| Author: | Guo, Chang |
| Title: | Axial soil-pipeline interaction under different soil moisture conditions : physical and dem modelling |
| Advisors: | Zhou, Chao (CEE) Yin, Jian-hua (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxxiii, 265 pages : color illustrations |
| Language: | English |
| Abstract: | Buried pipelines are called lifelines due to their critical role in economically and efficiently transporting natural resources. Soil-pipe relative displacements caused by landslides, earthquakes and thermal expansion/contraction can impose significant loads on pipeline systems, inducing the risk of failures. A thorough understanding of soil-pipe interaction behaviour is crucial for accurately predicting these loads. This study focuses on the axial soil-pipeline interaction (ASPI). Current design guidelines and previous research have not fully addressed three key factors of ASPI, potentially underestimating soil loads or pipe deformations. Firstly, surface roughness and coating hardness are critical determinants of soil-interface shearing behaviours, including friction angle and dilatancy, but the dilatancy effect on axial resistance hasn’t been fully considered. Secondly, pipelines carrying hot fluids are frequently subjected to cyclic displacement relative to the surrounding soil due to thermal variations. This cyclic loading can significantly affect axial resistance on pipes during and after loading. Thirdly, pipelines are often buried in partially saturated soils with negative water pressure (matric suction). Matric suction can make unsaturated soils stiffer and stronger, increasing the interface shearing strength, dilatancy and, thus, axial resistance. To enhance the understanding of ASPI, this study aims to (i) investigate how surface roughness, coating hardness and cyclic displacement amplitude affect axial resistance of pipes buried in dry soils through physical modelling; (ii) reveal how matric suction and water content affect axial resistance of pipe buried in unsaturated soils through physical modelling; (iii) uncover the mechanisms of ASPI by developing a new code using Discrete Element Method (DEM); and (iv) develop a new method for predicting the axial resistance of buried pipes. This study developed a large-scale physical modelling system to explore ASPI. A single-point-type tactile pressure sensor was developed to measure interface contact and earth pressures. Twenty-seven large-scale physical modelling tests were conducted at varying moisture conditions, roughness and coatings, and numbers and amplitudes of cyclic loading. An advanced 3D DEM code was developed to provide microscopic insights into ASPI. The pipe is modelled as a clump of overlapping particles to simulate roughness. A scaling factor for the surface tension coefficient, equalling to that for particle size, is used in calculating capillary forces, addressing particle size scaling effects in the unsaturated soil simulation. Based on the results of physical and DEM modelling, the major conclusions are as follows: Roughness and hardness are decisive for ASPI. The monotonic axial resistance of rough pipes in dry sand is 2.70~2.85 times that of smooth pipes. 72~79% of this increase is due to the interface friction coefficient increasing with the roughness increasing. The remaining is due to the interface contact pressure increase induced by constrained dilation and negative soil arching, as evidenced by soil particle movement and strong force chains developed from the pipe crown and invert in the DEM simulation. A critical hardness was observed (around 35 HRA). Pipes with lower hardness behave like rough pipes due to equivalent roughness from particle embedding. With the cyclic loading, axial resistance degrades with increasing cycles and stabilises at 32~62% of the monotonic resistance. This is mainly because ongoing pipe settlement translates negative soil arching to positive soil arching, reducing interface contact pressure at the crown and shoulders and thus lowering the average interface contact pressure as predicted. The post-cyclic resistance exceeds the monotonic resistance when the cyclic displacement is smaller than 5 mm due to cyclic loading-induced soil densification. The axial resistance at unsaturated conditions increases with matric suction under constant nominal pressure. At a suction of 70.2 kPa, the resistance was 1.69 times greater than in the saturated condition. 68% of this increment is attributed to the additional interface contact pressure induced by interface capillary forces. The remaining is related to net interface contact pressure increase due to higher constrained dilation under unsaturated conditions, as evidenced by larger particle displacements and stronger force chains in the DEM simulation. Finally, a new and simple equation was proposed to predict axial resistance by considering roughness effects on the interface friction angle, constrained dilation effects on interface contact pressure, and suction effects on Bishop’s interface contact pressure and constrained dilation. The new equation successfully predicts results from previous and current studies. |
| Rights: | All rights reserved |
| Access: | open access |
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