Scale model test of highway tunnel construction underlying mined-out thin coal seam

The construction of a highway tunnel adjacent to an overlying mined-out thin coal seam disturbs the surrounding rocks, thus affecting the stability of tunnel structures. This paper presents scale model tests in a 1g gravitational field on the tunnel excavation undercrossing the caved zone of the goaf. In the tests, different caved zone–tunnel distances (d) were considered as well as the inclined coal seam with different dip angles. Settlement on the roof and floor of the caved zone and vault settlement of the tunnel during excavation were tested and compared. It was shown that in the caved zone the settlement on the roof was greater than that on the floor. The caved zone also affected the vertical and lateral loads on the primary support of the tunnel. The results showed that d has great influence on the vertical pressure in the case of a horizontal coal seam while the dip angle affects the lateral pressure significantly for an inclined coal seam. This means that in the case of a horizontal coal seam, the closer the caved zone, the greater the average vertical pressure and the maximum bending moment, and the smaller the corresponding axial force. As for the inclined coal seam, the greater the dip angle, the larger the moment and axial force of primary support. A significant pressure bias was also revealed in the case of an inclined coal seam with the dip angle of 30°.