In the framework of the engineering of the gas pipeline project W ijngaarden – Beverwijk from N.V.Nederlandse Gasunie new insights were developed about the modeling of soil friction along horizontal directional drillings during operation.Modeling of the soil friction is a vital part of the stress assessment of HDD crossings. The soil friction determines in a large amount the expansion forces (due to temperature and pressure)on the upper vertical bends of the HDD.Normally those bends are cold or hot formed (not elastic). Based on the current status of modeling according to NEN 3650-1 and NEN 3650-2 (Dutch pipeline design codes)studies have been carried out to determine how the soil friction acts on HDD’s.Based on these outcomes a refined way of modeling soil friction along HDD’s is proposed. Elaboration on this subject is approached by:

• an introduction and motivation for study
• a description of the current calculation model according to Dutch pipeline code (NEN 3650-1: 2003)and its backgrounds;
• a break down of current calculation model along HDD’s,discussed per typical HDD pipeline crossing: 1xDN100 (Ø 114,3mm),1xDN600 (Ø 610 mm),1xDN1200 (Ø1219mm).
• a description of the amended model and theoretical verification;
• a discussion of results and conclusion.

Based on the gained insights of the study and the proposed approach for modeling soil friction along HDDs a verifiable and realistic approach is deduced which is applicable for small and large diameter pipelines in soft and sandy soils under elevated temperature conditions. The presented approach is on main issues already aligned with permit stakeholders on project level (for instance Rijkswaterstaat)and after final approval will be formally issued to the Dutch Norm Committee for transport pipelines as amendment in the current revision process of NEN 3650-1: 2003.