A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level Two (Advanced FAQs) Part I: Design of Tunnels 1. What is the difference in two common approaches in tunnel support, i.e. support approach and reinforcement approach? (DT1) For support approach it involves the application of reaction force at the face of excavation by using heavy structures, primarily ribs and lagging. For reinforcement approach, it involves the overall improvement of rock mass performance by techniques such as rock dowels, rock bolts and ground anchors. The target of reinforcement approach is to keep the rock and blocks from moving and loosening so that a large dead load of rock would not be exerted onto the support system. In fact, it holds the rock together and causes the ground around the opening to form a self-supporting ground arch around the opening. There is a trend of tunneling industry to move from support approach to reinforcement approach because it requires less amount of structural steel support. 2. What are the major factors affecting ground-support interaction in tunnels? (DT2) In general, the in-situ ground stress reduces with an increase in inward radial displacement in an unloaded ground/tunnel. The major factors affecting ground-support interaction in tunnels are as follows: (i) The relative stiffness of ground and linings plays an important role in ground-support interaction. For instance, a stiff support could support the unloaded ground at lower deformation than a flexible support. (ii) Delay of support to tunnels results in the ground to be deformed by a certain degree before the installation of linings. Consequently, the linings take up less loads subsequently owing to less deformation of ground generated after the action of delay of support. (iii) As the ground is unloaded, stress is redistributed during excavation. In case the ground is delayed in support and the ground respond from elastic region to yielding region, it results in larger displacement and supporting load. 344
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 3. Is NATM a tunneling method or a tunneling concept? (DT3) 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. 4. What is the difference between NATM and Sprayed Concrete Lining? (DT3) 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. 5. Can pipe jacking be implemented in a fast manner? (DP1) Energy is a function of load and speed. Damage may occur on the jacking pipes if it is incapable of absorbing the energy. Pipe jacking requires large energy in the process. Jacking pipes are low speed energy absorbers. To cater for the low-speed-energy absorbing characteristics of jacking pipes, hydraulic jacks should be designed to provide high loads with low speeds. Otherwise the excessive high speed generated by hydraulic jacks would cause kinetic damage to the pipes. 345
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A Self-Learning Manual - Institution of Engineers Mauritius

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