Thông thường các phương pháp khai quật: khoan và nổ, cứng đá TBM, bàn tay khai quật ở 3 vùng đất sét. Đá gia cố tạm thời và hỗ trợ đường hầm thường xuyên có thể là bất cứ điều nào sau đây: CCA, S (fr) + RRS + B, B + S (fr), B + S, B, S (fr), S, sb, (NONE). | 186 Tunnelling in weak rocks the slot-wedge i the length of the hole need not be precisely equal to that of the bolt and ii the bolt can be used in soft rocks also. For example anchorage capacity of expansion shell for 19 mm bolt ranges from 3 to 10 tonnes for soft to medium shales. However borehole diameter has to be slightly larger than that for slot and wedge type bolt of the same diameter. In practice surface of the excavation is rarely flat and perpendicular to the axis of the bolt. As such steel bearing plates of size 10 X 10 cm or 15 X 15 cm are used to bridge irregularities on the rock surface and provide firm bearing surface for the washer and the nut Fig. . As the bolt is tensioned the rock asperities are crushed to provide the required bearing area. With blasting vibrations the crushed material tends to become loose and at times spalling of the rock above the plate occurs leaving the bolt to hang in the air. Thus the bolt should be checked periodically and retightened. This is a rule which should be strictly followed in the practice. If rock bolts are desired to be a permanent system of support all boltholes must be grouted completely with cement grout Fig. or resin. This is for preserving the pretension and preventing corrosion of steel. Steel ribs are also encased in concrete lining for the same reasons. For this purpose either an air tube or hollow bar of high strength is used. While grouting a bolt the rubber grout seal is used to center the bolt in the hole and to seal the collar of the hole against grout leakage. Grout injection is stopped when air has been displaced and the grout flows out from the return tube Fig. . A site engineer should check the flowing out of return grout to ensure the full-column grouting of rock bolts. Cement 25 mm 5 Rebar 25mm dia Rebar 19 mm dia c. Resin Bolt b. Perfo Bolt Resin rẽcartridges Fig. Process of installation of grouted bolts. Rock bolting 187 In situation where very long bolts are .
