Loose earth pressure of tunnels considering progressive failure of loosen zone

The loose earth pressure of shallow shield tunnel is closely related to the soil arch effect and the progressive failure of the loosen zone. Based on the ellipsoidal theory, the elliptic loosen zone model is established, and the relationship between the ground loss and the loosen zone height is proposed. The process of progressive failure and the limit state are defined. Considering the soil cohesion and the ellipse shape of the loosen zone, the lateral earth pressure coefficient under arbitrary dip angle of slip surface is obtained by means of the large principal stress trace method. The formula for calculating the loose earth pressure at tunnel top is derived and verified. The parameter analysis is carried out for the limit state and non-limit state, and the research results show that: (1) The loose soil pressure at tunnel top decreases with the increase of the internal friction angle and cohesion. (2) With the increase of the loosen zone height, the loose earth pressure at tunnel top decreases sharply first, then increases gradually, and finally tends to be stable.