Support method of preset internal cable for in-situ tunnel expansion project

A support method of preset internal cable for in-situ tunnel expansion project, comprising a cable, a lockset and an anchoring agent, and is characterized in that:

• before excavation of the newly-expanded tunnel, a borehole is drilled into the surrounding rock from the in-situ tunnel, and the cable is installed in the borehole and pre-stressed, so as to play a supporting role at the moment of excavation of the newly-expanded tunnel;
• the cable is fixed in a depth of the surrounding rock by the anchoring agent, and the lockset of the cable is set at a position of the excavation contour line of the newly-expanded tunnel inside the surrounding rock;
• the support method specifically comprises the following steps:
• (1) drilling small-aperture boreholes at a lining of the in-situ tunnel, and the small-aperture boreholes extend from the lining of the in-situ tunnel to the depth of surrounding rocks outside the excavation contour line of the newly-expanded tunnel;
• (2) constructing an expanded borehole at a position of the lining borehole of the in-situ tunnel, and the expanded borehole extends from the lining of the in-situ tunnel to the excavation contour line of the newly-expanded tunnel, and the diameter of the expanded borehole is larger than that the lockset of the cable, so that the lockset of the cable can be directly installed at the excavation contour line of the newly-expanded tunnel;
• (3) one end of the cable is anchored in the small-aperture borehole by means of the anchoring agent, and then the lockset of the cable is installed at the bottom end of the expanded borehole;
• (4) after the anchoring agent has reached a certain strength, the outer end of the cable is tensioned by a tensioning device, and the lockset is pushed internally by a retractable thrusting device to prevent the lockset from moving outwards, thus applying prestress to the cable;
• (5) after the excavation of the newly-built tunnel, the part of the cable remaining outside the surrounding rock is cut off, and the rest part is used as a permanent supporting structure.