•Propose the experimental and particle flow numerical models for tunnel leakage.•Predict the settlement of sand layer and laws of water and sand loss.•Explore the distribution of seepage fields by the design of a seepage tracer device.•Discuss the migration and soil arching effect of sand particles from the fine scale.•Conduct the parametric analyses for different influencing factors of tunnel leakage. Most of urban ground collapses and tunnel failures are caused by joint leakage of shield tunnel lining. This paper conducts a series of model tests and particle flow simulations aiming to investigate the geo-hazards and explore the associated failure mechanism due to tunnel leaking. Based on critical width of leaky joint, taking leaky joint location, overburden depth and height of overlying water level as influencing factors, the settlement of the sand layer and the law of the water and sand loss is effectively predicted. Then, the focus is on designing a seepage tracer device to explore the distribution of the seepage field and its flow line variation. Finally, the particle flow numerical model of tunnel-sand is employed for the soil arching effect and flow line changes during seepage erosion to discover the fine scale influences of sand particle migration and loss around the shield tunnel on the surrounding environment.