## 1 Introduction  
Urban water transportation heavily relies on dense underground networks of pipelines. However, these pipelines often suffer from leakage due to various factors, leading to soil erosion and ground subsidence [1; 2; 3; 4]. According to a statistical analysis by [PERSON] et al. [5], 55% of urban subsidence incidents in China are caused by pipeline damage, and the likelihood of leakage significantly increases when the pipeline's service life exceeds 25 years [6]. This escalating issue poses a serious geotechnical hazard, endangering residents, buildings, and infrastructure. It has become increasingly prevalent across various regions, resulting in significant socio-economic losses and becoming a destabilizing factor. Currently, most attention is directed towards the rupture of pressurized pipelines. Some studies suggest that ground subsidence originates from the erosive action of high-pressure jets on the soil, creating liquefaction phenomena in the surrounding soil layers due to high-pressure fluid scouring [7; 8; 9]. However, researchers have also highlighted the complexity of the interaction between pipelines and soil, owing to changes in the geometry of unconfined flow regimes, piping, and hydraulic fracturing [10]. Unpressurized pipelines for urban rainwater and wastewater drainage are equally critical infrastructure for modern cities. When these pipelines experience leakage, they cause damage to the surrounding soil. [PERSON] et al. [11] summarized the damage process caused by seepage leakage in sewer pipelines, where the soil at the leakage point becomes fluidized and eroded, forming an erosion zone. Extensive studies have also been conducted on the infiltration rate of water through the pipeline leakage point [12; 13; 14; 15]. The erosion process may last for an extended period, forming a low-strength zone at the leakage point, with groundwater gradually propagating towards the surface [16]. [PERSON] [17] identified four variables influencing the rate of soil loss after damage to unpressurized pipelines: (1) size of the leakage point; (2) groundwater level; (3) amplitude and frequency of surcharge; and (4) soil properties. In coastal and river areas, or after continuous rainfall, cities often encounter high groundwater levels. [PERSON] et al. [18; 19] conducted an in-depth investigation into the intricacies of groundwater models. with unpressurized pipelines typically located below the water table. In such cases, pipeline leakage leads to the gradual formation of a cavity as soil particles are washed into the pipeline with groundwater [20; 21; 22; 23]. The formation of cavities not only causes ground subsidence but also weakens the support for the pipeline, resulting in structural damage [24; 25; 26; 27]. Previous studies have predominantly relied on macroscopic physical model experiments. Currently, the DEM-CFD approach has been extensively applied across various fields for fluid-solid interaction problems. [PERSON] et al. [28] validated open-source DEM and DEM-CFD coupling models and algorithms. [PERSON] et al. [29; 30] established mechanics-based models incorporating fluid volume and discrete element coupling, along with a fluid-solid coupling model elucidating the MFSV vibration mechanism, investigating multiphase mixed transport mechanisms. [PERSON] et al. [31] explored the Two-Fluid Model (TFM) approach. [PERSON] et al. [32] utilized the DEM-CFD method to investigate the impact of landslides blocking river channels and shock waves. [PERSON] et al. [33] analyzed the kinematic behavior of underwater concrete during pouring, while [PERSON] [34] investigated rock fracturing under the concurrent influences of thermal and hydraulic actions. Concurrently, the advancement in granular mechanics has laid the foundation for a finer-scale exploration of soil failure resulting from unpressurized pipeline rupture [35].