Displacements of buried pipelines due to sheet pile extraction in soft clay - A study on pipeline displacement using numerical modelling

Abstract

Large parts of the Gothenburg region consist of soft and sensitive soils, making the ground prone to settlements when ground conditions are altered. Such changes commonly occur during installation and replacement of water and wastewater pipelines. These projects are often considered as a low-budget project, which often results in insufficient geotechnical investigations and measurements, and have in many cases led to disputes and expensive settlements following soil deformations. Pipeline installation is often carried out in trenches supported by retaining walls, often sheet pile walls, and while settlements caused by the unloading and reloading during excavation are relatively well understood, available studies on the effects of the sheet pile extraction remain scarce. The aim of this study is to investigate how sheet pile extraction influences soil deformations, how the void created from the sheet pile is filled and further enlighten that sheet pile extraction can cause significant pipeline settlements in soft clay. The work was carried out by using numerical modelling in PLAXIS 2D together with comparison with a previously conducted full-scale field test in the Gothenburg region. The numerical model in PLAXIS 2D was calibrated against observed horizontal and vertical displacements of the full-scale field test for the initial phases leading up to the sheet pile extraction, in order to capture the soil behaviour. Thereafter, different modelling approaches for simulating sheet pile extraction in PLAXIS were evaluated based on their ability to reproduce the observed horizontal displacements. The selected method, where low stiffness- and strength- parameters are applied to the soil surrounding the sheet pile, was then used to simulate the vertical settlements induced by the sheet pile extraction. The results show that the numerical model was able to reproduce the general soil behaviour for all phases during pipeline construction. Furthermore, the results indicate that the majority of settlements originate from reloading the excavation with heavier material, but that the extraction of the sheet pile contributes to considerable settlements that should be taken into consideration. Lastly, this study contributes to an improved understanding of the additional settlements induced by sheet pile extraction in soft soils, although further and more extensive research is still considered necessary.