The Engineering Geology of Tunnels
Dr Michael de Freitas
Imperial College, London.
A tunnel is dynamic and never static. The ground in which it is constructed responds to the local geology and there are structural reactions such as surface changes and the type of material used to build it. The tunnel has to be supported and this is achieved in one of several ways, by using top and bottom arches, a double bridge, segments or by using reinforced concrete to form the tube. All these methods are able to support the ground. There are stresses and the intensity of load on the tunnel wall cause compression and relaxation problems.
This commenced originally at Folkestone and in the 1950s a drilling rig from the National Coal Board was used to examine the sea bed of the Channel to determine if the chalk, which was obvious at Dover, actually crossed the Channel. The cliffs at Dover are a lot higher than at Sangatte in France and this lead to more problems in France. The level of chalk also varied a lot under the sea. The UK pumping station was comprised of 3 tunnels each 7m in diameter and made of segments.
Here the country is rising and there is little alluvium. Bedding and joint sets create blocks which need support in the tunnel. This is done by using hard rock with soft joints between.
Lisbon is similar in many ways to S England, and has a high concentration of marl. Here a cut and cover tunnel, similar to the District Line, was used. However, in Lisbon a series of stresses around the tunnel brought about damage to the water main and to an ornamental pond in the zoo resulting in monkeys escaping and going up the tunnel.
Round Hill in Kent
The geology of this area is galt clay at the bottom superimposed with chalk. Here there is some relaxation in the rock and blocks of chalk have opened.
This is a region of very old granite where the blocks have been subjected previously to movement, tropical weathering and the hydro-thermal effect.
Here is yet another form of construction where the metro is fully self-supporting.
As with all our talks, this was a well illustrated lecture with clear slides and a knowledgeable and enthusiastic speaker. It proved popular with members and stimulated a lot of questions.
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