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Tuesday, 13 January 2015

171) The “25 Euro Silver-Niobium Coin Series": (xi) 2003 onwards minted by the Austrian Mint: Eleventh Coin in the Series: “Tunnel Construction” (2013):

171) The “25 Euro Silver-Niobium Coin Series": Part (xi): 2003 onwards minted by the Austrian Mint by using Niobium and Niobium metal insertion technology for the first time anywhere in the World of Numismatics:  The Eleventh coin in the Series: “Tunnel Construction”:

Tunnel Construction:

A tunnel is an underground or underwater passage dug through the surrounding soil/earth/rock and enclosed except for an entrance and exit at each end.

Tunnel construction is basically of three types:

-      Cut-and-cover tunnels constructed in a shallow trench and then covered over.

-      Bored tunnels, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.

-      Immersed tube tunnels, sunk into a body of water and sit on, or are buried just under its bed.

One of the most mountainous countries in Europe, Austria has been a pioneer in the development of tunnels.

Built between 1848 and 1854, the vertex tunnel of the Semmering Alpine Railway was the World’s first Alpine tunnel. The “New Austrian Tunnelling Method”, which uses the geographical stress of surrounding rock to strengthen a tunnel, was developed from 1957 – 1965 by Ladislaus von Rabcewicz, Leopold Muller and Franz Pacher, which has gone a long way in revolutionising tunnel construction around the World.

Milestones in Tunnel-construction:

-      Tunnels were hand-dug by several ancient civilisations in the Indian and Mediterranean regions. In addition to digging tools and copper rock saws, fire was sometimes used to heat a rock obstruction before dousing it with water to crack it apart. The cut-and cover method – digging a deep trench, constructing a roof at a predetermined height within the trench and covering the trench above the roof (a tunnelling technique still employed today) was used in ancient Babylon some 4000 years ago.

-      In 1681, the first advancement over hand digging came with the use of gunpowder to blast a 515 ft. (160 metres) long canal tunnel in France.

      This photo taken at Mahaballipuram , near Chennai during our recent trip in August 2014, reminded me of  this process. Notice the small holes on this rock, through which controlled amounts of gunpowder can be inserted to have a controlled explosion/blast to split the rock into a   pre-determined size.                                                                                                        

-      Around 1850, Nitro-glycerine (stabilised in the form of dynamite) replaced the less powerful “black powder” (gunpowder) in tunnel blasting. Steam and compressed air were used to power drills to create holes for the explosive charges. This mechanisation eventually replaced the manual process by using heavy sledge hammers and pounding steel chisels which could cut deep holes into solid rock.

-      Between 1820 and 1865, tunnelling shields were developed which enabled construction of two tunnels under the Thames River in London. A rectangular or circular enclosure (the shield) was divided horizontally and vertically into several compartments. A worker in each compartment could remove one plank at a time from the face of the shield, dig ahead a few inches and replace the plank. When space had been dug away from the entire front surface, the shield was pushed forward and the digging process was repeated. A team of workers at the rear of the shield lined the tunnel with bricks or cast iron rings.

-      In 1873, a compressed air technique was used to keep water from seeping into a railroad tunnel under construction below the Hudson river. This technique, though fraught with risks, is still in use today. Workers have to spend time in decompression chambers at the end of their shifts, hence in the case of an emergency evacuation, there could be casualties. The pressure within the tunnel must be carefully balanced with the surrounding earth and water pressure otherwise any imbalance can cause the tunnel to collapse or burst which will end up in flooding.

-      Soft soil is prone to collapse and it can clog digging equipment. To stabilise the soil, it is frozen by circulating coolant through pipes embedded at intervals throughout the area. This technique has been in use since the early 1900s. Another stabilisation and water-proofing technique used since the 1970s is to inject grout (liquid bonding agent) into soil or fractured rock surrounding the tunnel route.

-      Shotcrete is a liquid concrete that is sprayed on surfaces since 1907, it has been used as both a preliminary and a final lining for tunnels since the 1920s.

-      In 1931, the first drilling jumbos were devised to dig tunnels. These jumbos consisted of 24-30 pneumatic drills mounted on a frame welded to the bed of a truck. Modern Jumbos allow a single operator to control several drills mounted on hydraulically controlled arms.

-      In 1954, a Tunnel Boring Machine (TBM) was introduced which is a cylindrical device with digging or cutting heads mounted on a rotating front face that grinds away rock and soil as the machine moves forward slowly. Modern TBMs are customised for each project by matching the types and arrangement of the cutting heads to the site’s geology. The diameter of the TBM is also required to be equal to the diameter of the designed tunnel (including its lining).

New Austrian Tunnelling Method (NATM) – Salient Principles:

NATM integrates the principles of the behaviour of rock masses under load, as well as, monitoring the performance of underground construction during tunnel building. Of the 22 Principles that define NATM, the undernoted ones are most significant:

-      Mobilisation/exploitation of the strength of rock mass: The inherent strength of the soil or rock around the tunnel domain should be preserved and deliberately mobilised to the maximum extent possible. The primary support is directed to enable the rock to support itself.

