, and reduces P throughout all loading stages, whereas the fully surrounding RSM configuration (R-A) shows highest response. In both sand and RSM backfills, D and P spread laterally away from the loading center, but this redistribution is more pronounced in RSM. For cyclic response amplitude, both ΔD and ΔP are higher in RSM than in sand. Overall, the findings indicate that RSM is most effective when used as a top-layer backfill above the pipeline. Within typical service loads, this layout supporting its application as a sustainable protective backfill for buried pipelines.