Vincent T. H. CHU

When the process of pipe jacking stops, building up of resistance is very fast in some soil. For instance, increase in jacking force of 20%-40% is required for a stoppage of pipe jacking for just several hours. Hence, it is recommended that pipe jacking should be carried out in a continuous operation.

For a long pipeline, the frictional forces established between the jacking pipes and soil is high. Sometimes, such resisting forces may be so high that they can hardly be overcome by the jacks in jacking pits. Moreover, even if the jacks can overcome the high frictional forces induced during jacking, high loads are experienced in jacking pipes during driving. Jacking pipe’s material e.g. concrete may not have sufficient strength to resist these stresses and hence pipe strength is another factor that governs the need for consideration of using intermediate jacks.

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.

Grout holes are present in precast jacking pipes for the following reasons:

(i) They serve as the locations for injection of bentonite or other lubricant. Lubricant is used for both granular soils and cohesive soils to trim down the frictional resistance. For cohesive soils, the soils cannot get onto the pipes by the presence of lubricant and the shearing plane lies within the lubricant as suggested by R. N. Craig (1983). On the other hand, for granular soils, the lubricant mixes with soils with a significantly reduced friction. With the use of lubricant, longer pipe lengths can be jacked without the use of intermediate jacking station.

(ii) They provide the inlet locations for subsequent grouting works after completion of pipe jacking to fill completely the void space between the pipes and surrounding soils.

(iii) They are used as lifting holes when placing the precast jacking pipes into rails inside the jacking pits.

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.

Joint stress is induced in pipe joints during pipe jacking. Packers are normally installed in pipe joints to avoid localized stressing of joints leading to concrete crushing. In essence, packers should be elastic enough to take the reloading jacking force. Moreover, it should be thick enough to take the compression of maximum joint stress. Theoretically speaking, packers should be provided in all of the joint area except 20 mm back from the edge of concrete. The reason of such provision is to reduce the risk of local spalling of side edges.

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.

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.

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.

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.

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.

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.

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.

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.

(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.

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.

In the past, it was believed that the formation of frost heave was related to the volumetric expansion of soil water which changed from liquid state to solid state. However, the increase of volume of changes in states for water at zero degree Celsius is only about 9% and the observed heaving is far more than this quantum.

In fact, the mechanism of frost heave is best explained by the formation of ice lenses. In cold weather, ice lenses develop in the freezing zone in soils where there is an adequate supply of soil water. Soil particles are surrounded by a film of water which separates the soil particles from ice lenses. The moisture adhered to soil particles gets absorbed to the ice lenses on top of the soils and in turn water is obtained from other soil pores to replenish the loss of water to ice lenses. This process continues and results in pushing up of soils on top of the lenses and subsequently the formation of frost heave.

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.

During the construction of shotcrete, it is aimed at gunning the full thickness in one single operation and this helps to reduce the occurrence of possible delamination and formation of planes of weakness. Moreover, the nozzles should be held about 0.6m to 1.8m from the surface and normal to the receiving surface. The reason of gunning perpendicular to the receiving surface is to avoid the possible rebound and rolling resultingfrom gunning at an angle deviated from the perpendicular. The rolled shotcrete creates a non-uniform surface which serves to trap overspray and shotcrete resulting from the rebounding action. This is undesirable because of the wastage of materials and the generation of uneven and rough surface

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.

For the construction of diaphragm walls, bentonite slurry is commonly used to form a filter cake on walls of trenches to support earth pressure. The use of bentonite solely is based on its thixotrophic gel viscosity to provide support.

Though the cost of polymer is generally more expensive than bentonite, the use of polymer is increasing because polymer is generally infinitely re-usable and very small amount of polymer is normally required for construction works. The disposal cost of bentonite is quite high while the disposal of polymer can be readily conducted by adding agglomerator.

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.