Uplift resistance mechanism of pipes in lightweight backfill material of ceramsite

Abstract

Buried pipelines crossing active fault zones undergo significant seismic displacement, inducing substantial soil resistance that can damage the pipe structure. Employing lightweight backfill materials offers a potential mitigation strategy. This study investigates the suitability of ceramsite—a lightweight material characterized by its smooth surface and low density—as a novel backfill, specifically examining why it generates lower resistance than conventional silica sand. Comprehensive evaluation combined element tests comparing ceramsite and silica sand properties with model experiments and discrete element method (DEM) simulations of pipe uplift. Results demonstrate significantly lower uplift resistance in ceramsite compared to silica sand, indicating its potential to enhance the seismic performance of pipelines in fault zones. DEM simulations attribute this reduced resistance to ceramsite's lower self-weight, more convergent slip surfaces, and lower mobilized shear stresses on the slip surface. The convergent slip surfaces stem from ceramsite's lower internal friction angle, enabling surrounding particles to slide into the void beneath the uplifted pipe, causing the slip surfaces to converge centrally. Stress monitoring confirmed significantly lower mobilized shear stresses in ceramsite. Furthermore, the stress path followed by both materials on the slip surface was found to nearly follow drained paths up to peak resistance. This observation challenges the constant-mean stress trajectory assumption prevalent in prior studies, which tends to underestimate uplift resistance.