Vincent T. H. CHU

The shield connected to lead pipe is normally sharp-edged in design because:

(i) It helps to reduce the resistance when the shield enters into soils;

(ii) It reduces the amount of soil dropping into the shield.

Sometimes the shield is equipped with jacks so that it allows tilting of the shield and adjustment could be made to the direction of pipe jacking.

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 Slurry Shield TBMs, the slurry forms a filter cake on the face of excavation which has the following purposes:

(i) It provides the surface for slurry pressure to act on.

(ii) It acts as a seal against the y of groundwater into the tunnel.

(iii) In case the TBM breaks down, this filter cake serves as a sealing membrane at the tunnel face which allows man-entry into the excavation and working chamber upon provision go compressed air.

Slurry Shield TBMs are widely used for non-cohesive soils ranging from fine-grained sand to coarse-grained gravel. It is less suitable when operating in clay because:

(i) Most slurry separation plant could not separate clay from slurry. As such, the cost of frequent replacement of bentonite slurry is substantially increased.

(ii) Clayey materials to clog and cause blockages in slurry system leading to sudden pressure surges.

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.

Lubrication performs effectively in pipe jacking by maintaining a layer of lubricant between the outside surface of jacking pipes and the adjacent soil. However, once the ground has collapsed onto the jacking pipe, the effect of lubrication would be greatly reduced. Hence, it is important to maintain sufficient pressure to avoid these occurrences.

Bentonite slurries are commonly used in silty, sandy and granular soils but it is not recommended to use in clayey soils because it may cause swelling in clays by absorption of water and this results in increased contact between the jacking pipe and soils. Hence, non-aqueous lubricants should be considered for cohesive soils.

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.

Lubricants are introduced in pipe jacking to reduce the jacking loads. However, for pipe jacking in clay the swelling of clay is accelerated by water-based lubricant as the free water in the lubricant is readily absorbed by clay. To address the problem, swelling inhibitors are added to lubricants which enhance a reduction of free swelling for clays. In essence, the ability of swelling inhibitors to reduce free swelling of clay is achieved by the alternation of clay properties and formation of barrier on cavity 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.

Earth Pressure Balance (EPB) TBMs are used in excavating and advancing tunnels through any type of soft ground or soil condition, particularly below the water table. The EPB method consists of a cutting chamber located behind the cutterhead. This chamber is used to mix the soil with water foam. It is maintained under pressure by the mucking system. The ground at the cutting face is supported by earth pressure by balancing the advancement of the tunnel with the discharge rate of the excavated soil.

The underlying principle of the EPB method is that the excavated soil itself is used to provide continuous support to the tunnel face by balancing earth pressure against the forward pressure of the machine. The thrust forces generated from rear section of TBM is transferred to the earth in the cutterhead chamber so as to prevent uncontrolled intrusion of excavated materials into the chamber. When the shield advances at the face of excavation, the excavated soil is then mixed together with a special foam material which changes its viscosity or thickness and transforms it into a flowing material. This muck is then stored and is used to provide support and to balance pressure at the tunnel face during the excavation process.

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 segmental tunneling, the jacks are installed at the shield so that it is not necessary to install thrust wall at the jacking pit. This provides the opportunity for smaller size of the pit because of the absence of thrust wall. Moreover, as the jacking operation involves the jacking of small length of segmental liner plates and hence smaller force would be required for pipe jacking when compared with traditional pipe jacking (jacks at jacking pits) where the jacking force is needed to overcome long lengths of pipe drives. On the other hand, the use of segmental tunneling offers better control in alignment because the steering operation could be performed at the shield.

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.

It is commonly recognized that ground settlement is one of the major concern in pipe jacking operation. However, engineer should also pay attention to the problem of ground heaving during grouting work of pipe-jacking. For instance, if excessive slurry or grout pressure is applied so as to exceed the overburden pressure, ground heaving would result. Alternatively, if the ground contains loose soils with high porosity, the same phenomenon also occurs. Proper control on the applied pressure and viscosity of grout/slurry is necessary to prevent such occurrence.

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.

It is reported by Lake (1992) that settlements are expected at the ground surface at a distance of 1-2 times of tunnel depth ahead of the tunnel face and 80% to 90% of settlement to be completed at a similar distance behind the face. However, in the paper “Monitoring of ground response associated with pipe jacking works – recent experience in Hong Kong”, the author pointed out that based on their experience, development of longitudinal settlement was observed at a distance of 3-4 times of tunnel depth behind the tunnel face and little settlements were reported immediately above the tunneling face.

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.

When the pipejacking machine moves from a region of soil to very hard rock, it will be subject to damage of cutting disc. To break and loosen the rock, the pipejacking machine applies a large torque on cutting wheels.

However, with the change of soft region to hard region, the pipejacking machine is still under the same jacking load. As such, this results in insufficient or little space for the movement of the machine against the rock face, leading to damage and exhaustion of the pipejacking machine.

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.

The rate of cutting through soils is faster than that of cutting through boulders for pipe-jacking machine. As such, when pipe-jacking machine enters a region of mixed ground with soils and boulders, the machine has the tendency to move towards the direction of soft soils because of the difference of rate of advancement of pipe-jacking machine for soils and boulders. Consequently, migration of soft soils occur which contributes to ground settlement. The degree of settlement is dependent of the depth of soil cover, soil property and the size of boulders.

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.