Probably million years ago our earliest ancestors went underground into caves to get protection from bad weather and wild animals. Over time man learned to use tools to adapt the underground to human needs.Many thousand years ago early cultures in China, India, Egypt and Central and South America build voids, galleries and tunnels for water transport or for religious or defence purposes. Close to Naples a tunnel build by the Romans is still in use. In medieval times adits allowed access to ore deposits. Since then St. Barbara is worshiped for protecting miners and tunnelling staff.  Modern tunnelling started only about 150 years ago as big railway tunnel projects where conveyed in the Alps and construction engineers replaced miners in the planning and the building of tunnels.
Today tunnelling is an area of great economic importance. As the world's population grows, with rapid economic expansion, governments aim to improve the infrastructure as a prerequisite for mobility of goods and people as well as to reach more viable conditions in urban centres. Over the next one to two decades more than 5000 km of new transport tunnels with diametres greater than 1 m have been planned worldwide.

St. BarbaraSt. Barbara is worshiped for protecting tunnelling staff .

Excavation techniques

Where the surface allows easy access shallow tunnels are build in a cut-and-cover fashion: The ground is excavated from the surface and the completed underground construction is covered again or the protection towards the floor is part of the construction. In other cases, closed tunnel construction is required: Beginning at a portal the tunnel is advanced into the underground and excavated material is transported away from the tunnel face to the tunnel entrance. In weak but stable rock like many sandstones tunnel excavators can simply dredge away the ground.In stiffer rock roadheaders may replace conventional excavators. Roadheaders have rotating steel tools that scratch away rock from the tunnel face. In addition a tunnel hammer may remove sections of harder rock.

cut and cover tunnel constructionCut and cover construction of a tunnel. The tunnel is built from the surface and covered afterwards. Closed tunnel construction via roadheaderClosed tunnel construction via roadheader.

In hard rock where the ground is fairly stable, for non-circular profiles or for short tunnels where tunnelling machines would be to expensive tunnels are often advanced via drill-and-blast: Every few metres explosives are mounted in small boreholes at the tunnel face and ignited to loosen and remove rock. Drilling the shotholes must be done as quickly as possible because the number of shotholes may be of the order of one hundred. Therefore they are drilled by special vehicles with bore jumbos. The spatial distribution and depth of the bore holes and the choice of explosives aims at maximum local loosening of rock and therefore depends strongly on the geology. Typical shotholes have diameters of 40 mm and depths of few metres. Cartridges of dynamite are shifted into the holes with tamping rods and mostly filled with water or other material to contain the detonating gas front into the rock. Electric detonators ignite the explosives one after the other typically starting at the centre and moving outwards at time differences of few milliseconds. The time delay between the explosions increases rock destruction because it allows pressure waves to interfere and already separated blocks to collide.

vehicle with bore jumbosVehicle with bore jumbos to drill shotholes for a later blast. explosion scheme for drill and blast tunnellingExample of an explosion scheme for drill and blast tunnelling.

Recently usage of emulsion explosives, and non-electric initiation methods have become more popular for ecological and safety reasons. The explosives are mixed onsite and the initiation results from the reaction of a highly active substance within a tube. Excavators shovel the loosened material away and conveyors or trains bring it to the surface. As always in tunnelling the perfect timing between removal and transportation of rock is key to economical success. For economic and safety reasons an increasing portion of tunnels are nowadays drilled by tunnel boring machines (TBMs, also called tunnelling machines), particularly in soft ground, i.e. less consolidated sediments. TBMs remove soil and rock from the tunnel face as the cutting wheel rotates and the machine pushes forward. The rubble is being transported away while drilling via conveyors or in circulating suspensions. Tunnelling machines also secure the tunnel by building a wire mesh lining or closed rings of concrete. They may also implant anchors into the rock around the tunnel or inject concrete to strengthen the surrounding rocks. Tunnelling machines have diameters ranging from a few decimetres (for cable shafts) to about 15 metres (for large traffic tunnels) and yield circular profiles. The machines weigh from a few hundred kilograms up to a few thousand tons, need electrical power up to a few megawatts, cost up to several tens of millions of euros to build and may be longer than 100m. The cutter wheel of the TBM is equipped with dozens of special steel tools for scratching away soft rock and rolling and chiselling away hard rock as the wheel rotates. The tools are distributed on the cutting wheel such that the whole tunnel face is being worked on. At few revolutions per minute and advance rates of several metres a day big machines remove up to 1000 m³ of rubble per day. TBMs are typically designed individually for each tunnel. Different types are used for different geologies.

