Ground subsidence induced by internal erosion around defective sewer pipes commonly occurs in urban areas of many cities over the world, which has caused substantial socioeconomic loss. Professional pipe design, inspection and maintenance demand a comprehensive understanding of the internal erosion mechanism due to defective pipes. This paper conducts a series of experimental, theoretical and numerical analyses aiming to propose a new method quantifying the erosion process around defective pipes. In this study, a laboratory experimental test of internal erosion was conducted using gap-graded soils, followed by the derivation of a nonlinear incremental governing equation for quantifying the erosion rate of soil on the basis of experimental results. After that, a new type of element was established to incorporate the proposed governing equation of erosion into Mohr-Coulomb constitutive soil model. Finally, this new element was adopted in a 2D soil-fluid coupled finite element analysis of soil erosion due to pipe defect. Results indicate that (1) the hydraulic head greatly affects cavity formation and soil loss, whereas the defect types have a little effect on eroded region, (2) visual images during test show that the soil erosion gradually slows down and eventually stabilizes along with the forming of soil skeleton, (3) the numerical results match well with the experimental results, demonstrating the effectiveness of both the theoretical method and numerical tool for the analysis of soil erosion around defective pipes.