A Closer Look at Prevailing Civil Engineering Practice - European ...

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A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU enhances the cutterhead to be extended. The gripper in the middle section of TBM is mobilized so that it pushes against the tunnel walls to react the boring forces. As these forces are dissipated, it allows the installation of lining segments during tunnel so that it increases the speed of tunneling. Upon completion of a trust stroke, the grippers are retracted and the end portion of TMB is pushed against the front shield by thrust cylinders. Double shielded TBM is normally used in rock strata with geological fault zones and when a high rate of advancement is required. Single shielded TBM is more suitable to hard rock strata. 13. In Slurry Shield TBMs, should high or low slurry pressure be maintained to support excavated face In Slurry Shield TBMs, the slurry supports the wall of tunnel face in a manner similar to diaphragm wall. The bentonite forms a filter cake on the tunnel face on which the slurry exerts its pressure. The face of a slurry shield tunnel boring machine is stabilized by bentonite slurry, which is kept under pressure. If the slurry pressures provided are too low, instability of the face results with occurrence of large settlements. On the contrary, in case the slurry pressures are kept too high, it leads to an excessive loss of bentonite and significant soil disturbance would occur. 14. Is NATM a tunneling method or a tunneling concept In the original version of the New Austrian Tunneling Method, it is a tunneling concept with a set of main principles as follows: (i) Application of thin-spayed concrete lining (ii) Closure at invert to form a complete ring as soon as possible (iii) Measurement of deformation until equilibrium is attained It is a design concept in which the ground (soil or rock) enclosing the opening becomes a load-bearing element through formation of ring-like body. It uses all available means to develop maximum self-supporting capacity to provide support of the underground opening. In essence, it makes use of geological stress of surrounding soils to stabilize the opening. Therefore, NATM is a tunneling philosophy and concept rather than excavation techniques. 64
A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU 15. What is the difference between NATM and Sprayed Concrete Lining The New Austrian Tunneling Method originated from rock tunnels and it requires the use of rockbolts and shotcrete swiftly after blasting. Institution of Civil Engineers has renamed it as Sprayed Concrete Lining for construction of tunneling linings by this method in soft ground. There are some distinct differences in design philosophy between NATM and Sprayed Concrete Lining. In fact, it is not practical to mobilize inherent soil strength through deformation in soft ground. For lower strength of soils in soft ground, there should not be any delay in completing primary support and ground deformation should be reduced as much as possible. 65
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