Hard rock tunnel bore machines

Hard rock tunnelling machines employ nearly closed shields to protect the tunnelling personnel. Since the rock stands by itself staff may reach the space ahead of the shield after the wheel has been retracted, for example to repair the chiselling tools of the cutting wheel. The machine pushes forward while it presses against the rock to the sides. Rubble is transported away via conveyors. The tunnel is secured by wire meshs,  by injection of sprayed concrete or by mounting of segmental concrete or steel arches. Anchors with lengths of several metres are usually placed into the rock. The number of anchors and their material strongly depend on the local geology. Note that even very hard rock creeps over time and any tunnel eventually would collapse if supporting elements would be withdrawn.

hard rock tbmHard rock tunnelling machine. The machine drills with its cutting wheel, transports the rubble away via conveyor belt, and secures the tunnel itself by wire meshs and also the surrounding rock by anchors. A shotcrete robot is located just behind the white container right of the center of the picture.

hard rock tbmHard rock tunnelling machine. The machine drills with its cutting wheel (left) which is followed by a tensioning shield (1) and a short finger shield (2) and the ring erector (3) which pre-installs arch-segments. The anchor  drilling device (4) is operated from a working cage (5).The wire mesh erector (6) is located in front of the gripper plates (7). The gripper (bottom left) presses against the rock to allow for forward pressure. The TBM also transports the rubble away via conveyor belt.

Soft ground tunnel bore machines

In shallow softground tunnelling the pressure in front of the cutting wheel must be greater than the lithostatic stress. and is of the order of several bars.  Therefore the drilling zone cannot easily be accessed by man.  With earth pressure balanced shields (EPBs) the excavated material is used as the support medium and rubble is transported away via conveyors. EPB shield are a good choice in bonding ground like clay. Often the ground is preconditioned to make it more pulpy. If foam is injected under high pressure several centrimetres into the ground from jets in the cutting wheel, the friction of the cutting wheel rotation can be reduced by an order of magnitude. In fluid shields, a bentonite suspension serves as a support and transport medium. Fluid shields are used for many different geologies but they require expensive separation facilities to recycle the bentonite suspension.Mixed-shield tunnelling machines allow EPB and fluid modes of operation. About every two metres the advance halts for 1-2 hours to build a new tunnel ring. At this point the pushing cylinders that create the forward pressure, by pressing against the tunnel rings which have already been installed, are close to their maximum stroke. An erector adds concrete segments to build a new tunnel ring while the pushing cylinders at these segments are retracted. The erector grabs the concrete segments either mechanically or it sucks them on. Its highly sensitive mechanics with six degrees of freedom assure a smooth and precise operation. About a dozen concrete segments make up one ring of about 2m length. The prefabricated tunnel segments secure the tunnel against rock and water. They weigh several tonnes each but their size has tolerances below one millimetre to make sure that they fit together. The joints between the segments are filled by elastomers. After each tunnel ring has been built, boring recommences.
earth pressure balanced tbmCutting wheel of an earth pressure balanced TBM. Foam injection is being tested. mix shield tbmMix shield TBM. Cutting wheel functionality is being tested at the fabrication site.

sketch of earth pressure balanced tbmSketch of an earth pressure balanced TBM. Note that drilled material passes a screw conveyor before it drops on a conveyor belt to be carried away. Pushing cylinders (yellow) press the TBM forward against the concrete segmental lining. The concrete segments are transported via trains. sketch of a mix shield tbmSketch of a fluid shield TBM. Note that the cutting wheel is flooded by a bentonite suspension (light brown). Bentonite pressure is controlled by a pressurised air reservoir (light blue). An erector grabs the segments to build concrete rings.
starting shaft of a tunnel boring machineStarting shaft of a tunnel boring machine. The shaft has been secured by a concrete wall and metal anchors. Note that the cutting wheel has already disappeared into the ground. The final TBM segments still have to be assembled. tunnel under constructionTunnel under construction seen from 150m behind the cutting wheel. Note the tunnel tube made up of segmental concrete linings and the pipelines for fresh air, pressurised air, electric power, bentonite suspension etc.. The railway tracks serve the transport of the concrete segments and the TBM.

Last updated August 21, 2005. © 2005, Tunnelseis. All rights reserved.