<p>Progression and critical conditions of cavitation in underdense soil due to drainage leakage remain unelucidated. This study investigates the evolution process and collapse morphology of soil under varying density conditions—dense, loose, and void—through physical model testing and engineering analysis. Additionally, it examines the critical conditions that lead to soil cavitation. The results indicate that water has an erosive effect on soil, with pronounced impacts on less dense soil. The cavity evolution follows four stages: hydraulic erosion, cavity development, cavity formation, and eventual road collapse. In underdense soil, reduced cohesion and increased permeability accelerate cavity formation. The resulting cavities typically exhibit ellipsoidal cross-sections with a trumpet-shaped distribution. Their longitudinal slope angles are asymmetrical, influenced by the kinetic energy of fluid within the drainage pipe. Furthermore, the rate of porosity change in the soil decreases gradually with increasing distance from the cavity. A sudden shift in this rate can serve as an early indicator of imminent soil cavitation. For silty clay, the cavitation threshold is marked by a sudden increase in the porosity change rate to 50–80% under the tested conditions. These findings provide data and theoretical support for monitoring and predicting road collapse caused by drainage pipe leakage.</p>