CFD-DEM simulation of sand erosion into defective gravity pipes under constant groundwater table

Abstract

As sewer pipes age, cracks and other forms of structural damage may develop leading to soil infiltration into the pipes that can cause sinkholes and damage the overlying roads and moving vehicles. The fluctuations in the groundwater table following snow melt or heavy rainfall events can exacerbate soil erosion posing risk to the surrounding infrastructure. To understand and adequately characterize erosion in such cases, microscale analysis of both sand and water is needed to resolve the complex interactions around the pipe defects. In this study, a detailed numerical investigation is carried out using a series of coupled Computational Fluid Dynamics-Discrete Element method (CFD-DEM) simulations to evaluate the effects of groundwater table, crack size, defect angle, and the height of sand fill above the pipe on erosion. It is found that raising the groundwater table accelerates erosion only to a certain limit, above which, raising the groundwater level does not have a tangible effect on erosion. This helps explain some of the misunderstandings reported in previous investigations regarding the effect of water level on the rate of erosion. With respect to the location of defects, it was found that, when groundwater is involved, the erosion rate is fastest when the defect is located at the pipe crown. However, inclined defects ultimately resulted in a larger spatial extent of eroded mass. In addition, the thickness of the sand fill above the pipe was found to have little effect on the rate of erosion as a freefall arch regime was dominant in all simulations. Finally, simplified relationships were developed for estimating the total eroded mass for given sand properties, defect geometry, and location of the groundwater table.