Abstract
During the excavation of the Minxian tunnel, problems of large deformations of surrounding rocks and failure of support structures appeared frequently, which caused serious influences on construction safety and costs of the tunnel. Based on laboratory analysis of mineral composition and field investigations on deformation characteristics of the surrounding rocks, the large deformation mechanism of surrounding rocks of the tunnel was considered as water‐absorbing swelling molecules of carbonaceous slate and stress‐induced asymmetric structural deformations of the surrounding rocks. The structural deformations of surrounding rocks mainly include bending deformation, interlayer sliding, and crushing failure of local rock blocks. Then, a new constant resistance and yielding support technology based on the constant resistance and large deformation (CRLD) anchor cable was proposed to control large deformations of surrounding rocks. The field tests and deformation monitoring were carried out. The monitoring results showed that compared with original support measure, the surrounding rock deformations, stresses of primary supports, and permanent lining using new support technology decreased greatly. Among them, the maximum deformation of surrounding rock was only 73 mm. The effects of field application and results of deformation monitoring showed that the new support technology can effectively control large deformations of the surrounding rocks in the Minxian tunnel.
Highlights
In recent years, with the economic development of western China, transportation networks including railway and expressway have been developed rapidly
Several reinforced long constant resistance and large deformation (CRLD) anchor cables were installed on left or right shoulder of the first step according to the dip direction of the surrounding rocks to improve the deformation stiffness of key positions. e group of long CRLD anchor cables can absorb the deformation energy of rock mass and release the stress of surrounding rock through slippage of sleeve pipe, and apply constant resistance to the surrounding rock to limit its continuous deformation at the same time
There is a period of downtime for 30 days. e monitoring results of seven points are shown in Figure 16. e total convergence deformation of surrounding rocks decreased significantly under the control of new yielding support with constant resistance. e deformations of all monitoring points were within 100 mm after the surrounding rock being stable. It means that this new support measure can effectively control large deformation of surrounding rock in the Minxian tunnel. e maximum deformation appeared at the right shoulder since the angle between the CRLD anchor cable and strata inclination was smallest
Summary
With the economic development of western China, transportation networks including railway and expressway have been developed rapidly. A combined support countermeasure including grouting reinforcement, steel tubes advanced support, shaped steel frame, doublelayer steel meshes, and secondary lining was proposed and applied successfully in the tunnel It can be seen from the cases above that the combination of flexible and rigid supports is unified control principle for large deformation problem of the tunnel in the world. The existing rigid support technologies (e.g., traditional rock bolt, heavy steel frame, or sprayed concrete) have high strength but small deformation stiffness, which will lead to failure of the supports when the deformation of surrounding rock is bigger than the allowable value Both flexible and rigid supports are usually constructed separately in the field, which reduces effect of their combination. The monitoring results show that the large deformation of surrounding rock was effectively controlled by this new support method. is successful case may be a valuable reference for similar tunnels in the future
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