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Mechanisms of Soil Erosion due to Defective Sewer Pipes

  • Author / Creator
    Tang, Yao
  • Sinkholes are frequently reported around the world, and soil erosion around defective sewer pipe is found to be one of the possible cause of sinkholes. As water infiltrates through the defect in a pipe, soil can be washed into the pipe leading to a cavity or even sinkhole formation. Besides, water exfiltration through pipe defect can fluidize the adjacent soil leading to erosion. This thesis is focused on the mechanisms of soil erosion due to the defective sewer pipes. Experiments were conducted to simulate soil erosion through a slot on a pipe for the two-dimensional condition, and the erosion through an orifice under three-dimensional condition was also studied. In the erosion process, it shows a steady relationship between the sand and water flow rate before the erosion void reaches the defect, and the relationship is dependent upon the sand particle size and defect size. The position of the defect on the pipe will affect the formation of erosion void while it has little effect on the sand flow rate in the erosion process. A coupled three-dimensional discrete element model has been developed to simulate water/sand flow through an orifice. The water flow is simulated based on Darcy's model, which indicates this numerical model is valid if Reynolds number is less than 10, and the interaction between the fluid and solid phase is taken into account. The ‘supply layer’ is incorporated to study the continuous erosion process, which can simulate laboratory experiments on sand flow using this coupled model considering the current computing capacity. If the sand is assumed to be the uniform sized circular particle, an analytical model is developed to predict the free-fall arch formation as the granular particles flow through a two-dimensional opening. Based on numerical simulations using discrete element method, the assumption of free-fall arch is shown to be reasonable. Based on this free-fall arch theory, an analytical method is proposed to account for the effect of water flow on the granular discharge, which is developed using Stokes’ law. A numerical model based on computational fluid dynamics and kinetic theory of granular material is used to investigate the sand-bed erosion by an upward water jet. The numerical simulation shows that the inlet water velocity causing the sand-bed erosion increases as sand particle size increases. The increase in sand-bed height also increases the critical water velocity, whereas the critical velocity decreases as the decrease of sand friction angle. An analytical model based on the force equilibrium was developed to predict the critical water velocity. In this analytical model, the water flow was assumed as a uniform distribution over the mobilized zone in one-dimensional condition. The particle size in the sand bed was assumed to be uniformly distributed, whereas the shape and angularity of particle were taken into account using the sphericity coefficient. From this thesis study, the sand particle size and defect size are the key factors on the soil erosion due to defective sewer pipes. In the soil erosion by water infiltration, the particle size can significantly affect the water flow due to the change of soil permeability. The steady relationship between sand and water in the erosion process can be explained by the free-fall arch theory and Stokes’ law. The sand particle velocity reaches a small value as it moves to a specific boundary close to the defect, and the particle velocity will be significantly increased due to the gravity and drag force by water flow. From the theoretical derivation, it has been found the size of this boundary is dependent on the particle size and defect size, which is independent of the stress state above this boundary. In the study of sand-bed erosion by an upward water flow, the particle size can affect the seepage force on the mobilized sand bed, while the defect size controls the spread of water jet. Therefore, the particle size and defect size are also essential in the analysis of sand-bed erosion by an upward water jet. The numerical and analytical models in thesis provide effective approaches to predict the soil erosion due to the defective sewer pipe, which also provides methods to carefully determine the particle size around the sewer pipe to reduce the soil erosion.

  • Subjects / Keywords
  • Graduation date
    2017-11:Fall 2017
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R32N4ZX83
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Doctoral
  • Department
    • Department of Civil and Environmental Engineering
  • Specialization
    • Geotechnical Engineering
  • Supervisor / co-supervisor and their department(s)
    • Zhu, David (Civil and Environmental Engineering)
    • Chan, Dave (Civil and Environmental Engineering)
  • Examining committee members and their departments
    • Nouri, Alireza (Civil and Environmental Engineering)
    • Indraratna, Buddhima (Civil Engineering, University of Wollongong)
    • Deng, Lijun (Civil and Environmental Engineering)
    • Bindiganavile, Vivek (Civil and Environmental Engineering)