During tunnel construction, deformation often happens. Observing from tunnel surface, there are cracks, shedding phenomenon or underground water dropping appearing. If reason doesn’t get analyzed and maintained timely, worse deformation will happen.
There are four reasons causing deformation of tunnel:
1. Creep effect.
Time factor has many aspects’ influence on tunnel’s deformation and the most important one is creep effect under the condition of constant load for a long time. Fractured rock mass, developmental joint, unstable creep property and non-homogeneous characteristics will cause stress concentration under the effect of external force. Then some parts will appear plastic deformation and transfer stress to adjacent parts, gradually cause deformation of rock mass and even the tunnel. As time going by, deformation is increasing and it’s easy to cause collapse.
2. Inadequate primary support strength.
The geological exploration can’t reflect situation of reflective surrounding rock accurately, it may cause designed parameter small, such as specification and distance of I-steel, thickness of sprayed concrete and predicted settlement can’t reach standard and can’t bear load of big deformation.
3. The function of water.
The underground water in deformation sections is mainly poured out in guttate-shape and rain-shape, partial sections is strain-shape, which has larger influence on deformation. Once there is mud in rock mass, water sliding surface will be formed, then floating force and dynamic water pressure are increasing, accordingly, shearing strength is reduced and deformation is intensified.
4. Construction factor.
During tunnel construction, there exist different factors influencing construction and causing deformation, such as nonstandard parameters, imperfect sprayed concrete, insufficient rigidity, not prompt construction, etc.
According to different geological conditions, different measures should be chosen to prevent further deterioration. Nowadays, self drilling bolt has great effect on supporting project.