Tunnel Design and Construction Methods

The method of construction chosen for any tunnel is the result of several design considerations. The ground conditions, the type of ground or water to be crossed, the geometrical configuration of the crossing, and the environmental and regulatory requirements of construction must all be taken into account before the initial design can begin.

Detailed analysis and research into site conditions and the geology of the specific and surrounding areas must be undertaken; whilst social and cultural impact on the immediate area and population should also be considered prior to design. Further to this the functionality, life-span, durability, safety and security, and long term maintenance need to be fully planned as an integral part of the final design. Some of the preferred construction methods currently in use are detailed below, and we will look at the Coentunnel Project in Amsterdam as an example of one style of construction.

Cut and Cover Tunnels


© Copyright Paul Dixon and licensed for reuse under this Creative Commons Licence

Taken in 2005, the image above shows a clear example of a cut and cover tunnel. The Baldock bypass in the chalk hills of Weston, England, was designed in such a way as to retain the contour of the hills throughout construction.

Cut and cover tunnels, as the name suggests, are excavated into the ground then backfilled with material once the tunnel structure is in place. The design is generally used on tunnels which have a shallow profile and where excavation from the surface is viable. There are two types of cut and cover construction; bottom-up and top-down construction.

Bottom-up construction, the more conventional method, involves excavating a trench from the surface within which the tunnel is constructed, before the trench is refilled and the surface replaced. The trench may be cut with open cuts, where the sides slope back and have minimal support; or cut with vertical faces which require excavation support systems. Bottom-up is a well understood method of construction, and advantages are that waterproofing can be done on the outer surface of the structure, drainage systems can be implemented outside of the tunnel, and the inside of the structure is easily accessible for vehicles and materials. Disadvantages are that the surface cannot be restored until construction is complete, it may require relocation of utilities or temporary support, and any dewatering of the trench could have an effect on the local environment and infrastructure.

Top-down construction involves first installing the supporting tunnel walls, usually slurry walls or secant pile walls, then adding the top slab section of the tunnel. Waterproofing of the tunnel roof and back filling can be completed to restore the surface much quicker than when using the bottom-up method. Finally excavation is completed under the cover of the top section, before inner walls and floors are installed. Advantages of top-down construction are that the overall width of the construction site should be smaller, the top surface can be replaced much earlier, and both the cost and duration of the build can be reduced compared with bottom-up construction. The disadvantages are that it is not possible to waterproof the outside of the supporting walls, access for excavation is limited to shafts in the roof, and the connections for roof, floor and walls are more complicated.

Bored and Mined Tunnels

Bored or mined tunnels may be a preferred choice of construction method for some sites, as conditions play a pivotal role in design. TBM’s (Tunnel Boring Machines) can be utilised in a number of different types of ground from rock, to soft ground, to a combination of sediments often encountered in mountain tunnels. Some of the typical construction methods for rock and soft tunnelling are detailed below. TBM’s can be several metres in diameter, and the cutting teeth on the circular plate chisel into the rock, which falls through gaps in the plate onto a conveyor system dispensing it to the rear of the machine. While the main cutter excavates, two drills behind it bore holes into the rock which operators fill with grout and bolt, to hold the tunnel in place until a final lining can be installed. A TBM carries this out by attaching segments of the tunnel lining with a powerful erecting arm.

INTERESTED IN LEARNING MORE ABOUT THIS TOPIC? Tunnel Design & Construction Europe 2011 Don't miss the "Tunnel Design & Construction Europe 2011" Congress, taking place 16-18 November 2011 at the Dorint Hotel Amsterdam-Airport, NL. --> DOWNLOAD THE FREE BROCHURE