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Fiber reinforced shotcrete or wire mesh for tunnel support

When tunnels are excavated with conventional drill and blast operations or via mechanical excavation for softer material, the quickest way to support is the use of shotcrete. This method is called Sprayed Concrete Lining (SCL) in the UK and in other countries it is named as New Austrian Tunneling Method (NATM). In reality NATM is more than just the sprayed concrete lining and erroneously every tunnel support utilizing shotcete is named NATM but another post will cover this issue.

In this post I would like to make some points regarding the use of fibers or wire mesh in the reinforcement of shotcrete used for tunnel support. A great amount of literature exist regarding this issue and even more laboratory tests verifying that it is better to use fibers to reinforce shotcrete. This is because the shotcrete becomes more ductile when fibers are used in relation to just plain shotcrete and ductility is good in tunnel support.

The issue is what happens in larger displacements? When the shotcrete will crack? Would we want shotcrete to crack? How much cracking is acceptable? And if cracks occur does fibers or wire mesh do a better job?

When you excavate a tunnel you would like to have a ductile support that can accommodate some displacements. In this way you stabilize your tunnel using the ground as a supporting element and at the same time you gain in cost by using a lighter tunnel support. This is clearly demonstrated with the convergence – confinement diagrams (fig 1).

Convergence – confinement diagrams for tunnel support

In the stiffer support the yield point (failure of support) is where the stress – displacement becomes horizontal, in the less stiffer and more ductile the yield point is not clearly defined  but you could argue that at some point the displacements become too large with little offered additional support.

The equilibrium point is when the support stress – displacement curve meets the rock stress – displacement curve. If the support has not reached the yielding point, then you have “supported” your tunnel. If the yield point (for simplicity, the horizontal portion of the line) is beyond the rock curve then your support has failed to support the tunnel.

Back in our issue, what type of reinforcement to use? Steel fibers or wire mesh for tunnel support?

In the following photograph an area in the tunnel can be seen where too much displacement has taken place. The shotcrete has been severely cracked but is still standing and some support is offered due to the presence of the wire mesh (and bolts).

Shotcrete with wire mesh  for tunnel support

If the shotcrete was reinforced with fibers and such displacement had taken place, large chunks of shotcrete would had detached and fallen. This could harm personnel and equipment. This can be seen in the next photo where a crack has formed in fiber reinforced shotcrete and a gap where the fibers have been detached from the shotcrete.

steel fiber reinforced shotcrete for tunnel support

During construction, the use of fibers is more easily executed because the labor to erect the mesh is more time consuming. Also the mesh may not be able to follow the profile if inappropriate blasting has been executed in hard rock. On the other hand steel fibers are abrasive and can produce maintenance problems to the shotcrete equipment, are more dangerous for injuries during spraying and can more easily be “reduced” by the contractor without anybody knowing.

So coming back to the question of what type to use in tunnel support, one could argue that when you anticipate large displacements you should use steel wire mesh (maybe in collaboration with fibers) and when you anticipate small displacements steel fibers are appropriate and adequate.

In any case the primary support selection for tunnel construction requires careful and meticulous planning. The use of wire mesh can at least protect workers of uncontrolled collapse of shotcrete chunks in severely displacing rock masses.

Please comment for a fruitful technical discussion…

Post update 06/06/2013:

David Oliveira has posted in his popular and highly scientific LinkedIn group  Underground Geomechanics some very interesting comments and has promoted significantly the discussion. Please visit and contribute if you like. Thanks David!

Mining software: new category in the database

Geotechpedia today proudly presents the new category for mining software. Mining is a highly profitable industry worldwide. For this reason several companies have developed mining related software i.e. mine stability design, 3D visualization of mines, cost estimation, underground mining, surface mining etc.


Geotechpedia team has worked in order to create a user friendly mining software category. The classification subcategories are the following: General, Mine management software, Surface mining software, Underground mining software. The General subcategory also includes mining software packages that deal with both surface and underground mining.

Additional information in Geotechpedia database concerning software packages is whether the license is commercial or free and whether you can try a demo version.

In order to optimize our new mining software category we welcome any comments and feedback you care to provide us!


Huge colliery landslide Hatfield Stainforth, UK

People interested in geotechnical engineering are watching closely the colliery landslide at Hatfield Stainforth, in Northern England that is still in progress. Since February 13nth reports and photos on the landslide’s progress are coming in.

Thanks to Prof. Dave Petley’s (Durham University UK) landslide blog, the info and the photographs are really spectacular. According to his first estimation the size and the geometry of the toe bulge suggest a bearing capacity failure. The very wet weather of last few months, triggered the landslide with a rotational geometry.

Due to the size of landslide mass, stabilization will be highly difficult. The railway line is expected to be closed for some time.

In these cases where the amount of soil mass is prohibiting, it might be better to wait for the landslide system to find progressively its own balance, before intervening with further stabilization measures.



Etna’s spectacular volcano eruption 19 Feb 2013

Another eruption of Etna volcano started on Tuesday Feb 19, 2013 from the SE crater. Volcano eruption of Etna happened with 4 paroxysms so far, with lava fountains, lava flows and tall ash plumes.


Mount Etna is an active volcano on the island of Sicily. The volcanic activity of the area is a result of the convergence of the African Plate and the Eurasian Plate.  Mount Etna is one of the most active volcanos on Earth.  Numerous volcano eruptions of Etna have taken place in historical and recent years.

During historical years is reported that in 396 BC the volcano eruption of Etna prevented the Carthaginians to advance on Syracuse. One of Etna’s most destructive eruptions took place on 11 March 1669. In recent history the 1928 volcano eruption led to the destruction of village Mascali. Other major 20th – century eruptions occurred in 1949, 1971,1981, 1991, 2001, 2007 and 2011.


Getting social

You might have noticed that some familiar buttons have popped-up in the site. social1

Yep, these are the ones!

Since every site nowadays needs a social presence in order to be more noticable, we wouldn’t want to be left behind. In other words, we are very eager for you to introduce us to your friends!

However, you probably have noticed these as well right?

So what’s the point of having two sets of the same buttons on a page? Well it’s not actually the same set. The set of buttons on the footer of the site have to do with the social aspect of Geotechpedia in whole. If you like the site, send a kind word for us over the internet kind of thing.

The latter set of buttons, appear in the Item view of a Publication/Software/Equipment record. The purpose of this set of buttons, is to share the information about the actual record. If for example, while browsing thourgh the site you come across theExtended Nucleation of the 1999 MW 7.6 IZMIT Erthquake, and find it interesting, you might want to share it with other people.

Just head over to the right pane of the Publication and click on the share option you prefer. If you choose more than one you will make us even happier!

Happy sharing!