Stone Indentations in District Heating Pipes Caused by Lateral Displacement of the Pipeline - Experimental Studies


Gunnar Bergström and Stefan Nilsson

SP Swedish National Testing and Research Institute,
Box 24036, SE-400 22 Gothenburg, Sweden


Email: Nilsson



The strength of district heating pipes with respect to localised loads from stones pressing against the casing pipe wall is of growing concern as the district heating industry is striving towards the reuse of excavated material as backfill. Unspecified backfill material can contain large stones which, when located adjacent to the pipe wall, may induce severe indentations in the casing pipe which ultimately lead to failure by crack propagation through the polyethylene material. Experimental tests were run aimed at studying the interactions between a laterally displaced buried district heating pipe, a stone situated adjacent to the pipe wall and the surrounding backfill. In particular, the indentation caused by the force concentration from the stone and how the risk for indentations depends on the stiffness of the backfill was investigated. It was seen that a moderately stiff backfill is clearly the least beneficial in terms of limiting the indentation depth. In a very loose backfill, the reaction forces on the stone will not be large enough to produce significant indentations. On the other hand, if the backfill is densely compacted and the wedge between the stone and the pipe is filled with compacted sand, there will be no force concentration effect and thus no indentation.
Keywords: district heating pipe, stone indentation, backfill material

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