-      Shotcrete protection: The mobilisation can be achieved by controlled deformation of the ground. Excessive deformation that will result in loss of strength or high surface settlement must be avoided. This is achieved by applying a thin layer of shotcrete immediately after surface advance. Initial and primary support systems consisting of systematic rock bolting or anchoring and thin semi-flexible sprayed concrete lining are used to achieve the particular purposes given in permanent support works are usually carried out at a later stage.

-      Closing of invert: The closure of the invert and creating a load bearing ring should be adjusted with an appropriate timing that can vary, depending on the soil or rock conditions. It is particularly crucial in soft ground tunnels where no section of the tunnel should be left open even temporarily.

-      Measurement & Monitoring: Laboratory tests and monitoring of the deformation of supports and ground should be carried out without delay. Every deformation of the excavation needs to be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in the lining, ground and bore-holes.

-      Flexible support: The primary lining is thin and reflects recent strata conditions. Active, rather than passive support is used and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock bolts, wire-mesh and steel ribs.

-      Flexible Contractual arrangements & ongoing cooperation during building of the tunnel: Those involved in the execution, design and supervision of NATM construction should understand and implement the NATM and react cooperatively in resolving any problems. Changes in support and construction method, as tunnel construction work progresses, are possible and the contractual arrangements should embody clauses to enable such changes.

-      Rock mass classification will determine support measures: There are several main rock classes for tunnels and corresponding support systems for each one, which serve as guidelines for tunnel reinforcement. The measured rock properties lead to the appropriate tools for tunnel strengthening. Nevertheless, the length of the unsupported span should be left as short as possible.

The NATM concept focuses on using geological stress of the surrounding rock mass to provide stabilisation and support to the tunnel itself. Originally developed for use in the Alps, where tunnels are commonly excavated at depth and in high stress situations, the Principles of NATM have become fundamental to modern-day tunnelling.

Philosophy and controversial names:

Interestingly, there is a paradox which has led to difference of opinions – whether the NATM is a philosophical approach or it is an actual technique/method in tunnel excavation and construction.

For example, NATM has also been referred to by such diverse names as the Sprayed Concrete Method (SCL - This method consists of a thin sprayed concrete lining, closure of the ring at the earliest possible moment by an invert to a complete ring – called an “auxiliary arch”, the deformation of which is measured as a function of time until equilibrium is obtained. The 3 key points are – application of thin-sprayed concrete lining known as “shotcrete”, closure of the ring as soon as possible and the systematic deformation measurement), the Sequential Excavation Method (SEM – which also refers to the NATM, is a concept that is based on the understanding of the behaviour of the ground as it reacts to the creation of an underground opening), the Cross Diaphragm Method (CDM), the Centre Dividing Wall or Cross Diaphragm Method (CDW or CRD-NATM) and the Upper half Vertical Subdivision (UHVS).

Interestingly, for this widely used Tunnelling method, the scientific community is not able to arrive at a single unified name.

In 1980, the definition of NATM was redefined by the Austrian National Committee in Underground Construction of the International Tunneling Association as “a concept whereby the ground surrounding an underground opening becomes a load bearing structural component through activation of a ring like body of supporting ground”.

NATM is a Design Philosophy or a Method?

The above nomenclatures have led to some confusion in the exact definition and principles of NATM leading to three group of thought – one group which supports the NATM, anther which opposes it and a third neutral group.

NATM as a Design Philosophy:

Leopold Muller, one of the pioneers of this method, suggested that NATM was a tunnelling concept defined by a set of principles. It was not to be viewed as a method for construction, as this actually implied a means by which to advance or drive a tunnel. This led to a school of thought that believes that NATM is more of a Philosophy rather than a set of excavation and support techniques.

NATM is termed a Design Philosophy because of the under mentioned features:

-      The strength of the ground around a tunnel is deliberately mobilized to the maximum extent possible.

-      Mobilisation of the ground strength is achieved by allowing controlled deformation of the ground.

-      Initial primary support is installed having load-deformation characteristics appropriate to the ground conditions and installation is timed in relation to ground deformations.

-      Instrumentation is installed to monitor deformations in the initial support system, as well as to form the basis of varying the initial support design and the sequence of excavation.

Nevertheless, whichever way one looks at it – method or philosophy, , NATM has several beneficial features in comparison to the more conventional tunnelling methods and is widely being accepted in the construction of tunnels today. NATM has helped revolutionise the modern tunnelling concepts. Many of the most famous tunnels all over the world have used this excavation technique.

NATM and Soft-ground Tunnel Construction:

NATM is being widely used in soft-ground tunnel excavation and construction around the World.

