Abstract: The looseness range of surrounding rock is easy to overlay and enlarge when tunnelling adjacently underneath triple-decker mined-out area, the asymmetric movement of formation may lead to asymmetric loads to tunnel structure. Indoor similarity model test of tunnel excavation underlying triple-decker mined-out area was carried out; the ground movement of the mined-out area and internal forces of the primary supporting structure were gauged with the displacement meters, the earth pressure cells and the electric resistance strain gauges embedded in the earth. Test result of displacement predicts that in a specific dip angle range the smaller dip angle of mined-out area will cause larger settlement rate, steeper curve, slower convergence speed later and bigger settlement value. With the increase of distance between the strata and tunnel, strata disturbance degree first increases and then decreases; a certain angle of mined-out area will make the earth pressure of one side of the inverted arch where relatively far from the mined-out area significantly larger than another side; therefore, the inverted arch’s structure design should be strengthened. In the range of 15° to 30°, the existence of dip angle has little influence on distribution form and size of surrounding rock pressure; but greatly affects the internal forces of the steel arch; the larger the angle is, the bigger the primary support eccentricity is. For the fourth grade of surrounding rock, when two-lane tunnel is built adjacently(2 m) underneath triple-decker mined-out area, it’s strongly recommended to support in advance, then excavate by steps method, excavation footage should be not more than 1m, steel arch span should be not more than 0.5 m.

Key words: highway tunnel, triple-decker mined-out area, model test, ground movement, primary support