Abstarct: Sudden tunnel collapses, such as cave-ins, are among the most critical and severe forms of underground instability. They occur abruptly and can lead to consequences such as construction delays, increased costs, threats to worker safety, and even the overall failure of the project. Therefore, precise geomechanical and numerical analysis of this phenomenon is an undeniable necessity in tunnel design and construction. The occurrence of a collapse in Tunnel 32 (eastern section) of the Tehran–North Freeway provided the motivation for this research. To simulate the collapse process, a high-resolution three-dimensional graphical model was constructed baxsed on photogrammetric data from Tunnel 32. This model offered a realistic representation of the actual geometry and collapse pattern. Subsequently, using the Distinct Element Method (DEM) in 3DEC software, a three-dimensional numerical model of the rock mass conditions around the tunnel was developed. In this model, boundary conditions, in-situ stresses, mechanical properties of the rocks and joints, and staged excavation were carefully defined to enable realistic representation of the instability mechanism. Additionally, a specific approach was adopted to define a weak zone in order to simulate the progressive development of the collapse. Finally, comparison between the numerical model and the graphical model showed that the numerical simulations successfully reproduced the collapse pattern, deformations, and failed volume with a high degree of accuracy, achieving the closest agreement with field observations. This consistency not only validates the numerical modeling approach but also provides a basis for more detailed analysis of the factors influencing the initiation and propagation of collapse, thereby offering valuable insights for improving the accuracy of future tunnel designs.