Discussion. Design and Construction of the Cargo Tunnel at Heathrow Airport, London.

Abstract

The principal object in designing this tunnel was to keep it as near to the surface as possible, not only to minimize its overall length but also to avoid steeper than necessary approaches in the congested area of the south apex of the Central Area of Heathrow Airport. The extent of cover over this large diameter tunnel is probably near to the acceptable minimum for this type of ground, within the limitations of present knowledge of the behaviour of a tunnel in conjunction with the ground.66. The cargo tunnel at Heathrow is one in a succession of tunnels adopting the principle of rings of elements expanded and stressed directly against the ground.The first large tunnel to adopt this principle was the Potters Bar tunnel built 13 years ago where, for a variety of reasons including technical development, the lining thickness represented about 8 5 % of the tunnel diameter against about 3% for the cargo tunnel. 67. Not only was the total cover at Heathrow small, but the clay over the crown was very much less; the principle of being able to build a tunnel economically in this form requires a continuous band of clay outside the tunnel against which the ring can be stressed. The line of the tunnel was explored with a number of shallow boreholes to establish the profile of the clay, to ensure within an acceptable margin of risk that the tunnel throughout would have at least 4 ft of clay cover.68. There are a number of problems which arise in designing a large tunnel so close to the surface. First, it was necessary to explore possible failure mechanisms to ensure adequate safety margins, second it was necessary to consider tolerable distortions of the tunnel after construction and third, since the settlement trough above the tunnel would be very narrow on account of proximity to the surface, it was necessary to ensure that this was not deep. As the Paper explains, the scheme appears effectively to have satisfied these requirements. 69. We should like to emphasize the degree of continuity between the original concept and the actual construction. We believe that in tunnelling this degree of cooperation is essential for good engineering and we do not think that the degree of co-operation in the technical sense that this principle requires need in any way upset contractual responsibilities. The construction of the tunnel consisted of three main activities:(U) tunnel excavation and concrete lining; (6) in situ road construction; (c) secondary lining.One of the unusual features of the method of construction was that the three operations were started in succession and eventually were carried out simultaneously. This required careful planning and detailing of the work to minimize the impingement of one activity on another.71. The main factors which determined the method of tunnel excavation and lining were the limited clay cover and the need to limit ground movements. The face excavation was carried out by hand to ensure that good support was always maintained thus limiting settlement. To ensure early support of the crown we used...