Tunneling

In Slurry Shield TBMs, the slurry supports the wall of tunnel face in a manner similar to diaphragm wall. The bentonite forms a filter cake on the tunnel face on which the slurry exerts its pressure.

The face of a slurry shield tunnel boring machine is stabilized by bentonite slurry, which is kept under pressure. If the slurry pressures provided are too low, instability of the face results with occurrence of large settlements. On the contrary, in case the slurry pressures are kept too high, it leads to an excessive loss of bentonite and significant soil disturbance would occur.

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 single shield TBM, it extends and moves forward by thrust cylinders on the last segment ring installed.

In double shield TBM, it consists of an extendable front shield which enhances the cutterhead to be extended. The gripper in the middle section of TBM is mobilized so that it pushes against the tunnel walls to react the boring forces. As these forces are dissipated, it allows the installation of lining segments during tunnel so that it increases the speed of tunneling. Upon completion of a trust stroke, the grippers are retracted and the end portion of TMB is pushed against the front shield by thrust cylinders.

Double shielded TBM is normally used in rock strata with geological fault zones and when a high rate of advancement is required. Single shielded TBM is more suitable to hard rock strata.

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.

Open shield type TBM refers to those providing lateral support only. They can be further classified into single shield and double shield.

Closed shield type TBM refers to those providing lateral support and frontal support. Some common TBM method under this category includes compressed air TBM, slurry shield TBM, earth pressure balance machine and mixed confinement shield.

Compressed air TBM is suitable for cohesive soils under water table (e.g. ground with low permeability with no major discontinuities). Slurry shield TBM is suitable for soft ground and soft rock under water table and also for ground for high permeability. Earth pressure balance machine is suitable for soft ground and soft rock under water table. It is not recommended for very abrasive and hard ground.

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.

There are two main distinct advantages of pipe ramming:

(i) Settlement and heaving of existing ground.
Where a pipe have to be installed under an existing railway or heavily-trafficked highway, it is almost impossible to install the pipes by open excavation. In particular, if the pipes to be installed are of shallow depth, the use of some trenchless methods (e.g. pipe jacking and heading) may cause considerable ground settlement because the soil loss within shallow zone would induce larger settlement. As such, the use of pipe ramming could resolve this concern. Pipe ramming is a displacement method which generally would not result in ground loss. For open-ended steel casing, the soils inside the pipe are not removed until the entire casing is installed in place.

(ii) Muck Disposal
For microtunneling, spoils are removed from the excavation face in a slurry so that the spoil are wet. Hence, sufficient space has to be provided to allow for drying of spoils or the wet spoils have to be removed off site immediately. For pipe ramming slurry is not used so that the spoil retains only it natural moisture content. Therefore, it is easier to handle in-situ soils than wet spoils. On the other hand, the amount of spoil produced by pipe ramming is smaller when compared with pipe jacking and microtunneling

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.

Consider a certain cross section along the line of pipe ramming. When the pipe is rammed close to the cross section, the horizontal and vertical soil pressure would increase owing to the effect of soil compaction brought about by dynamic ramming operation. Upon reaching the cross section, soil pressure is redistributed around the pipe such that the vertical pressure above the pipes is reduced while the vertical pressure in pipe abutment locations is increased. When the pipe is advanced further, the load on pipes tend to increase owing to reorientation of soils around pipe wall. Finally, when the pipe is rammed some distance away from the cross section, a stable state is achieved in which there is smaller earth pressure on the pipe’s top and higher vertical soil pressure on soils at both sides of the pipe.

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 more common to adopt open-end pipes in pipe ramming because it is not readily to undergo surface heaving or pipe deflection when compared with closed-end pipes. Moreover, the use of open-end pipes requires lower ramming force.

Closed-end pipes are only used in the following conditions:

(i) Ground with poor self-support so that inflow of soils inside the pipes would render ground settlement and loss of support to utility services.

(ii) Small size of pipes.

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 pipe jacking/microtunneling, the causes of large settlement are loss of face stability, failure to stabilize ground around shafts, presence of annular space around pipes and shield, drag along pipe joints, etc. The settlement mechanism of shallow depths of pipe jacking/microtunneling is the formation of a settlement trough on top of the jacking pipes. The width of the trough depends on soil properties; the larger is the cover depth of jacking pipes, the larger is the width of settlement trough. For the same soil volume loss due to pipe jacking/microtunneling, the width of settlement trough for shallow cover depth is smaller and therefore it results in a larger vertical maximum settlement.

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.

Settlements occur in microtunneling, or other tunnel construction methods in two forms: large settlements and systematic settlements. The cause of large settlements is the over-excavation by microtunneling machine leading to the loss of stability at the tunnel face and the formation of empty space above the tunnel. The occurrence of large settlements is attributed to the improper operation of the tunneling machine or rapid unexpected changes in ground conditions.

Systematic settlements are mainly caused by the collapse of the radial overcut between the jacking pipe and the excavation. The annular space between the jacking pipe and the excavation is essential in microtunneling and pipe jacking for the following purposes:

(i) Reduction of jacking forces
(ii) Injection of the lubrication
(iii)Steering of the microtunneling boring machine

During tunneling, the soils may collapse onto the pipe, resulting in subsidence at the ground surface. Systematic settlements can be controlled by limiting the radial overcut and by filling the annulus with bentonite lubricant during tunneling, and with cement grout after tunneling is completed.

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.

There are two main advantages of microtunneling:

(i) Difficult ground conditions
Microtunnels could operate under a water head of 30m or more. It is capable of handling a wide range of soils such as cobble, boulders and rock without the need of dewatering.

(ii) Surface settlement
Surface settlement could be minimized by using microtunneling. For example, the use of earth pressure balance method in microtunneling helps balance the external soil loads and groundwater. Moreover, the rate of advancement of machine and the rate of excavation of tunnel face can be readily controlled so that it reduces the occurrence of over-excavation at tunnel face and hence the ground settlement.

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 international definition of microtunneling includes pipe with diameter up to 1,000mm only. However, in the United States, it allows for larger pipe size when defining microtunneling in which pipe with diameters up to 144 inches are also counted within the ambit of microtunneling.

The basic concept of microtunneling has changed gradually owing to recent technological developments and past experience. For instance, the use of alignment control system with advanced surveying techniques allows for longer drives with good control and curved microtunneling. The use of automatic lubrication system enhances lower jacking forces. The employment of gripper in microtunneling machines helps develop adequate loads on cutter discs for cutting rock.

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.