Middle wall is the main bearing component of the double arch tunnel. Non-dense backfill on the top of the middle wall can easily lead to large deformation or collapse of surrounding rocks. In response to this phenomenon

we developed a similar model material and the test equipment and carried out the corresponding destructive model test

by which the evolution process of the collapse was studied. The whole process of collapse has been reproduced by UDEC simulation; and the numerical results are in good agreement with the model test. Collapse mechanism has been derived from the distribution of displacement and plastic zones. Finally

we propose targeted prevention measures which are of reference value for similar projects.