The instability even collapse of tunnel would occur during the construction in soft ground. A collapse of expressway tunnel which suddenly encountered soft ground took place just after completing the anchor bolts was investigated as the study background. Based on the plastic and damage constitutive laws, the numerical method of finite element was funded for analyzing the failure mechanism of anchored rock mass and support effects of bolts. Some results were concluded as following by comparing the numerical calculations and field investigations, which would be adopted in further similar engineering. The tunnel collapse is a progressive process with the damage evolution and stress redistributions. The failure zone concentrated near the tunnel crown which can be considered to be the main load on the tunnel structure. According to the length of bolts increases or the spacing distance of bolts decreases, the loosening zones, damage zones, and failure zones will simultaneously become less and less. Nevertheless, those effects become less obvious when the length increases more than 5m or the spacing decreases less than 1m in practice.
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