tunnels

tunnels The earliest recorded tunnels are the quanats found in the Middle East and North Africa. These tunnels, some dating from 4000 bc, conducted water from springs in the foothills to communities in the desert. The use of tunnels, some of 60 km length, made it possible to even out gradients and prevented the evaporation that would have taken place in an open watercourse. Many quanats operate to this day.

The Romans developed tunnelling further, and many Roman water-supply systems used a combination of aqueducts and tunnels to negotiate uneven terrain at a constant gradient. The Romans also developed tunnelling for military purposes, either by breaking through behind enemy defences or by undermining fortifications to cause their collapse.

Much early tunnelling was carried out in association with mineral mining, either to provide access to the body of ore or to provide drainage as workings became deeper. Excavation was carried out using pick and shovel. Harder rocks were broken down by ‘fire-setting’, a process in which the rock face was heated by burning faggots against it, and then quenched with water, causing the rock to fracture.

Modern tunnelling developed with the canal age, which required tunnels of relatively large cross-section to pass beneath the watersheds between valley systems. Arched brickwork linings were developed and the use of gunpowder eased excavation in hard ground. Railways created a further demand for tunnelling, and by the mid-nineteenth century the construction of urban sewers, which now form the greatest length of tunnels constructed, began in earnest.

The nineteenth century saw the development of compressed air tunnelling, in which the entire tunnel was pressurized in order to hold back groundwater and support the face. The elder Brunel ( Marc Isambard) also developed the tunnelling shield, a steel cylinder from within which excavation could safely take place. These techniques enabled softer ground to be tackled.

Machines to excavate tunnels began to appear in the latter half of the nineteenth century. A machine drove over a mile of a Channel tunnel in the late 1800s before the work was stopped by political problems. Preformed linings for tunnels were first developed using cast iron. Later, concrete segments which could be bolted together to form a circular lining were used to provide support to ground and watertightness.

Modern tunnelling can take place in virtually any ground conditions. Machines 15 m in diameter bore through the hardest rock for two-lane highways. Sealed-face machines tackle soft silts, sands and water pressure beneath river crossings. Remotely controlled machines enable tunnels below man-entry size to be constructed, and at the smaller sizes the boundary between tunnelling and drilling techniques is becoming increasingly blurred. The growth of modern cities has greatly increased the demand for tunnelling for road transport and metro systems and for water, gas, electricity, and telecommunications routes. The high land costs of alternative routes more than offset the additional costs of tunnelling.

Nowadays tunnels are mainly lined with concrete segments or with concrete pumped in between the excavated ground and internal shuttering. Any voids between the lining and the ground are filled by pressure grouting to improve watertightness and ensure that the loading on the lining is even. Excavated material is removed by rail or, in some systems, by mixing the spoil with a bentonite clay slurry, which is then pumped to the surface. After settling, the slurry is reused. Lining segments are brought in by rail and erected using mechanical handling systems. Excavation is carried out either by a cutting head the size of the finished tunnel or by boom cutters which can be operated over the complete tunnel face by the driver. In more variable rock conditions, drill and blast techniques are used for excavations. Steel arches or sprayed concrete keep the tunnel stable until a final lining is put in place.

The Channel Tunnel, with 102 km of main tunnels and 51 km of service tunnels, achieved progress rates of 400m a week on each tunnel face. Rates will continue to improve as machines and materials are developed further.

Hamish J. Orr-ewing