There are a lot of complex problems involving a number of conflicting factors when planning a TBM drive in squeezing ground. In this respect, numerical analyses represent a helpful decision aid. In the present paper, Beheshtabad water transmission tunnel is introduced, and mechanized full face and drill and blast excavation challenges in squeezing ground are investigated. Then the geomechanical rock mass parameters in the 19th zone are determined. In following, the approach to thrust evaluation and effective parameters on that is discussed, and the required thrust for penetration in the work face on the basis of advance rate, cutterhead geometry and compressive and tensile strength of the host rock is calculated. Then the numerical analysis pattern and the achieved results are investigated. According to these results, if three and ten cm overcutting were performed in the most and least favorable geotechnical condition respectively, double shield TBMs could be used in the 29030–31600 and the 34900–37490 Km of the tunnel. In the 31600–34900 Km, utilization of single and double shield TBMs is not possible, and decision on the usage of double and single shield TBMs in this section could be made if an exploration gallery were planned, and additional insitu tests were done for the determination of long term parameters of the host rock.
The requirement of water in the central part of Iran has increased because of Increasing of population, industry concentration, decreasing water quality and underground water level reduction in this area. To provide the water necessities for human, industrial and agricultural, the Beheshtabad water transmission tunnel which its length and lined Diameter is 65 Km and 6 meter respectively is under investigation. In accordance with overground and underground investigations, geophysics studies, drilled boreholes and engineering geology mapping, the tunnel route is broken up to 29 zones. The high squeezing conditions in the 19th zone will take place according to high tunnel convergence, which is obtained by close form solution, because of high overburden and weak geotechnical conditions. The tunnel in this chainage will be excavated by TBM and for machine type selection, calculation of the required thrust for excavation is essential, so in this research, according to 3, 5 and 10 Cm over excavation in the 19th zone, the necessary thrust and appropriate method of excavation are determined.
Squeezing refers to the phenomenon of large time dependent deformation during tunnel excavation. It occurs mostly in week rocks with high deformability and a low strength, often in combination with a high overburden. In addition, there are considerable challenges for tunnel excavation if tunnel convergence exceeds 5 percent due to high squeezing conditions . Various types of TBMs have different challenges in squeezing ground according to their thrust and existence or inexistence of the shield. Also drill and blast technique in contrast to mechanized full face excavation has flexibility in over-excavation quantity, but on the downside, the advance rate of this method is very slow.