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 Element Design of Tunnels Description Objective No. Design of Tunnels Support approach and reinforcement approach DT1 Ground-support interaction DT2 New Austrian Tunneling Method, DT3 “Cut-and-cover” method and “cover-and-cut” method DT4 Ground freezing DT5 Design Pipejacking of Speed of jacking DP1 Stress concentration in pipe joints DP2 Lubricants DP3 Pipe joints DP4 General DP5 326
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level One (Core FAQs) Part I: Pipejacking 1. What is the difference between close mode, open mode and mixed mode of trenchless methods? (TP1) (i) Close mode It refers to mechanically operated TBM using bulkhead or slurry to balance earth pressure and groundwater. There is no manual access to the face of excavation. (ii) Open mode It refers free air hand-dug tunnel or compressed air handing tunneling with manual access to face of excavation. (iii) Mixed mode It is similar to close mode except that it allows access to the face of TBM for manual removal of obstructions. 2. Which method of pipe jacking is better, pressure balance method or compressed air method? (TP2) Pressure balance method normally requires the use of mechanically operated tunnel-boring machine at its cutting head in pipe jacking. Slurry or steel bulkhead is commonly adopted to provide the balance of earth pressure and groundwater in front of the boring machine. Slurry used in balancing earth pressure and ground water pressure is constantly supplied to the face of the cutting wheel through slurry pipes. The excavated materials drop into a crusher for reduction in material size. Later, the debris and spoils will enter the spoil removal chamber near the invert of the shield and will be transported to ground level through slurry discharge pipes. This method of construction is normally adopted in sand and gravel. However, it suffers from the demerit that it is quite difficult to remove large rock boulders during the advancement of the machine. It is quite time-consuming for workers to go inside the relatively small airlock chamber and remove large bounders by hand tools. The other type of pressure balance technique is called earth pressure balance method which is commonly used in clay and silty soils. It makes use of the principle of maintaining the pressure of excavation chamber the same as the pressure in ground. The excavated materials are transported through screw conveyor to the jacking pit. 327
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A Self-Learning Manual - Institution of Engineers Mauritius

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