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Is thrust wall an essential element in pipe jacking? Can it be omitted if there is insufficient depth for constructing normal thrust wall?

Thrust wall is an essential element in pipe jacking and it provides the reaction against the pipe jacking operation. In poor ground, consideration may be given to using piling or other methods to increase the stiffness of thrust wall. When there is insufficient depth to construct thrust wall (e.g. jacking through an embankment), a structure has still be constructed to provide the reaction to pipe jacking. In this case, the resistance to horizontal jacking loads is resisted by piles, ground anchors or other methods to reinforce the structure.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the function of packing materials in the joint of concrete pipes in pipe jacking?

Packing materials are about 10mm to 20mm thick and are normally made of plywood, fibreboard or other materials. In case packing materials are absent in pipe joints for pipe jacking, then any deflection in the joints reduces the contact area of the concrete and it leads to spalling of joints due to high stresses induced. With the insertion of packing material inside the pipe joints, the allowable deflection without damaging the joint during the pipe jacking process can be increased.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Which method of pipe jacking is better, pressure balance method or compressed air method?

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.

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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.

Compressed air method in pipe jacking is commonly adopted in locations where groundwater table is high. An air pressure of less than 1 bar is usually maintained to provide the face support and to avoid water ingress. Pressurization and depressurization has to be conducted for workers entering and leaving the pipe-jacked tunnels. In case of porous ground, certain ground treatment like grouting has to be carried out. The removal of boulders by this method is convenient but it has the disadvantages of slow progress and significant noise problem generated by generators and compressors.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the difference between close mode, open mode and mixed mode of trenchless methods?

(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.

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(iii) Mixed mode
It is similar to close mode except that it allows access to the face of TBM for manual removal of obstructions.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Filed under Tunneling | 0 Comments