In soft ground tunnels, shotcrete in combination with lattice girders and some form of ground support are installed as excavation takes place, followed by the installation of a final lining at a later date.

Tunnel Construction: Soft Rock and Underwater:

Tunneling through soft rock and tunnelling underground require different approaches – Blasting in soft, firm rock such as shale or limestone is difficult to control.

Therefore, Tunnel-Boring Machines (TBMs) or Moles are used in creating a tunnel.  TBMs are enormous pieces of equipment with a circular plate on one end. The circular plate is covered with disk cutters which are chisel shaped cutting teeth, steel disks or a combination of both. As the circular plate rotates slowly, the disk cutters slice into the rock, which falls through spaces in the cutting head onto a conveyor system. The conveyor system carries the debris to the rear of the machine. Hydraulic cylinders attached to the spine of the TBM propel it forward, a few feet at a time.

TBMs also provide support while boring the tunnel. As the machine excavates, two drills just behind the cutters bore into the rock. Then the Tunneling team pumps materials into the holes & attaches bolts to hold everything in place until the permanent lining is installed. The TBM assists by using a massive erector arm which raises segments of the tunnel lining into place.

             An image of a Tunnel boring Machine (TBM)
An image of a model of a Tunnel Boring Machine which has been taken to the reverse of this Silver-Niobium coin.

On the other hand, for underwater tunnel construction, the cut-and-cover method is used. Construction is carried out by dredging a trench in the riverbed or ocean floor. Long pre-fabricated tube sections made of steel or concrete and sealed to keep out water are floated to the site and sunk into the prepared trench. Then workers dive into the water and connect the sections and remove the seals. Any excess water is pumped out and the entire tunnel is covered with backfill.

The 25 Euro Silver-Niobium coin titled “Tunnel Construction”:

On the Obverse of this coin is shown a present day Tunnel Boring Machine (TBM) in the Niobium Core, its rotating motion symbolised by three arrows. The outer silver ring shows the mountains through which the machine is drilling. On the upper left periphery is mentioned the denomination of the coin “25 EURO” and the year if issue “2013”. On the lower periphery is depicted the name of the issuing country “REPUBLIK OSTERREICH” (meaning “Republic of Austria”).

The colour of the Niobium Core is a brilliant ice blue.

On the Reverse of this coin is shown one of the many road tunnels that are found on Austria’s Alpine landscape. A tunnel worker is shown using a pneumatic drill to loosen rock in the silver ring. The word “TUNNELBAU” (meaning “Tunnel construction”) is inscribed on the lower periphery of the coin in the outer silver ring.

The specifications of the coin are:

Face value: 25 Euros; Metallic composition: Outer ring: Silver (Ag) 900 – 9 gms, Niobium 998 – 6.50 gms; Diameter: 34 mm; Weight: 16.50 gms; Edge: smooth.

The mintage of this coin was limited to a maximum of 65000 pieces. 

 This coin has won the best Bimetallic Coin in the prestigious Krause Publications Competition held in 2014:
Every year, since 1984, Krause Publications holds a competition for “Coin of the Year” Awards in which there are 10 sub-categories (Best Gold Coin, Best Bimetallic Coin, Most Artistic Coin, Most Historically Significant Coin, Best Contemporary Event Coin, Best Silver Coin, Most Innovative Coin, Most Inspirational Coin, Best Crown, Best Circulating Coin) apart from the overall “Coin Of The Year” (COTY).

The coins nominated for the Competition should have elegant and diverse styles, themes and technology used by mints from across the Globe.
The recently concluded Competition saw 94 elegant coins being nominated from 45 different countries which were all issued in 2013. Out of the ten categories, Austrian Mint coins won best coin in three categories - Most Artistic Coin Category (Austrian Mint – 2013 “Wildlife in Our Sights” Red Deer 100 Euro Gold Coin – KM No. 3225), Best Bimetallic Coin Category (Austria – 2013 Tunnelling or Tunnel Construction – KM 321) Silver & Niobium 25 Euro Coin, and Best Gold Coin (Austria – 2013 “Klimt and His Women –  The Expectation” 50 Euro Gold Coin – KM 3218).

The following coins have been issued in this Series:

2003 – 700 years old Hall City in Tyrol or Tirol

2004 – 150 years Semmering Alpine Railway

2005 – 50 years of Television

2006 – The European Satellite Navigation

2007 – Austrian Aviators

2008 – Fascinating Light

2009 – Year of Astronomy

2010 – Renewable Energy Sources

2011 – Robotics

2012 – Bionics

2013 – Drilling tunnels

2014 – Evolution


  1. Sumita Chaudhry has commented:
    "Wow ! I will never be able to JUST pass through a tunnel now!! That was a lot to process! "

    1. Even I was fascinated going through the reference material for this post.

  2. Ramchandra Lalingkar has commented:
    " Very interesting information about 'Tunnel' construction which otherwise would not have been read. Thanks for such an informative sharing ! "