Construction Phase – Part 1: In the Bowels of the Earth

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Good day fellow railway colleagues and enthusiasts!

I have asked Martyn Gomersall to elaborate on one of the points in a previous blog post:

Today, we will be looking at the construction phase when building a modern railway.


I was asked to expand a little on the construction phase of building a railway and particularly an Urban Metro Railway.

Essentially there are three levels of running a railway,:

           · Underground

           · At Grade

           · Elevated

Each of these have their pros and cons which may include:

· Cost (to build and maintain)

· Land Issues

· Right of Way

· Geo-Technical consideration (i.e. ground types, underground water courses etc. etc.)

· Cultural Issues (old buildings, consecrated ground etc.)

For this first segment I will talk of underground construction which includes:

· Cut & Cover

· Drill & Blast

· Tunnel Boring Machine (TBM)

Cut & Cover

This method is in essence digging a hole, laying concrete in the floor and then building a sturdy concrete roof and replacing roads, building etc. back onto the roof. This methodology is usually quite disruptive to public because of the need to divert traffic and pedestrians, usually for long periods of time.

This method is usually only suitable for shallow running tunnels and station development. Much of the early London Underground was constructed using this method and the disruption to street life is well documented.

For this method of construction of stations, there are two methods of construction,

· Top Down; and

· Bottom Up

The reason for choosing one or the other is based on many technical issues but from a public point of view the former is the most preferable. Top Down means as the hole is dug we can cover the area with deck plates and traffic and pedestrians may then resume moving over the area while work continues safely underneath; this is not possible with the Bottom Up method.

 Drill and Blast

This method is only suitable for tunnel alignments are through hard rock. Essentially holes are drilled in a particular pattern and then charged with explosives which are then ignited. Once the dust settles the blasted rock is removed, roof support is installed and then the process cycle begins again. 

At some stage in this process the tunnel will be lined with a waterproof membrane and then given a concrete lining for stability of the tunnel over its lifetime.

The drill and blast method is quite a hazardous operation and requires great skill in executing this methodology. In the South African context there are some excellent skills available due to the reputable skills South Africans have in mining technology. (I was able to speak to Tunnelling personnel here in India in Afrikaans such is the demand for SA Tunnel people around the world).

The use of Drill and Blast is not recommended in fairly shallow alignments in urban areas due to possibility of damage to buildings and roads through shock waves from the blasting operation.

Tunnel Boring Machine (TBM)

The TBM is an impressive and expensive piece of machinery and a relatively safe method of boring tunnels in urban areas. A TBM can be used in most ground conditions ranging from pure sand, clay weathered rock or even hard rock (provided of course that you select the correct machine type – but that is an entirely long subject which maybe can be explored further sometime else).

 The TBM has a rotating cutter head in front of the machine which drills through the strata whilst supporting the face with pressure to prevent the hole from collapsing until a segmented concrete ring is installed which is then grouted into position with a cement mixture. This pressure can be supplied through keeping the cutting chamber filled with the material being cut (an Earth Pressure Balance Machine), through a slurry of Bentonite and water (a Slurry Machine) or, on occasion, simply with compressed air.

These machines can bore tunnels up to around 17 metre in diameter though for metro railways they are usually working at diameters of around 6 metre. The machines are guided in the vertical and horizontal plane by Laser guidance systems and these days the machines are loaded with on board computers to make the operation easier and safer.

The TBM is a complex piece of machinery and requires experienced people to operate it and to maintain the various systems. In ideal conditions the TBM can move 50 metre or more in a single 24 hour shift but there is always the fear that difficult ground conditions may be encountered where the productivity of the TBM falls to just a few feet per day or even grinds to a complete standstill…!

Surface Monitoring

Whilst any of these methods are being carried out it is absolutely essential that monitoring of the surface area and buildings is carried out. This monitoring includes:

· Verticality / Inclination of buildings

· Building Crack Propagation

· Ground subsidence

· Water Table Levels.

· Vibrations

Usually the tunnelling contractor will employ surveyors and geo-technical experts to monitor these conditions as well as those underground, and make decisions when any variation to the standards are indicated.

Ground subsidence or building movement can usually be mitigated by pressure grouting or under-pinning a building but once again this is a rather technical issue but rest assured tunnelling contractors have a whole array of solutions to any anticipated challenges which arise.

There are other tunnelling methods which are used during construction but these are usually just variations on the above methods.

Next we will look at Elevated construction and At Grade construction.


Martyn is currently the “Chief Safety Expert for General Consultant to Delhi Metro”

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