Numerical Investigations on District Heating Pipelines under Combined Axial and Lateral Loading

Within the design process of district heating networks, the soil resistances in axial and lateral pipeline direction are commonly treated independently as friction resistance and bedding pressure. However, at curved segments or near ellbows, these resistances occur simultaneously and affect each other. The state of knowledge regarding this topic is summarized, and it is shown that only limited information exists for this case of loading. Therefore, a three-dimensional finite element model was developed, using the sophisticated concept of hypoplasticity as an advanced constitutive model for the bedding material. This soil model is able to account for dilatancy, barotropy and pycnotropy of granular soils. Subsequently, variations of the loading direction were performed for a reference system. The investigations give a good insight into the behaviour of district heating pipelines under combined loading, showing the interdependency of skin friction resistance and bedding pressure. We present a design approach which incorporates interaction terms, derived from the presented investigations. Results gained from these investigation are then transferred to the academic district heating network design tool IGtH-Heat, to evaluate in which manner the incorporation of coupling terms between bedding and friction resistance influences the pipe-soil interaction. Additionally, a temperature dependent formulation of maximum friction resistance is adopted to incorporate the effects of radial pipe displacement. Thereby we demonstrate that the predicted pipeline's displacement significantly change when these effects are taken into account. Using this new formulation, model predictions are compared to data from full scale field measurements