Internal erosion in soil can be initiated by suffusion, which involves selective erosion of fine particles within the matrix of coarse soil particles under seepage flow. Suffusion in an internally unstable soil will occur when the hydraulic gradient exceeds a certain critical value. Often the critical hydraulic gradients are found using conventional 1-D seepage tests. The core wall and filters in an earth dam are however typically under complex shear stress states. As internal erosion propagates backward in an earth dam, the soil pipe may collapse and the dam failure process may then evolve into an overtopping process. Most landslide dams fail by overtopping directly. The erodibility of soils, therefore, also plays an important role in evaluating the development of breaching of dams. The main objectives of this thesis are to investigate the internal erosion process under complex stress states, characterize the erodibility of soils, and model the dam breaching process. A stress-controlled erosion apparatus was developed to investigate the initiation and development of internal erosion under complex stress states and to study the mechanical response of soil to internal erosion. It allows independent control of hydraulic gradient and stress state. The hydraulic gradient is controlled using a water-head control method. The eroded soil and the outflow rate are measured using a soil collection system and a water collection system, respectively. Extensive internal erosion tests were conducted on a gap-graded soil under complex stress states following three stress paths: isotropic, drained triaxial compression and triaxial extension stress paths. After loss of a significant amount of fine particles in the soil, the original dilative stress-strain behaviour changes to be contractive one and the shear strength decreases. The entire internal erosion process can be divided into four phases: stable, initiation, development, and failure. Accordingly, three critical gradients named initiation, skeleton-deformation, and failure ...