Tunneling

Pipe jacking is a general technique of the installation of pipes with a tunneling shield in front and the pipes are jacked from a jacking pit to a receiving pit. The tunneling shield for pipe jacking can be electrical and mechanical equipment for conducting the excavation work or it can be a manual shield for workers going inside the shield to carry out manual excavation. For microtunneling, it is a kind of pipe jacking of small sized non-man-entry pipes which are remotely controlled. In general, there are two common types of micro-tunneling machines:

(i) Pressurised slurry
Similar to the Pressurised slurry TBM, excavated material is transported from the excavation face to the surface suspended in a slurry.

(ii) Auger machine
Excavated material is transported from the excavation face to the drive pit through a cased screw auger.

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.

Butt end type with mild steel band is commonly used for storm water application. The mild steel band serves to prevent lateral displacement if pipe joint during pipe jacking.

Socketed in-wall rubber ring type is normally used for sewer and pressure pipe situation and for pipe size exceeding 1200mm in diameter. The rubber ring is designed to provide a seal to pipe joint to ensure water tightness, which is essential in sewage pipelines.

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 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.