ВЛИЯНИЕ СТРАТИФИКАЦИИ ГРУНТА НА ПОДЪЕМНУЮ СИЛУ, ДЕЙСТВУЮЩУЮ НА ТРУБОПРОВОД ПРИ ПРОТАИВАНИИ МЕРЗЛОТЫ

During the operation of underground pipelines in permafrost soils, cases of soil thawing around the pipe and its subsequent upward floatation are widely reported. This phenomenon is attributed to winter construction activities dictated by local natural and climatic conditions, where backfill soil differs in its key properties (ice content, moisture, thermal conductivity) from the soil properties considered in the design calculations. As a result of operations, underground pipelines become flooded and are forced to the surface with insufficient ballasting. The equilibrium between buoyant forces and restraining loads is disrupted, and the pipeline stability against floatation is not ensured.Practice shows that the buoyant force should be determined considering the variation in liquefied soil density along the height of the silted trench.The article presents computational models for the interaction of an underground pipeline laid in a trench silted up with liquefied soil during permafrost thawing. The most critical case regarding underground pipeline stability loss was selected for analysis: water-saturated trench completely filled with slumped soil and silt. The sedimentation of mineral particles and the formation of a silted zone are characterized by non-uniform density along the trench depth, reaching a maximum value beneath the lower generatrix of the pipe. In the presence of substantial amount of meltwater, a layer of settled water may form above the pipeline.Based on Archimedes' principle, a formula was derived to determine the buoyant force acting on an underground pipeline situated in the water-saturated heterogeneous soil within a silted trench. Calculations for the case under consideration, accounting for the linear change in density of the trench fill with height, showed that the buoyant force increases by 1.7 times compared to the value determined using standard methodology (based on a water medium); this necessitates a corresponding increase in pipeline ballasting.The most challenging scenario involves the stratification of water-saturated frost-heaving soils around a floating pipeline during the autumn-winter period, when the growth of a frost heave mound and its ice core causes the pipeline to arch upward and the ejection of the pipeline from the trench.Accounting for soil heterogeneity along the trench depth when calculating the buoyant force will allow for the appropriate ballasting for the pipeline section, thereby improving its resistance to floatation. This will ensure that the pipeline remains in its design position, enhancing both its safety and operational reliability.